diff --git a/java/com/google/gerrit/plugins/codeowners/backend/BackendModule.java b/java/com/google/gerrit/plugins/codeowners/backend/BackendModule.java
index 50e5f68..3bdb518 100644
--- a/java/com/google/gerrit/plugins/codeowners/backend/BackendModule.java
+++ b/java/com/google/gerrit/plugins/codeowners/backend/BackendModule.java
@@ -47,7 +47,8 @@
     install(new CodeOwnerSubmitRule.Module());
 
     DynamicSet.bind(binder(), ExceptionHook.class).to(CodeOwnersExceptionHook.class);
-    DynamicSet.bind(binder(), OnPostReview.class).to(CodeOwnersOnPostReview.class);
+    DynamicSet.bind(binder(), OnPostReview.class).to(OnCodeOwnerApproval.class);
+    DynamicSet.bind(binder(), OnPostReview.class).to(OnCodeOwnerOverride.class);
     DynamicSet.bind(binder(), ReviewerAddedListener.class).to(CodeOwnersOnAddReviewer.class);
   }
 
diff --git a/java/com/google/gerrit/plugins/codeowners/backend/CodeOwnersOnPostReview.java b/java/com/google/gerrit/plugins/codeowners/backend/OnCodeOwnerApproval.java
similarity index 97%
rename from java/com/google/gerrit/plugins/codeowners/backend/CodeOwnersOnPostReview.java
rename to java/com/google/gerrit/plugins/codeowners/backend/OnCodeOwnerApproval.java
index 1596e23..e818163 100644
--- a/java/com/google/gerrit/plugins/codeowners/backend/CodeOwnersOnPostReview.java
+++ b/java/com/google/gerrit/plugins/codeowners/backend/OnCodeOwnerApproval.java
@@ -35,7 +35,8 @@
 import java.util.stream.Stream;
 
 /**
- * Callback that is invoked on post review.
+ * Callback that is invoked on post review and that extends the change message if a code owner
+ * approval was changed.
  *
  * <p>If a code owner approval was added, removed or changed, include in the change message that is
  * being posted on vote, which of the files:
@@ -47,14 +48,14 @@
  * </ul>
  */
 @Singleton
-class CodeOwnersOnPostReview implements OnPostReview {
+class OnCodeOwnerApproval implements OnPostReview {
   private static final FluentLogger logger = FluentLogger.forEnclosingClass();
 
   private final CodeOwnersPluginConfiguration codeOwnersPluginConfiguration;
   private final CodeOwnerApprovalCheck codeOwnerApprovalCheck;
 
   @Inject
-  CodeOwnersOnPostReview(
+  OnCodeOwnerApproval(
       CodeOwnersPluginConfiguration codeOwnersPluginConfiguration,
       CodeOwnerApprovalCheck codeOwnerApprovalCheck) {
     this.codeOwnersPluginConfiguration = codeOwnersPluginConfiguration;
diff --git a/java/com/google/gerrit/plugins/codeowners/backend/OnCodeOwnerOverride.java b/java/com/google/gerrit/plugins/codeowners/backend/OnCodeOwnerOverride.java
new file mode 100644
index 0000000..df24fbb
--- /dev/null
+++ b/java/com/google/gerrit/plugins/codeowners/backend/OnCodeOwnerOverride.java
@@ -0,0 +1,167 @@
+// Copyright (C) 2021 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.plugins.codeowners.backend;
+
+import static com.google.common.base.Preconditions.checkState;
+
+import com.google.common.base.Joiner;
+import com.google.common.collect.ImmutableSet;
+import com.google.common.flogger.FluentLogger;
+import com.google.gerrit.entities.PatchSet;
+import com.google.gerrit.plugins.codeowners.backend.config.CodeOwnersPluginConfiguration;
+import com.google.gerrit.plugins.codeowners.backend.config.RequiredApproval;
+import com.google.gerrit.server.IdentifiedUser;
+import com.google.gerrit.server.notedb.ChangeNotes;
+import com.google.gerrit.server.restapi.change.OnPostReview;
+import com.google.gerrit.server.util.LabelVote;
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+
+/**
+ * Callback that is invoked on post review and that extends the change message if a code owner
+ * override was changed.
+ *
+ * <p>If a code owner override was added, removed or changed, include in the change message that is
+ * being posted on vote, that the vote is a code owner override to let users know about its effect.
+ */
+@Singleton
+class OnCodeOwnerOverride implements OnPostReview {
+  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
+
+  private final CodeOwnersPluginConfiguration codeOwnersPluginConfiguration;
+
+  @Inject
+  OnCodeOwnerOverride(CodeOwnersPluginConfiguration codeOwnersPluginConfiguration) {
+    this.codeOwnersPluginConfiguration = codeOwnersPluginConfiguration;
+  }
+
+  @Override
+  public Optional<String> getChangeMessageAddOn(
+      IdentifiedUser user,
+      ChangeNotes changeNotes,
+      PatchSet patchSet,
+      Map<String, Short> oldApprovals,
+      Map<String, Short> approvals) {
+    if (codeOwnersPluginConfiguration.isDisabled(changeNotes.getChange().getDest())) {
+      return Optional.empty();
+    }
+
+    // code owner overrides are only relevant for the current patch set
+    if (!changeNotes.getChange().currentPatchSetId().equals(patchSet.id())) {
+      return Optional.empty();
+    }
+
+    ImmutableSet<RequiredApproval> overrideApprovals =
+        codeOwnersPluginConfiguration.getOverrideApproval(changeNotes.getProjectName());
+
+    List<String> messages = new ArrayList<>();
+    for (RequiredApproval overrideApproval : overrideApprovals) {
+      if (oldApprovals.get(overrideApproval.labelType().getName()) == null) {
+        // If oldApprovals doesn't contain the label or if the labels value in it is null, the label
+        // was not changed.
+        continue;
+      }
+
+      buildMessageForCodeOwnerOverride(
+              user, changeNotes, patchSet, oldApprovals, approvals, overrideApproval)
+          .ifPresent(messages::add);
+    }
+
+    if (messages.isEmpty()) {
+      return Optional.empty();
+    }
+    return Optional.of(Joiner.on("\n\n").join(messages));
+  }
+
+  private Optional<String> buildMessageForCodeOwnerOverride(
+      IdentifiedUser user,
+      ChangeNotes changeNotes,
+      PatchSet patchSet,
+      Map<String, Short> oldApprovals,
+      Map<String, Short> approvals,
+      RequiredApproval overrideApproval) {
+    LabelVote newVote = getNewVote(overrideApproval, approvals);
+
+    if (isCodeOwnerOverrideNewlyApplied(overrideApproval, oldApprovals, newVote)) {
+      return Optional.of(
+          String.format(
+              "By voting %s the code-owners submit requirement is overridden by %s",
+              newVote, user.getName()));
+    } else if (isCodeOwnerOverrideRemoved(overrideApproval, oldApprovals, newVote)) {
+      if (newVote.value() == 0) {
+        return Optional.of(
+            String.format(
+                "By removing the %s vote the code-owners submit requirement is no longer overridden"
+                    + " by %s",
+                newVote.label(), user.getName()));
+      }
+      return Optional.of(
+          String.format(
+              "By voting %s the code-owners submit requirement is no longer overridden by %s",
+              newVote, user.getName()));
+    } else if (isCodeOwnerOverrideUpOrDowngraded(overrideApproval, oldApprovals, newVote)) {
+      return Optional.of(
+          String.format(
+              "By voting %s the code-owners submit requirement is still overridden by %s",
+              newVote, user.getName()));
+    }
+    logger.atSevere().log(
+        "code owner override was neither newly applied, removed, upgraded nor downgraded:"
+            + " project = %s, change = %s, patch set = %d, oldApprvals = %s, approvals = %s,"
+            + " requiredApproval = %s",
+        changeNotes.getProjectName(),
+        changeNotes.getChangeId(),
+        patchSet.id().get(),
+        oldApprovals,
+        approvals,
+        overrideApproval);
+    return Optional.empty();
+  }
+
+  private boolean isCodeOwnerOverrideNewlyApplied(
+      RequiredApproval requiredApproval, Map<String, Short> oldApprovals, LabelVote newVote) {
+    String labelName = requiredApproval.labelType().getName();
+    return oldApprovals.get(labelName) < requiredApproval.value()
+        && newVote.value() >= requiredApproval.value();
+  }
+
+  private boolean isCodeOwnerOverrideRemoved(
+      RequiredApproval requiredApproval, Map<String, Short> oldApprovals, LabelVote newVote) {
+    String labelName = requiredApproval.labelType().getName();
+    return oldApprovals.get(labelName) >= requiredApproval.value()
+        && newVote.value() < requiredApproval.value();
+  }
+
+  private boolean isCodeOwnerOverrideUpOrDowngraded(
+      RequiredApproval requiredApproval, Map<String, Short> oldApprovals, LabelVote newVote) {
+    String labelName = requiredApproval.labelType().getName();
+    return oldApprovals.get(labelName) >= requiredApproval.value()
+        && newVote.value() >= requiredApproval.value();
+  }
+
+  private LabelVote getNewVote(RequiredApproval requiredApproval, Map<String, Short> approvals) {
+    String labelName = requiredApproval.labelType().getName();
+    checkState(
+        approvals.containsKey(labelName),
+        "expected that approval on label %s exists (approvals = %s)",
+        labelName,
+        approvals);
+    return LabelVote.create(labelName, approvals.get(labelName));
+  }
+}
diff --git a/java/com/google/gerrit/plugins/codeowners/validation/CodeOwnerConfigValidator.java b/java/com/google/gerrit/plugins/codeowners/validation/CodeOwnerConfigValidator.java
index e4714bd..cef6d52 100644
--- a/java/com/google/gerrit/plugins/codeowners/validation/CodeOwnerConfigValidator.java
+++ b/java/com/google/gerrit/plugins/codeowners/validation/CodeOwnerConfigValidator.java
@@ -855,6 +855,14 @@
         PathCodeOwners.createKeyForImportedCodeOwnerConfig(
             keyOfImportingCodeOwnerConfig, codeOwnerConfigReference);
 
+    if (isSelfImport(keyOfImportingCodeOwnerConfig, keyOfImportedCodeOwnerConfig)) {
+      return nonResolvableImport(
+          importType,
+          codeOwnerConfigFilePath,
+          "code owner config imports itself",
+          ValidationMessage.Type.WARNING);
+    }
+
     Optional<ProjectState> projectState = projectCache.get(keyOfImportedCodeOwnerConfig.project());
     if (!projectState.isPresent() || !isProjectReadable(keyOfImportedCodeOwnerConfig)) {
       // we intentionally use the same error message for non-existing and non-readable projects so
@@ -944,6 +952,21 @@
     return Optional.empty();
   }
 
+  /** Whether the importing code owner config is the same as the imported code owner config. */
+  private boolean isSelfImport(
+      CodeOwnerConfig.Key keyOfImportingCodeOwnerConfig,
+      CodeOwnerConfig.Key keyOfImportedCodeOwnerConfig) {
+    return keyOfImportingCodeOwnerConfig.project().equals(keyOfImportedCodeOwnerConfig.project())
+        && keyOfImportingCodeOwnerConfig.ref().equals(keyOfImportedCodeOwnerConfig.ref())
+        && codeOwnersPluginConfiguration
+            .getBackend(keyOfImportingCodeOwnerConfig.branchNameKey())
+            .getFilePath(keyOfImportingCodeOwnerConfig)
+            .equals(
+                codeOwnersPluginConfiguration
+                    .getBackend(keyOfImportedCodeOwnerConfig.branchNameKey())
+                    .getFilePath(keyOfImportedCodeOwnerConfig));
+  }
+
   private boolean isProjectReadable(CodeOwnerConfig.Key keyOfImportedCodeOwnerConfig) {
     try {
       return permissionBackend
@@ -994,14 +1017,26 @@
       CodeOwnerConfigImportType importType,
       Path codeOwnerConfigFilePath,
       String message) {
+    return nonResolvableImport(
+        importType,
+        codeOwnerConfigFilePath,
+        message,
+        codeOwnersPluginConfiguration.rejectNonResolvableImports(project)
+            ? ValidationMessage.Type.ERROR
+            : ValidationMessage.Type.WARNING);
+  }
+
+  private Optional<CommitValidationMessage> nonResolvableImport(
+      CodeOwnerConfigImportType importType,
+      Path codeOwnerConfigFilePath,
+      String message,
+      ValidationMessage.Type validationMessageType) {
     return Optional.of(
         new CommitValidationMessage(
             String.format(
                 "invalid %s import in '%s': %s",
                 importType.getType(), codeOwnerConfigFilePath, message),
-            codeOwnersPluginConfiguration.rejectNonResolvableImports(project)
-                ? ValidationMessage.Type.ERROR
-                : ValidationMessage.Type.WARNING));
+            validationMessageType));
   }
 
   private Optional<CommitValidationMessage> nonResolvableCodeOwner(
diff --git a/javatests/com/google/gerrit/plugins/codeowners/acceptance/api/CodeOwnerConfigValidatorIT.java b/javatests/com/google/gerrit/plugins/codeowners/acceptance/api/CodeOwnerConfigValidatorIT.java
index 12b3360..3dfa9db 100644
--- a/javatests/com/google/gerrit/plugins/codeowners/acceptance/api/CodeOwnerConfigValidatorIT.java
+++ b/javatests/com/google/gerrit/plugins/codeowners/acceptance/api/CodeOwnerConfigValidatorIT.java
@@ -1023,6 +1023,102 @@
   }
 
   @Test
+  public void uploadConfigWithGlobalSelfImportReportsAWarning() throws Exception {
+    testUploadConfigWithSelfImport(CodeOwnerConfigImportType.GLOBAL);
+  }
+
+  @Test
+  public void uploadConfigWithPerFileSelfImportReportsAWarning() throws Exception {
+    testUploadConfigWithSelfImport(CodeOwnerConfigImportType.PER_FILE);
+  }
+
+  private void testUploadConfigWithSelfImport(CodeOwnerConfigImportType importType)
+      throws Exception {
+    skipTestIfImportsNotSupportedByCodeOwnersBackend();
+
+    // create a code owner config that imports itself
+    CodeOwnerConfig.Key keyOfImportingCodeOwnerConfig = createCodeOwnerConfigKey("/");
+    CodeOwnerConfigReference codeOwnerConfigReference =
+        CodeOwnerConfigReference.builder(
+                CodeOwnerConfigImportMode.GLOBAL_CODE_OWNER_SETS_ONLY,
+                codeOwnerConfigOperations
+                    .codeOwnerConfig(keyOfImportingCodeOwnerConfig)
+                    .getFilePath())
+            .setProject(project)
+            .build();
+    CodeOwnerConfig codeOwnerConfig =
+        createCodeOwnerConfigWithImport(
+            keyOfImportingCodeOwnerConfig, importType, codeOwnerConfigReference);
+
+    PushOneCommit.Result r =
+        createChange(
+            "Add code owners",
+            codeOwnerConfigOperations
+                .codeOwnerConfig(keyOfImportingCodeOwnerConfig)
+                .getJGitFilePath(),
+            format(codeOwnerConfig));
+    assertOkWithWarnings(
+        r,
+        "invalid code owner config files",
+        String.format(
+            "invalid %s import in '%s': code owner config imports itself",
+            importType.getType(),
+            codeOwnerConfigOperations
+                .codeOwnerConfig(keyOfImportingCodeOwnerConfig)
+                .getFilePath()));
+  }
+
+  @Test
+  public void canUploadConfigWithGlobalImportOfFileWithExtensionFromSameFolder() throws Exception {
+    testUploadConfigWithImportOfFileWithExtensionFromSameFolder(CodeOwnerConfigImportType.GLOBAL);
+  }
+
+  @Test
+  public void canUploadConfigWithPerFileImportOfFileWithExtensionFromSameFolder() throws Exception {
+    testUploadConfigWithImportOfFileWithExtensionFromSameFolder(CodeOwnerConfigImportType.PER_FILE);
+  }
+
+  private void testUploadConfigWithImportOfFileWithExtensionFromSameFolder(
+      CodeOwnerConfigImportType importType) throws Exception {
+    skipTestIfImportsNotSupportedByCodeOwnersBackend();
+
+    // create a code owner config that imports a code owner config from the same folder but with an
+    // extension in the file name
+    CodeOwnerConfig.Key keyOfImportingCodeOwnerConfig = createCodeOwnerConfigKey("/");
+    CodeOwnerConfig.Key keyOfImportedCodeOwnerConfig =
+        codeOwnerConfigOperations
+            .newCodeOwnerConfig()
+            .project(project)
+            .branch("master")
+            .folderPath("/")
+            .fileName(getCodeOwnerConfigFileName() + "_extension")
+            .addCodeOwnerEmail(user.email())
+            .create();
+    GitUtil.fetch(testRepo, "refs/*:refs/*");
+    testRepo.reset(projectOperations.project(project).getHead("master"));
+    CodeOwnerConfigReference codeOwnerConfigReference =
+        CodeOwnerConfigReference.builder(
+                CodeOwnerConfigImportMode.GLOBAL_CODE_OWNER_SETS_ONLY,
+                codeOwnerConfigOperations
+                    .codeOwnerConfig(keyOfImportedCodeOwnerConfig)
+                    .getFilePath())
+            .setProject(project)
+            .build();
+    CodeOwnerConfig codeOwnerConfig =
+        createCodeOwnerConfigWithImport(
+            keyOfImportingCodeOwnerConfig, importType, codeOwnerConfigReference);
+
+    PushOneCommit.Result r =
+        createChange(
+            "Add code owners",
+            codeOwnerConfigOperations
+                .codeOwnerConfig(keyOfImportingCodeOwnerConfig)
+                .getJGitFilePath(),
+            format(codeOwnerConfig));
+    r.assertOkStatus();
+  }
+
+  @Test
   public void cannotUploadConfigWithGlobalImportFromNonExistingProject() throws Exception {
     testUploadConfigWithImportFromNonExistingProject(CodeOwnerConfigImportType.GLOBAL);
   }
@@ -1764,6 +1860,16 @@
     return ObjectIds.abbreviateName(id, testRepo.getRevWalk().getObjectReader());
   }
 
+  private String getCodeOwnerConfigFileName() {
+    CodeOwnerBackend backend = backendConfig.getDefaultBackend();
+    if (backend instanceof FindOwnersBackend) {
+      return FindOwnersBackend.CODE_OWNER_CONFIG_FILE_NAME;
+    } else if (backend instanceof ProtoBackend) {
+      return ProtoBackend.CODE_OWNER_CONFIG_FILE_NAME;
+    }
+    throw new IllegalStateException("unknown code owner backend: " + backend.getClass().getName());
+  }
+
   private static void assertOkWithoutMessages(PushOneCommit.Result pushResult) {
     pushResult.assertOkStatus();
     pushResult.assertNotMessage("fatal");
diff --git a/javatests/com/google/gerrit/plugins/codeowners/acceptance/api/CodeOwnersOnPostReviewIT.java b/javatests/com/google/gerrit/plugins/codeowners/acceptance/api/OnCodeOwnerApprovalIT.java
similarity index 99%
rename from javatests/com/google/gerrit/plugins/codeowners/acceptance/api/CodeOwnersOnPostReviewIT.java
rename to javatests/com/google/gerrit/plugins/codeowners/acceptance/api/OnCodeOwnerApprovalIT.java
index 9bae548..97f426a 100644
--- a/javatests/com/google/gerrit/plugins/codeowners/acceptance/api/CodeOwnersOnPostReviewIT.java
+++ b/javatests/com/google/gerrit/plugins/codeowners/acceptance/api/OnCodeOwnerApprovalIT.java
@@ -38,9 +38,9 @@
 import org.junit.Test;
 
 /**
- * Acceptance test for {@code com.google.gerrit.plugins.codeowners.backend.CodeOwnersOnPostReview}.
+ * Acceptance test for {@code com.google.gerrit.plugins.codeowners.backend.OnCodeOwnerApproval}.
  */
-public class CodeOwnersOnPostReviewIT extends AbstractCodeOwnersIT {
+public class OnCodeOwnerApprovalIT extends AbstractCodeOwnersIT {
   @Inject private RequestScopeOperations requestScopeOperations;
   @Inject private ProjectOperations projectOperations;
 
diff --git a/javatests/com/google/gerrit/plugins/codeowners/acceptance/api/OnCodeOwnerOverrrideIT.java b/javatests/com/google/gerrit/plugins/codeowners/acceptance/api/OnCodeOwnerOverrrideIT.java
new file mode 100644
index 0000000..f094f7b
--- /dev/null
+++ b/javatests/com/google/gerrit/plugins/codeowners/acceptance/api/OnCodeOwnerOverrrideIT.java
@@ -0,0 +1,437 @@
+// Copyright (C) 2021 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.plugins.codeowners.acceptance.api;
+
+import static com.google.common.truth.Truth.assertThat;
+import static com.google.gerrit.server.group.SystemGroupBackend.REGISTERED_USERS;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.Iterables;
+import com.google.common.collect.Lists;
+import com.google.gerrit.acceptance.config.GerritConfig;
+import com.google.gerrit.acceptance.testsuite.project.ProjectOperations;
+import com.google.gerrit.acceptance.testsuite.project.TestProjectUpdate;
+import com.google.gerrit.acceptance.testsuite.request.RequestScopeOperations;
+import com.google.gerrit.entities.BranchNameKey;
+import com.google.gerrit.extensions.api.changes.ReviewInput;
+import com.google.gerrit.extensions.api.projects.DeleteBranchesInput;
+import com.google.gerrit.extensions.common.ChangeMessageInfo;
+import com.google.gerrit.extensions.common.LabelDefinitionInput;
+import com.google.gerrit.plugins.codeowners.acceptance.AbstractCodeOwnersIT;
+import com.google.inject.Inject;
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.regex.Pattern;
+import org.junit.Test;
+
+/** Acceptance test for {@code com.google.gerrit.plugins.codeowners.backend.OnCodeOwnerOverride}. */
+public class OnCodeOwnerOverrrideIT extends AbstractCodeOwnersIT {
+  @Inject private RequestScopeOperations requestScopeOperations;
+  @Inject private ProjectOperations projectOperations;
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.disabled", value = "true")
+  @GerritConfig(name = "plugin.code-owners.overrideApproval", value = "Owners-Override+1")
+  public void changeMessageNotExtendedIfCodeOwnersFuctionalityIsDisabled() throws Exception {
+    createOwnersOverrideLabel();
+
+    String changeId = createChange().getChangeId();
+
+    gApi.changes().id(changeId).current().review(new ReviewInput().label("Owners-Override", 1));
+
+    Collection<ChangeMessageInfo> messages = gApi.changes().id(changeId).get().messages;
+    assertThat(Iterables.getLast(messages).message).isEqualTo("Patch Set 1: Owners-Override+1");
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.overrideApproval", value = "Owners-Override+1")
+  public void changeMessageExtendedIfCodeOwnersOverrideIsApplied() throws Exception {
+    createOwnersOverrideLabel();
+
+    String changeId = createChange().getChangeId();
+
+    gApi.changes().id(changeId).current().review(new ReviewInput().label("Owners-Override", 1));
+
+    Collection<ChangeMessageInfo> messages = gApi.changes().id(changeId).get().messages;
+    assertThat(Iterables.getLast(messages).message)
+        .isEqualTo(
+            String.format(
+                "Patch Set 1: Owners-Override+1\n\n"
+                    + "By voting Owners-Override+1 the code-owners submit requirement is overridden by %s\n",
+                admin.fullName()));
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.overrideApproval", value = "Owners-Override+1")
+  public void changeMessageExtendedIfCodeOwnersOverrideIsReApplied() throws Exception {
+    createOwnersOverrideLabel();
+
+    String changeId = createChange().getChangeId();
+
+    gApi.changes().id(changeId).current().review(new ReviewInput().label("Owners-Override", 1));
+
+    int messageCount = gApi.changes().id(changeId).get().messages.size();
+
+    // Apply the Owners-Override+1 approval again
+    gApi.changes().id(changeId).current().review(new ReviewInput().label("Owners-Override", 1));
+
+    // Check that a new change message was added.
+    Collection<ChangeMessageInfo> messages = gApi.changes().id(changeId).get().messages;
+    assertThat(messages.size()).isEqualTo(messageCount + 1);
+
+    assertThat(Iterables.getLast(messages).message)
+        .isEqualTo(
+            String.format(
+                "Patch Set 1: Owners-Override+1\n\n"
+                    + "By voting Owners-Override+1 the code-owners submit requirement is still overridden by %s\n",
+                admin.fullName()));
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.overrideApproval", value = "Owners-Override+1")
+  public void changeMessageExtendedIfCodeOwnersOverrideIsAppliedByOtherUser() throws Exception {
+    createOwnersOverrideLabel();
+
+    String changeId = createChange().getChangeId();
+
+    gApi.changes().id(changeId).current().review(new ReviewInput().label("Owners-Override", 1));
+
+    Collection<ChangeMessageInfo> messages = gApi.changes().id(changeId).get().messages;
+    assertThat(Iterables.getLast(messages).message)
+        .isEqualTo(
+            String.format(
+                "Patch Set 1: Owners-Override+1\n\n"
+                    + "By voting Owners-Override+1 the code-owners submit requirement is overridden by %s\n",
+                admin.fullName()));
+
+    // Apply the Owners-Override+1 approval by another user
+    requestScopeOperations.setApiUser(user.id());
+    gApi.changes().id(changeId).current().review(new ReviewInput().label("Owners-Override", 1));
+
+    messages = gApi.changes().id(changeId).get().messages;
+    assertThat(Iterables.getLast(messages).message)
+        .isEqualTo(
+            String.format(
+                "Patch Set 1: Owners-Override+1\n\n"
+                    + "By voting Owners-Override+1 the code-owners submit requirement is overridden by %s\n",
+                user.fullName()));
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.overrideApproval", value = "Owners-Override+1")
+  public void changeMessageExtendedIfCodeOwnersOverrideIsUpgraded() throws Exception {
+    LabelDefinitionInput input = new LabelDefinitionInput();
+    input.values = ImmutableMap.of("+2", "Override", "+1", "Override", " 0", "No Override");
+    gApi.projects().name(project.get()).label("Owners-Override").create(input).get();
+
+    // Allow to vote on the Owners-Override label.
+    projectOperations
+        .project(project)
+        .forUpdate()
+        .add(
+            TestProjectUpdate.allowLabel("Owners-Override")
+                .range(0, 2)
+                .ref("refs/*")
+                .group(REGISTERED_USERS)
+                .build())
+        .update();
+
+    String changeId = createChange().getChangeId();
+
+    gApi.changes().id(changeId).current().review(new ReviewInput().label("Owners-Override", 1));
+
+    // Upgrade the approval from Owners-Override+1 to Owners-Override+2
+    gApi.changes().id(changeId).current().review(new ReviewInput().label("Owners-Override", 2));
+
+    Collection<ChangeMessageInfo> messages = gApi.changes().id(changeId).get().messages;
+    assertThat(Iterables.getLast(messages).message)
+        .isEqualTo(
+            String.format(
+                "Patch Set 1: Owners-Override+2\n\n"
+                    + "By voting Owners-Override+2 the code-owners submit requirement is still"
+                    + " overridden by %s\n",
+                admin.fullName()));
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.overrideApproval", value = "Owners-Override+1")
+  public void changeMessageExtendedIfCodeOwnersOverrideIsDowngraded() throws Exception {
+    LabelDefinitionInput input = new LabelDefinitionInput();
+    input.values = ImmutableMap.of("+2", "Override", "+1", "Override", " 0", "No Override");
+    gApi.projects().name(project.get()).label("Owners-Override").create(input).get();
+
+    // Allow to vote on the Owners-Override label.
+    projectOperations
+        .project(project)
+        .forUpdate()
+        .add(
+            TestProjectUpdate.allowLabel("Owners-Override")
+                .range(0, 2)
+                .ref("refs/*")
+                .group(REGISTERED_USERS)
+                .build())
+        .update();
+
+    String changeId = createChange().getChangeId();
+
+    gApi.changes().id(changeId).current().review(new ReviewInput().label("Owners-Override", 2));
+
+    // Downgrade the approval from Owners-Override+2 to Owners-Override+1
+    gApi.changes().id(changeId).current().review(new ReviewInput().label("Owners-Override", 1));
+
+    Collection<ChangeMessageInfo> messages = gApi.changes().id(changeId).get().messages;
+    assertThat(Iterables.getLast(messages).message)
+        .isEqualTo(
+            String.format(
+                "Patch Set 1: Owners-Override+1\n\n"
+                    + "By voting Owners-Override+1 the code-owners submit requirement is still"
+                    + " overridden by %s\n",
+                admin.fullName()));
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.overrideApproval", value = "Owners-Override+1")
+  public void changeMessageExtendedIfCodeOwnersOverrideIsRemoved() throws Exception {
+    createOwnersOverrideLabel();
+
+    String changeId = createChange().getChangeId();
+
+    gApi.changes().id(changeId).current().review(new ReviewInput().label("Owners-Override", 1));
+
+    // Remove the override approval
+    gApi.changes().id(changeId).current().review(new ReviewInput().label("Owners-Override", 0));
+
+    Collection<ChangeMessageInfo> messages = gApi.changes().id(changeId).get().messages;
+    assertThat(Iterables.getLast(messages).message)
+        .isEqualTo(
+            String.format(
+                "Patch Set 1: -Owners-Override\n\n"
+                    + "By removing the Owners-Override vote the code-owners submit requirement is"
+                    + " no longer overridden by %s\n",
+                admin.fullName()));
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.overrideApproval", value = "Owners-Override+1")
+  public void changeMessageExtendedIfCodeOwnersOverrideIsChangedToNegativeValue() throws Exception {
+    LabelDefinitionInput input = new LabelDefinitionInput();
+    input.values = ImmutableMap.of("+1", "Override", " 0", "No Override", "-1", "No Override");
+    gApi.projects().name(project.get()).label("Owners-Override").create(input).get();
+
+    // Allow to vote on the Owners-Override label.
+    projectOperations
+        .project(project)
+        .forUpdate()
+        .add(
+            TestProjectUpdate.allowLabel("Owners-Override")
+                .range(-1, 1)
+                .ref("refs/*")
+                .group(REGISTERED_USERS)
+                .build())
+        .update();
+
+    String changeId = createChange().getChangeId();
+
+    gApi.changes().id(changeId).current().review(new ReviewInput().label("Owners-Override", 1));
+
+    // Vote with Owners-Override-1
+    gApi.changes().id(changeId).current().review(new ReviewInput().label("Owners-Override", -1));
+
+    Collection<ChangeMessageInfo> messages = gApi.changes().id(changeId).get().messages;
+    assertThat(Iterables.getLast(messages).message)
+        .isEqualTo(
+            String.format(
+                "Patch Set 1: Owners-Override-1\n\n"
+                    + "By voting Owners-Override-1 the code-owners submit requirement is no longer"
+                    + " overridden by %s\n",
+                admin.fullName()));
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.overrideApproval", value = "Owners-Override+1")
+  public void changeMessageNotExtendedIfNonCodeOwnersOverrideIsApplied() throws Exception {
+    LabelDefinitionInput input = new LabelDefinitionInput();
+    input.values = ImmutableMap.of("+1", "Approval", " 0", "No Approval");
+    gApi.projects().name(project.get()).label("Other").create(input).get();
+
+    // Allow to vote on the Owners-Override label.
+    projectOperations
+        .project(project)
+        .forUpdate()
+        .add(
+            TestProjectUpdate.allowLabel("Other")
+                .range(0, 1)
+                .ref("refs/*")
+                .group(REGISTERED_USERS)
+                .build())
+        .update();
+
+    String changeId = createChange().getChangeId();
+
+    gApi.changes().id(changeId).current().review(new ReviewInput().label("Other", 1));
+
+    Collection<ChangeMessageInfo> messages = gApi.changes().id(changeId).get().messages;
+    assertThat(Iterables.getLast(messages).message).isEqualTo("Patch Set 1: Other+1");
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.overrideApproval", value = "Owners-Override+1")
+  public void changeMessageExtendedIfCodeOwnersOverrideIsAppliedTogetherWithComment()
+      throws Exception {
+    createOwnersOverrideLabel();
+
+    String path = "foo/bar.baz";
+    String changeId = createChange("Test Change", path, "file content").getChangeId();
+
+    ReviewInput.CommentInput commentInput = new ReviewInput.CommentInput();
+    commentInput.line = 1;
+    commentInput.message = "some comment";
+    commentInput.path = path;
+    ReviewInput reviewInput = new ReviewInput();
+    reviewInput.labels = new HashMap<>();
+    reviewInput.labels.put("Owners-Override", (short) 1);
+    reviewInput.comments = new HashMap<>();
+    reviewInput.comments.put(commentInput.path, Lists.newArrayList(commentInput));
+    gApi.changes().id(changeId).current().review(reviewInput);
+
+    Collection<ChangeMessageInfo> messages = gApi.changes().id(changeId).get().messages;
+    assertThat(Iterables.getLast(messages).message)
+        .isEqualTo(
+            String.format(
+                "Patch Set 1: Owners-Override+1\n\n"
+                    + "(1 comment)\n\n"
+                    + "By voting Owners-Override+1 the code-owners submit requirement is"
+                    + " overridden by %s\n",
+                admin.fullName()));
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.overrideApproval", value = "Owners-Override+1")
+  public void changeMessageNotExtendedIfCodeOwnersOverrideIsAppliedOnOldPatchSet()
+      throws Exception {
+    createOwnersOverrideLabel();
+
+    String changeId = createChange().getChangeId();
+
+    // create a second patch set
+    amendChange(changeId);
+
+    // vote on the first patch set
+    gApi.changes().id(changeId).revision(1).review(new ReviewInput().label("Owners-Override", 1));
+
+    Collection<ChangeMessageInfo> messages = gApi.changes().id(changeId).get().messages;
+    assertThat(Iterables.getLast(messages).message).isEqualTo("Patch Set 1: Owners-Override+1");
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.overrideApproval", value = "Owners-Override+1")
+  public void changeMessageExtendedIfDestinationBranchWasDeleted() throws Exception {
+    createOwnersOverrideLabel();
+
+    String branchName = "tempBranch";
+    createBranch(BranchNameKey.create(project, branchName));
+
+    String changeId = createChange("refs/for/" + branchName).getChangeId();
+
+    DeleteBranchesInput input = new DeleteBranchesInput();
+    input.branches = ImmutableList.of(branchName);
+    gApi.projects().name(project.get()).deleteBranches(input);
+
+    gApi.changes().id(changeId).current().review(new ReviewInput().label("Owners-Override", 1));
+
+    Collection<ChangeMessageInfo> messages = gApi.changes().id(changeId).get().messages;
+    assertThat(Iterables.getLast(messages).message)
+        .isEqualTo(
+            String.format(
+                "Patch Set 1: Owners-Override+1\n\n"
+                    + "By voting Owners-Override+1 the code-owners submit requirement is overridden by %s\n",
+                admin.fullName()));
+  }
+
+  @Test
+  @GerritConfig(
+      name = "plugin.code-owners.overrideApproval",
+      values = {"Owners-Override+1", "Global-Override+1"})
+  public void changeMessageExtendedIfMultipleCodeOwnersOverridesAreAppliedTogether()
+      throws Exception {
+    createOwnersOverrideLabel();
+    createOwnersOverrideLabel("Global-Override");
+
+    String changeId = createChange().getChangeId();
+
+    ReviewInput reviewInput = new ReviewInput();
+    reviewInput.labels = new HashMap<>();
+    reviewInput.labels.put("Owners-Override", (short) 1);
+    reviewInput.labels.put("Global-Override", (short) 1);
+    gApi.changes().id(changeId).current().review(reviewInput);
+
+    Collection<ChangeMessageInfo> messages = gApi.changes().id(changeId).get().messages;
+    assertThat(Iterables.getLast(messages).message)
+        .matches(
+            Pattern.quote("Patch Set 1: ")
+                + "("
+                + Pattern.quote("Owners-Override+1 Global-Override+1")
+                + "|"
+                + Pattern.quote("Global-Override+1 Owners-Override+1")
+                + ")"
+                + Pattern.quote(
+                    String.format(
+                        "\n\nBy voting Owners-Override+1 the code-owners submit requirement is"
+                            + " overridden by %s\n\n"
+                            + "By voting Global-Override+1 the code-owners submit requirement is"
+                            + " overridden by %s\n",
+                        admin.fullName(), admin.fullName())));
+  }
+
+  @Test
+  @GerritConfig(name = "plugin.code-owners.overrideApproval", value = "Owners-Override+1")
+  public void changeMessageExtendedIfCodeOwnersOverrideAndCodeOwnerApprovalAreAppliedTogether()
+      throws Exception {
+    createOwnersOverrideLabel();
+
+    String path = "foo/bar.baz";
+    String changeId = createChange("Test Change", path, "file content").getChangeId();
+
+    ReviewInput reviewInput = new ReviewInput();
+    reviewInput.labels = new HashMap<>();
+    reviewInput.labels.put("Owners-Override", (short) 1);
+    reviewInput.labels.put("Code-Review", (short) 1);
+    gApi.changes().id(changeId).current().review(reviewInput);
+
+    Collection<ChangeMessageInfo> messages = gApi.changes().id(changeId).get().messages;
+    assertThat(Iterables.getLast(messages).message).startsWith("Patch Set 1: ");
+    assertThat(Iterables.getLast(messages).message)
+        .containsMatch(
+            "("
+                + Pattern.quote("Owners-Override+1 Code-Review+1")
+                + "|"
+                + Pattern.quote("Code-Review+1 Owners-Override+1")
+                + ")");
+    assertThat(Iterables.getLast(messages).message)
+        .contains(
+            String.format(
+                "By voting Owners-Override+1 the code-owners submit requirement is"
+                    + " overridden by %s\n",
+                admin.fullName()));
+    assertThat(Iterables.getLast(messages).message)
+        .contains(
+            String.format(
+                "By voting Code-Review+1 the following files are now code-owner approved by"
+                    + " %s:\n"
+                    + "* %s\n",
+                admin.fullName(), path));
+  }
+}
diff --git a/resources/Documentation/build.md b/resources/Documentation/build.md
index 6c2db04..38d44ed 100644
--- a/resources/Documentation/build.md
+++ b/resources/Documentation/build.md
@@ -22,6 +22,14 @@
 ```
   bazel test //plugins/@PLUGIN@/...
 ```
+\
+To measure the test coverage run:
+
+```
+  bazel coverage --test_output=all plugins/code-owners/... --coverage_report_generator=@bazel_tools//tools/test:coverage_report_generator --combined_report=lcov --instrumentation_filter="^//plugins/code-owners/java/com/google/gerrit/plugins/codeowners/acceptance[/:],^//plugins/code-owners/java/com/google/gerrit/plugins/codeowners/acceptance/testsuite[/:],^//plugins/code-owners/java/com/google/gerrit/plugins/codeowners/api[/:],^//plugins/code-owners/java/com/google/gerrit/plugins/codeowners/api/impl[/:],^//plugins/code-owners/java/com/google/gerrit/plugins/codeowners/backend[/:],^//plugins/code-owners/java/com/google/gerrit/plugins/codeowners/common[/:],,^//plugins/code-owners/java/com/google/gerrit/plugins/codeowners/metrics[/:],^//plugins/code-owners/java/com/google/gerrit/plugins/codeowners/restapi[/:],^//plugins/code-owners/java/com/google/gerrit/plugins/codeowners/testing[/:],^//plugins/code-owners/java/com/google/gerrit/plugins/codeowners/util[/:],^//plugins/code-owners/java/com/google/gerrit/plugins/codeowners/validation[/:]" && genhtml -o . --ignore-errors source bazel-out/_coverage/_coverage_report.dat
+```
+\
+and then open the generated `index.html` in a browser.
 
 ---