diff --git a/build-system/pr-check/build-targets.js b/build-system/pr-check/build-targets.js
index af985276d567..0cce23a8f01e 100644
--- a/build-system/pr-check/build-targets.js
+++ b/build-system/pr-check/build-targets.js
@@ -14,6 +14,7 @@ const {getLoggingPrefix, logWithoutTimestamp} = require('../common/logging');
 const {gitDiffNameOnlyMain} = require('../common/git');
 const {ignoreListFiles} = require('../tasks/check-ignore-lists');
 const {isCiBuild} = require('../common/ci');
+const {shouldTriggerAva} = require('../tasks/ava');
 
 /**
  * Used to prevent the repeated recomputing of build targets during PR jobs.
@@ -115,16 +116,7 @@ const targetMatchers = {
     if (isOwnersFile(file)) {
       return false;
     }
-    return (
-      file == 'build-system/tasks/ava.js' ||
-      file.startsWith('build-system/release-tagger/') ||
-      file.startsWith('build-system/server/') ||
-      file.startsWith('build-system/tasks/css/') ||
-      file.startsWith('build-system/tasks/get-zindex/') ||
-      file.startsWith('build-system/tasks/make-extension/') ||
-      file.startsWith('build-system/tasks/markdown-toc/') ||
-      file.startsWith('build-system/tasks/prepend-global/')
-    );
+    return shouldTriggerAva(file);
   },
   [Targets.BABEL_PLUGIN]: (file) => {
     if (isOwnersFile(file)) {
diff --git a/build-system/tasks/ava.js b/build-system/tasks/ava.js
index 6e56909e8c17..eec6178a4bf2 100644
--- a/build-system/tasks/ava.js
+++ b/build-system/tasks/ava.js
@@ -1,25 +1,46 @@
 'use strict';
 
 const argv = require('minimist')(process.argv.slice(2));
+const {dirname, relative} = require('path');
 const {execOrDie} = require('../common/exec');
+const {sync: globbySync} = require('globby');
+
+const testFiles = [
+  'build-system/release-tagger/test/*test*.js',
+  'build-system/server/app-index/test/*test*.js',
+  'build-system/server/test/app-utils.test.js',
+  'build-system/tasks/css/test/bento-css.test.js',
+  'build-system/tasks/get-zindex/get-zindex.test.js',
+  'build-system/tasks/make-extension/test/test.js',
+  'build-system/tasks/markdown-toc/test/test.js',
+  'build-system/tasks/prepend-global/prepend-global.test.js',
+];
+
+let targetFiles;
+
+/**
+ * Determines whether to trigger ava by a file change adjacent to a test file.
+ * Considered adjacent when the file changed is in the same directory as one of
+ * the listed test files, or its parent if the directory is named `test`.
+ * @param {string} changedFile
+ * @return {boolean}
+ */
+function shouldTriggerAva(changedFile) {
+  if (!targetFiles) {
+    const thisFile = relative(process.cwd(), __filename);
+    const patterns = testFiles.map(
+      (pattern) => dirname(pattern).replace(/\/test$/, '') + '/'
+    );
+    targetFiles = new Set([thisFile, ...globbySync(patterns)]);
+  }
+  return targetFiles.has(changedFile);
+}
 
 /**
  * Runs ava tests.
  * @return {Promise<void>}
  */
 async function ava() {
-  // These need equivalents for CI in build-system/pr-check/build-targets.js
-  // (see targetMatchers[Targets.AVA])
-  const testFiles = [
-    'build-system/release-tagger/test/*test*.js',
-    'build-system/server/app-index/test/*test*.js',
-    'build-system/server/test/app-utils.test.js',
-    'build-system/tasks/css/test/bento-css.test.js',
-    'build-system/tasks/get-zindex/get-zindex.test.js',
-    'build-system/tasks/make-extension/test/test.js',
-    'build-system/tasks/markdown-toc/test/test.js',
-    'build-system/tasks/prepend-global/prepend-global.test.js',
-  ];
   execOrDie(
     [
       'npx ava',
@@ -32,6 +53,7 @@ async function ava() {
 
 module.exports = {
   ava,
+  shouldTriggerAva,
 };
 
 ava.description = "Run ava tests for AMP's tasks";