diff -r a30cdf366c02 Lib/distutils/command/install_lib.py
--- a/Lib/distutils/command/install_lib.py	Tue Jan 10 11:30:44 2017 +0800
+++ b/Lib/distutils/command/install_lib.py	Tue Jan 10 16:41:30 2017 +0100
@@ -140,18 +140,31 @@
 
     # -- Utility methods -----------------------------------------------
 
-    def _mutate_outputs(self, has_any, build_cmd, cmd_option, output_dir):
-        if not has_any:
+    def _pure_outputs(self):
+        if not self.distribution.has_pure_modules():
             return []
 
-        build_cmd = self.get_finalized_command(build_cmd)
+        build_cmd = self.get_finalized_command('build_py')
         build_files = build_cmd.get_outputs()
-        build_dir = getattr(build_cmd, cmd_option)
+        build_dir = getattr(build_cmd, 'build_lib')
 
         prefix_len = len(build_dir) + len(os.sep)
         outputs = []
         for file in build_files:
-            outputs.append(os.path.join(output_dir, file[prefix_len:]))
+            outputs.append(os.path.join(self.install_dir, file[prefix_len:]))
+
+        return outputs
+
+    def _ext_outputs(self):
+        if not self.distribution.has_ext_modules():
+            return []
+
+        build_cmd = self.get_finalized_command('build_ext')
+
+        outputs = []
+        for ext in build_cmd.extensions:
+            filename = build_cmd.get_ext_filename(ext.name)
+            outputs.append(os.path.join(self.install_dir, filename))
 
         return outputs
 
@@ -182,19 +195,14 @@
         were actually run.  Not affected by the "dry-run" flag or whether
         modules have actually been built yet.
         """
-        pure_outputs = \
-            self._mutate_outputs(self.distribution.has_pure_modules(),
-                                 'build_py', 'build_lib',
-                                 self.install_dir)
+        pure_outputs = self._pure_outputs()
+
         if self.compile:
             bytecode_outputs = self._bytecode_filenames(pure_outputs)
         else:
             bytecode_outputs = []
 
-        ext_outputs = \
-            self._mutate_outputs(self.distribution.has_ext_modules(),
-                                 'build_ext', 'build_lib',
-                                 self.install_dir)
+        ext_outputs = self._ext_outputs()
 
         return pure_outputs + bytecode_outputs + ext_outputs
 
diff -r a30cdf366c02 Lib/distutils/tests/test_install_lib.py
--- a/Lib/distutils/tests/test_install_lib.py	Tue Jan 10 11:30:44 2017 +0800
+++ b/Lib/distutils/tests/test_install_lib.py	Tue Jan 10 16:41:30 2017 +0100
@@ -6,6 +6,7 @@
 
 from distutils.command.install_lib import install_lib
 from distutils.extension import Extension
+from distutils.sysconfig import get_config_var
 from distutils.tests import support
 from distutils.errors import DistutilsOptionError
 from test.support import run_unittest
@@ -70,6 +71,25 @@
         outputs = cmd.get_outputs()
         self.assertEqual(len(outputs), 4, outputs)
 
+    def test_get_outputs_inplace_ext(self):
+        # ensure get_outputs() is correct with extensions built inplace
+        project_dir, dist = self.create_dist()
+        build_ext_cmd = dist.get_command_obj('build_ext')
+        build_ext_cmd.inplace = 1
+        os.chdir(project_dir)
+        cmd = install_lib(dist)
+
+        # setting up a dist environment
+        cmd.install_dir = self.mkdtemp()
+        cmd.distribution.ext_modules = [Extension('foo', ['xxx'])]
+        cmd.distribution.script_name = 'setup.py'
+
+        # get_outputs should return foo.import-tag-abiflags.so
+        actual = cmd.get_outputs()
+        expected = [os.path.join(cmd.install_dir,
+                                 'foo' + get_config_var('EXT_SUFFIX'))]
+        self.assertEqual(expected, actual)
+
     def test_get_inputs(self):
         project_dir, dist = self.create_dist()
         os.chdir(project_dir)
diff -r a30cdf366c02 Misc/ACKS
--- a/Misc/ACKS	Tue Jan 10 11:30:44 2017 +0800
+++ b/Misc/ACKS	Tue Jan 10 16:41:30 2017 +0100
@@ -1453,6 +1453,7 @@
 Quentin Stafford-Fraser
 Frank Stajano
 Joel Stanley
+Elvis Stansvik
 Anthony Starks
 Oliver Steele
 Greg Stein
diff -r a30cdf366c02 Misc/NEWS
--- a/Misc/NEWS	Tue Jan 10 11:30:44 2017 +0800
+++ b/Misc/NEWS	Tue Jan 10 16:41:30 2017 +0100
@@ -547,6 +547,9 @@
 - Issue #24142: Reading a corrupt config file left configparser in an
   invalid state.  Original patch by Florian Höch.
 
+- Issue #29225: distutils.command.install_lib.get_outputs() wrong with
+  extensions built inplace.
+
 Windows
 -------