diff --git a/maven-project/src/main/java/org/apache/maven/project/processor/BuildProcessor.java b/maven-project/src/main/java/org/apache/maven/project/processor/BuildProcessor.java
index 86963ab88d..13faf89338 100644
--- a/maven-project/src/main/java/org/apache/maven/project/processor/BuildProcessor.java
+++ b/maven-project/src/main/java/org/apache/maven/project/processor/BuildProcessor.java
@@ -25,6 +25,7 @@ import java.util.List;
 
 import org.apache.maven.model.Build;
 import org.apache.maven.model.BuildBase;
+import org.apache.maven.model.Dependency;
 import org.apache.maven.model.Extension;
 import org.apache.maven.model.Model;
 import org.apache.maven.model.Plugin;
@@ -205,15 +206,50 @@ public class BuildProcessor
         if(source.getTestSourceDirectory() != null)
         {
             target.setTestSourceDirectory( source.getTestSourceDirectory() );    
-        }        
-          
+        }   
+        /*
+        List<Dependency> childDependencies = 
+            new ArrayList<Dependency>(dependencies.subList( length - 1 , dependencies.size() ) );
+        dependencies.removeAll( childDependencies );
+        dependencies.addAll( 0, childDependencies );   
+        */
+        int i = target.getExtensions().size();
+        
+        List<Extension> m = new ArrayList<Extension>();
         for(Extension extension : source.getExtensions())
         {
-            Extension e = new Extension();
-            e.setArtifactId( extension.getArtifactId() );
-            e.setGroupId( extension.getGroupId() );
-            e.setVersion( extension.getVersion() );
-            target.addExtension( e );
-        }
+            Extension match = isMatch(extension, target.getExtensions());
+            if(match != null)
+            {
+                match.setArtifactId( extension.getArtifactId() );
+                match.setGroupId( extension.getGroupId() );
+                match.setVersion( extension.getVersion() );  
+                m.add( match );
+            }
+            else
+            {
+                Extension e = new Extension();
+                e.setArtifactId( extension.getArtifactId() );
+                e.setGroupId( extension.getGroupId() );
+                e.setVersion( extension.getVersion() );
+                m.add( e );
+               // target.addExtension( e );               
+            }
+        } 
+        target.getExtensions().removeAll( m );
+        target.getExtensions().addAll( 0, m );
     }
+
+    private static Extension isMatch(Extension extension, List<Extension> extensions)
+    {
+        for(Extension e : extensions)
+        {
+            if(e.getGroupId().equals( extension.getGroupId() ) && e.getArtifactId().equals( extension.getArtifactId() ))
+            {
+                return e;
+            }
+        }
+        return null;
+    }
+
 }
diff --git a/maven-project/src/test/java/org/apache/maven/project/PomConstructionTest.java b/maven-project/src/test/java/org/apache/maven/project/PomConstructionTest.java
index 6c47ef59c9..b8d33f476e 100644
--- a/maven-project/src/test/java/org/apache/maven/project/PomConstructionTest.java
+++ b/maven-project/src/test/java/org/apache/maven/project/PomConstructionTest.java
@@ -1244,6 +1244,19 @@ public class PomConstructionTest
         PomTestWrapper pom = buildPom( "boolean-interpolation" );
     }    
     */ 
+    
+    /* MNG-3899 */
+    public void testBuildExtensionInheritance()
+        throws Exception
+    {
+        PomTestWrapper pom = buildPom( "build-extension-inheritance/sub" ); 
+        System.out.println(pom.getDomainModel().asString());
+        assertEquals(3, ( (List<?>) pom.getValue( "build/extensions" )).size() );
+        assertEquals("b", pom.getValue( "build/extensions[1]/artifactId" ) );
+        assertEquals("a", pom.getValue( "build/extensions[2]/artifactId" ) );
+        assertEquals("0.2", pom.getValue( "build/extensions[2]/version" ) );
+        assertEquals("c", pom.getValue( "build/extensions[3]/artifactId" ) );
+    }
  
     private void assertPathSuffixEquals( String expected, Object actual )
     {
diff --git a/maven-project/src/test/resources-project-builder/build-extension-inheritance/pom.xml b/maven-project/src/test/resources-project-builder/build-extension-inheritance/pom.xml
new file mode 100644
index 0000000000..2d95cb1c5d
--- /dev/null
+++ b/maven-project/src/test/resources-project-builder/build-extension-inheritance/pom.xml
@@ -0,0 +1,49 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you 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.
+-->
+
+<project>
+  <modelVersion>4.0.0</modelVersion>
+
+  <groupId>org.apache.maven.its.mng3899</groupId>
+  <artifactId>parent</artifactId>
+  <version>0.1</version>
+  <packaging>pom</packaging>
+
+  <name>Maven Integration Test :: MNG-3899</name> 
+  <description>
+    Test that build extensions are properly merged during inheritance.
+  </description>
+
+  <build>
+    <extensions>
+      <extension>
+        <groupId>org.apache.maven.its.mng3899</groupId>
+        <artifactId>a</artifactId>
+        <version>0.1</version>
+      </extension>
+      <extension>
+        <groupId>org.apache.maven.its.mng3899</groupId>
+        <artifactId>c</artifactId>
+        <version>0.1</version>
+      </extension>
+    </extensions>
+  </build>
+</project>
diff --git a/maven-project/src/test/resources-project-builder/build-extension-inheritance/sub/pom.xml b/maven-project/src/test/resources-project-builder/build-extension-inheritance/sub/pom.xml
new file mode 100644
index 0000000000..4411b8929f
--- /dev/null
+++ b/maven-project/src/test/resources-project-builder/build-extension-inheritance/sub/pom.xml
@@ -0,0 +1,75 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you 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.
+-->
+
+<project>
+  <modelVersion>4.0.0</modelVersion>
+
+  <parent>
+    <groupId>org.apache.maven.its.mng3899</groupId>
+    <artifactId>parent</artifactId>
+    <version>0.1</version>
+  </parent>
+
+  <artifactId>child</artifactId>
+
+  <name>Maven Integration Test :: MNG-3899</name> 
+  <description>
+    Test that build extensions are properly merged during inheritance.
+  </description>
+
+  <build>
+    <!-- project extensions should precede inherited extensions -->
+    <extensions>
+      <extension>
+        <groupId>org.apache.maven.its.mng3899</groupId>
+        <artifactId>b</artifactId>
+        <version>0.1</version>
+      </extension>
+      <extension>
+        <!-- project extensions should override inherited extension with equal gid:aid -->
+        <groupId>org.apache.maven.its.mng3899</groupId>
+        <artifactId>a</artifactId>
+        <version>0.2</version>
+      </extension>
+    </extensions>
+    <plugins>
+      <plugin>
+        <groupId>org.apache.maven.its.plugins</groupId>
+        <artifactId>maven-it-plugin-expression</artifactId>
+        <version>2.1-SNAPSHOT</version>
+        <executions>
+          <execution>
+            <phase>validate</phase>
+            <goals>
+              <goal>eval</goal>
+            </goals>
+            <configuration>
+              <outputFile>target/extension.properties</outputFile>
+              <expressions>
+                <expression>project/build/extensions</expression>
+              </expressions>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>
+  </build>
+</project>