diff --git a/lib/builder/webpack/base.js b/lib/builder/webpack/base.js
index 2606d55d05..3c96e04992 100644
--- a/lib/builder/webpack/base.js
+++ b/lib/builder/webpack/base.js
@@ -260,6 +260,21 @@ export default class WebpackBaseConfig {
     return plugins
   }
 
+  customize(config) {
+    if (typeof this.options.build.extend === 'function') {
+      const extendedConfig = this.options.build.extend.call(this.builder, config, {
+        isDev: this.options.dev,
+        isServer: this.isServer,
+        isClient: !this.isServer
+      })
+      // Only overwrite config when something is returned for backwards compatibility
+      if (extendedConfig !== undefined) {
+        return extendedConfig
+      }
+    }
+    return config
+  }
+
   config() {
     // Prioritize nested node_modules in webpack search path (#2558)
     const webpackModulesDir = ['node_modules'].concat(this.options.modulesDir)
diff --git a/lib/builder/webpack/client.js b/lib/builder/webpack/client.js
index 9694d87b75..d45cc39d4c 100644
--- a/lib/builder/webpack/client.js
+++ b/lib/builder/webpack/client.js
@@ -76,7 +76,7 @@ export default class WebpackClientConfig extends WebpackBaseConfig {
   }
 
   config() {
-    let config = super.config()
+    const config = super.config()
 
     // Entry points
     config.entry = path.resolve(this.options.buildDir, 'client.js')
@@ -119,20 +119,6 @@ export default class WebpackClientConfig extends WebpackBaseConfig {
       )
     }
 
-    // Extend config
-    if (typeof this.options.build.extend === 'function') {
-      const isDev = this.options.dev
-      const extendedConfig = this.options.build.extend.call(this.builder, config, {
-        isDev,
-        isClient: true
-      })
-
-      // Only overwrite config when something is returned for backwards compatibility
-      if (extendedConfig !== undefined) {
-        config = extendedConfig
-      }
-    }
-
     // Make uglifyjs faster
     if (!this.options.dev && !config.optimization.minimizer) {
       // https://github.com/webpack-contrib/uglifyjs-webpack-plugin
@@ -153,6 +139,6 @@ export default class WebpackClientConfig extends WebpackBaseConfig {
       ]
     }
 
-    return config
+    return this.customize(config)
   }
 }
diff --git a/lib/builder/webpack/server.js b/lib/builder/webpack/server.js
index ac98af48dc..7ca2570fd5 100644
--- a/lib/builder/webpack/server.js
+++ b/lib/builder/webpack/server.js
@@ -33,7 +33,7 @@ export default class WebpackServerConfig extends BaseConfig {
   }
 
   config() {
-    let config = super.config()
+    const config = super.config()
 
     // Config devtool
     config.devtool = 'cheap-source-map'
@@ -75,19 +75,6 @@ export default class WebpackServerConfig extends BaseConfig {
       }
     })
 
-    // Extend config
-    if (typeof this.options.build.extend === 'function') {
-      const isDev = this.options.dev
-      const extendedConfig = this.options.build.extend.call(this.builder, config, {
-        isDev,
-        isServer: true
-      })
-      // Only overwrite config when something is returned for backwards compatibility
-      if (extendedConfig !== undefined) {
-        config = extendedConfig
-      }
-    }
-
-    return config
+    return this.customize(config)
   }
 }