diff --git a/middleman-core/features/v3_extensions.feature b/middleman-core/features/v3_extensions.feature
new file mode 100644
index 00000000..134dd72f
--- /dev/null
+++ b/middleman-core/features/v3_extensions.feature
@@ -0,0 +1,20 @@
+Feature: v3 Modular Extension
+  Scenario: Registering and overwriting a system config option
+    Given a fixture app "large-build-app"
+    And a file named "config.rb" with:
+      """
+      module MyFeature
+        class << self
+          def registered(app)
+            app.set :css_dir, "lib/my/css"
+          end
+          alias :included :registered
+        end
+      end
+
+      ::Middleman::Extensions.register(:my_feature, MyFeature)
+      activate :my_feature
+      """
+    Given a successfully built app at "large-build-app"
+    When I cd to "build"
+    Then the file "link_test.html" should contain "lib/my/css/test.css"
\ No newline at end of file
diff --git a/middleman-core/fixtures/large-build-app/source/link_test.html.erb b/middleman-core/fixtures/large-build-app/source/link_test.html.erb
new file mode 100644
index 00000000..c5d6488e
--- /dev/null
+++ b/middleman-core/fixtures/large-build-app/source/link_test.html.erb
@@ -0,0 +1,5 @@
+---
+layout: false
+---
+
+<%= stylesheet_link_tag :test %>
\ No newline at end of file
diff --git a/middleman-core/lib/middleman-core/configuration.rb b/middleman-core/lib/middleman-core/configuration.rb
index b1893db5..0b2454fa 100644
--- a/middleman-core/lib/middleman-core/configuration.rb
+++ b/middleman-core/lib/middleman-core/configuration.rb
@@ -22,7 +22,7 @@ module Middleman
         # @param default Attribute value
         # @return [void]
         def set(key, default=nil, &block)
-          config.define_setting(key, default)
+          config.define_setting(key, default) unless config.defines_setting?(key)
           @inst.set(key, default, &block) if @inst
         end