diff --git a/Library/Homebrew/extend/ARGV.rb b/Library/Homebrew/extend/ARGV.rb
index a78073b9b6..3fef2dcd0e 100644
--- a/Library/Homebrew/extend/ARGV.rb
+++ b/Library/Homebrew/extend/ARGV.rb
@@ -64,7 +64,8 @@ module HomebrewArgvExtension
   end
 
   def build_from_source?
-    return true if flag? '--build-from-source' or ENV['HOMEBREW_BUILD_FROM_SOURCE'] or not MacOS.lion?
+    return true if flag? '--build-from-source' or ENV['HOMEBREW_BUILD_FROM_SOURCE'] \
+      or not MacOS.lion? or HOMEBREW_PREFIX.to_s != '/usr/local'
     options = options_only
     options.delete '--universal'
     not options.empty?