diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5ae77c47..7c2c8ed6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -114,7 +114,7 @@ option(ENABLE_TESTS     "Set to OFF to disable building SimGear's test applicati
 option(ENABLE_SOUND     "Set to OFF to disable building SimGear's sound support" ON)
 
 if (MSVC)
-  GET_FILENAME_COMPONENT(PARENT_DIR ${PROJECT_SOURCE_DIR} PATH)
+  GET_FILENAME_COMPONENT(PARENT_DIR ${PROJECT_BINARY_DIR} PATH)
   if (CMAKE_CL_64)
     SET(TEST_3RDPARTY_DIR "${PARENT_DIR}/3rdparty.x64")
   else (CMAKE_CL_64)