diff --git a/CMakeLists.txt b/CMakeLists.txt
index 16e97f835f..a19249bee0 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -107,8 +107,12 @@ add_warnings_target(warnings ${ALPAQA_WARNINGS_AS_ERRORS})
 add_library(alpaqa::warnings ALIAS warnings)
 
 # Compiler options
-set(CMAKE_CXX_EXTENSIONS ${ALPAQA_WITH_QUAD_PRECISION})
-set(CMAKE_C_EXTENSIONS ${ALPAQA_WITH_QUAD_PRECISION})
+if (NOT DEFINED CMAKE_CXX_EXTENSIONS)
+    set(CMAKE_CXX_EXTENSIONS ${ALPAQA_WITH_QUAD_PRECISION})
+endif()
+if (NOT DEFINED CMAKE_C_EXTENSIONS)
+    set(CMAKE_C_EXTENSIONS ${ALPAQA_WITH_QUAD_PRECISION})
+endif()
 if (ALPAQA_WITH_PYTHON OR ALPAQA_WITH_MATLAB)
     set(CMAKE_POSITION_INDEPENDENT_CODE On)
 endif()