diff --git a/CMakeLists.txt b/CMakeLists.txt
index 52c44ec..75620dc 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -5,8 +5,8 @@ option(PLASP_BUILD_TESTS "Build unit tests" OFF)
 
 find_package(Boost 1.55.0 COMPONENTS program_options iostreams system REQUIRED)
 
-set(CMAKE_CXX_FLAGS "-Wall -Wextra -Wpedantic -Werror")
-set(CMAKE_CXX_FLAGS_DEBUG "-g")
+set(CMAKE_CXX_FLAGS "-Wall -Wextra -Wpedantic -Werror ${CMAKE_CXX_FLAGS}")
+set(CMAKE_CXX_FLAGS_DEBUG "-g ${CMAKE_CXX_FLAGS_DEBUG}")
 
 set(CMAKE_CXX_STANDARD 14)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
diff --git a/lib/pddlparse/CMakeLists.txt b/lib/pddlparse/CMakeLists.txt
index 1f5ced9..60b674a 100644
--- a/lib/pddlparse/CMakeLists.txt
+++ b/lib/pddlparse/CMakeLists.txt
@@ -3,8 +3,8 @@ project(pddlparse)
 
 option(PDDLPARSE_BUILD_TESTS "Build unit tests" OFF)
 
-set(CMAKE_CXX_FLAGS "-Wall -Wextra -Wpedantic -Werror")
-set(CMAKE_CXX_FLAGS_DEBUG "-g")
+set(CMAKE_CXX_FLAGS "-Wall -Wextra -Wpedantic -Werror ${CMAKE_CXX_FLAGS}")
+set(CMAKE_CXX_FLAGS_DEBUG "-g ${CMAKE_CXX_FLAGS_DEBUG}")
 
 set(CMAKE_CXX_STANDARD 14)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)