diff --git a/cmake/CMakeLists.txt b/cmake/CMakeLists.txt
index 87c70e4483..9fff73f49e 100644
--- a/cmake/CMakeLists.txt
+++ b/cmake/CMakeLists.txt
@@ -260,16 +260,9 @@ if(ENABLE_IWYU)
   if (IWYU_EXE AND IWYU_TOOL)
     add_custom_target(
       iwyu
-      ${IWYU_TOOL} -o clang -p ${CMAKE_CURRENT_BINARY_DIR} -- -Xiwyu --mapping_file=${CMAKE_CURRENT_BINARY_DIR}/iwyu-extra-map.imp
+      ${IWYU_TOOL} -o clang -p ${CMAKE_CURRENT_BINARY_DIR} -- -Xiwyu --mapping_file=${CMAKE_CURRENT_SOURCE_DIR}/iwyu/iwyu-extra-map.imp
       COMMENT "Running IWYU")
     add_dependencies(iwyu lammps)
-    # generate file with additional mappings
-    file(WRITE  ${CMAKE_CURRENT_BINARY_DIR}/iwyu-extra-map.imp "[\n")
-    file(APPEND ${CMAKE_CURRENT_BINARY_DIR}/iwyu-extra-map.imp
-           "  { include: [ \"<bits/types/struct_rusage.h>\", private, \"<sys/resource.h>\", public ] },\n")
-    file(APPEND ${CMAKE_CURRENT_BINARY_DIR}/iwyu-extra-map.imp
-           "  { include: [ \"<bits/exception.h>\", public, \"<exception>\", public ] },\n")
-    file(APPEND ${CMAKE_CURRENT_BINARY_DIR}/iwyu-extra-map.imp "]\n")
   else()
     message(FATAL_ERROR "To use IWYU you need the include-what-you-use/iwyu executable"
       "and the iwyu-tool/iwyu_tool script installed in your PATH")
diff --git a/cmake/iwyu/iwyu-extra-map.imp b/cmake/iwyu/iwyu-extra-map.imp
new file mode 100644
index 0000000000..8985998704
--- /dev/null
+++ b/cmake/iwyu/iwyu-extra-map.imp
@@ -0,0 +1,4 @@
+[
+  { include: [ "<bits/types/struct_rusage.h>", private, "<sys/resource.h>", public ] }
+  { include: [ "<bits/exception.h>", public, "<exception>", public ] }
+]