diff --git a/Ladybird/CMakeLists.txt b/Ladybird/CMakeLists.txt
index e13b8a8307e..fb07b589569 100644
--- a/Ladybird/CMakeLists.txt
+++ b/Ladybird/CMakeLists.txt
@@ -55,15 +55,12 @@ if (HAIKU)
 endif()
 
 # Lagom
-# FIXME: PROJECT_IS_TOP_LEVEL with CMake 3.21+
-set(LADYBIRD_IS_TOP_LEVEL FALSE)
 set(LADYBIRD_CUSTOM_TARGET_SUFFIX "-ladybird")
-if ("${CMAKE_BINARY_DIR}" STREQUAL "${PROJECT_BINARY_DIR}")
-    set(LADYBIRD_IS_TOP_LEVEL TRUE)
+if (PROJECT_IS_TOP_LEVEL)
     set(LADYBIRD_CUSTOM_TARGET_SUFFIX "")
 endif()
 
-if (LADYBIRD_IS_TOP_LEVEL)
+if (PROJECT_IS_TOP_LEVEL)
     get_filename_component(
         SERENITY_SOURCE_DIR "${ladybird_SOURCE_DIR}/.."
         ABSOLUTE
diff --git a/Meta/CMake/code_generators.cmake b/Meta/CMake/code_generators.cmake
index 63b8e7fb62e..0c96f2d8f4f 100644
--- a/Meta/CMake/code_generators.cmake
+++ b/Meta/CMake/code_generators.cmake
@@ -62,10 +62,7 @@ function(compile_ipc source output)
     add_custom_target(generate_${output_name} DEPENDS ${output})
     add_dependencies(all_generated generate_${output_name})
 
-    # TODO: Use cmake_path() when we upgrade the minimum CMake version to 3.20
-    #       https://cmake.org/cmake/help/v3.23/command/cmake_path.html#relative-path
-    string(LENGTH ${SerenityOS_SOURCE_DIR} root_source_dir_length)
-    string(SUBSTRING ${CMAKE_CURRENT_SOURCE_DIR} ${root_source_dir_length} -1 current_source_dir_relative)
+    cmake_path(RELATIVE_PATH CMAKE_CURRENT_SOURCE_DIR BASE_DIRECTORY ${SerenityOS_SOURCE_DIR} OUTPUT_VARIABLE current_source_dir_relative)
     install(FILES ${CMAKE_CURRENT_BINARY_DIR}/${output} DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/${current_source_dir_relative}" OPTIONAL)
 endfunction()
 
diff --git a/Meta/CMake/utils.cmake b/Meta/CMake/utils.cmake
index 80c8fd43331..eecdff3e126 100644
--- a/Meta/CMake/utils.cmake
+++ b/Meta/CMake/utils.cmake
@@ -29,10 +29,7 @@ function(serenity_install_headers target_name)
 endfunction()
 
 function(serenity_install_sources)
-    # TODO: Use cmake_path() when we upgrade the minimum CMake version to 3.20
-    #       https://cmake.org/cmake/help/v3.23/command/cmake_path.html#relative-path
-    string(LENGTH ${SerenityOS_SOURCE_DIR} root_source_dir_length)
-    string(SUBSTRING ${CMAKE_CURRENT_SOURCE_DIR} ${root_source_dir_length} -1 current_source_dir_relative)
+    cmake_path(RELATIVE_PATH CMAKE_CURRENT_SOURCE_DIR BASE_DIRECTORY ${SerenityOS_SOURCE_DIR} OUTPUT_VARIABLE current_source_dir_relative)
     file(GLOB_RECURSE sources RELATIVE ${CMAKE_CURRENT_SOURCE_DIR} "*.h" "*.cpp" "*.gml")
     foreach(source ${sources})
         get_filename_component(subdirectory ${source} DIRECTORY)