diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index cf60065..61742c3 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -630,11 +630,65 @@ jobs:
         if-no-files-found: error
         path: |
             ${{ github.workspace }}/build/archive/*
+
+#------------------------------------------------------------------------------
+# BUILD INSIDE DOCKER INSIDE LLVM-SOURCE-TREE
+#------------------------------------------------------------------------------
+  in-source-tree:
+    runs-on: ubuntu-22.04
+    name: ${{ matrix.config.name }} LLVM source tree
+    container:
+      image: andreasfertig/cppinsights-builder
+    continue-on-error: false
+    strategy:
+      fail-fast: false
+      matrix:
+        arch:
+          - amd64
+
+        config:
+        # GCC 13 / LLVM 18
+        - {
+            name: "GCC 13 / LLVM 18 @ Ubuntu Release",
+            build_type: Release,
+            cxx: "g++-13",
+            llvm_version: "18.1.0",
+            llvm_config: "/usr/bin/llvm-config-18",
+            coverage: "No",
+            static: "Yes",
+            debug: "No",
+            libcxx: "No",
+          }
+
+    steps:
+    - uses: actions/checkout@v4
+      with:
+        path: cppinsights
+
+    - uses: actions/checkout@v4
+      name: Checkout Clang
+      with:
+        repository: llvm/llvm-project
+        ref: llvmorg-${{ env.LLVM_VERSION }}
+        path: llvm-project
+        fetch-tags: true
+
+    - name: Configure
+      id: cmake_configure
+      shell: bash
+      run: |
+          mkdir build
+          cd build
+
+          CXX=${{ matrix.config.cxx }}
+          cmake -G Ninja -D=CMAKE_BUILD_TYPE=Release -DLLVM_EXTERNAL_PROJECTS=cppinsights -DLLVM_EXTERNAL_CPPINSIGHTS_SOURCE_DIR=${GITHUB_WORKSPACE}/cppinsights  -DINSIGHTS_STATIC=${{ matrix.config.static }} -DDEBUG=${{ matrix.config.debug }} -DINSIGHTS_COVERAGE=${{ matrix.config.coverage }} -DINSIGHTS_USE_LIBCPP=${{ matrix.config.libcxx }} ${GITHUB_WORKSPACE}/llvm-project/llvm
+          # For now a successful configure is enough
+
 #------------------------------------------------------------------------------
 # DEPLOY
 #------------------------------------------------------------------------------
   deploy:
-    needs: [build, docker]
+    needs: [build, docker, in-source-tree]
     if: github.ref == 'refs/heads/main'
     name: Final Deploy
     continue-on-error: false
diff --git a/CMakeLists.txt b/CMakeLists.txt
index aee8b0b..6ef0b8b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -35,8 +35,18 @@ set(INSIGHTS_LLVM_CONFIG "llvm-config" CACHE STRING "LLVM config executable to u
 set(INSIGHTS_MIN_LLVM_MAJOR_VERSION 18)
 set(INSIGHTS_MIN_LLVM_VERSION ${INSIGHTS_MIN_LLVM_MAJOR_VERSION}.0)
 
-if(NOT DEFINED LLVM_VERSION_MAJOR)  # used when build inside the clang tool/extra folder
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   set(BUILD_INSIGHTS_OUTSIDE_LLVM On)
+else()
+  # used when build inside the clang tool/extra folder
+  set(BUILD_INSIGHTS_OUTSIDE_LLVM Off)
+  include_directories(BEFORE
+    ${LLVM_SOURCE_DIR}/include
+    ${LLVM_EXTERNAL_CLANG_SOURCE_DIR}/include
+    ${LLVM_BINARY_DIR}/include
+    ${LLVM_BINARY_DIR}/tools/clang/include
+  )
+
 endif()
 
 if(INSIGHTS_COVERAGE OR DEBUG)
@@ -367,11 +377,6 @@ if (BUILD_INSIGHTS_OUTSIDE_LLVM)
         set( CMAKE_EXE_LINKER_FLAGS  "${CMAKE_EXE_LINKER_FLAGS} ${COVERAGE_LINK_FLAGS}")
     endif()
 
-    # copied from: llvm/tools/clang/cmake/modules/AddClang.cmake
-    macro(add_clang_tool name)
-      add_executable( ${name} ${ARGN} )
-    endmacro(add_clang_tool)
-
     # additional libs required when building insights outside llvm
     set(ADDITIONAL_LIBS
         ${LLVM_LDFLAGS}
@@ -422,6 +427,12 @@ if (${LLVM_PACKAGE_VERSION} VERSION_LESS ${INSIGHTS_MIN_LLVM_VERSION})
     message(FATAL_ERROR "LLVM version ${INSIGHTS_MIN_LLVM_VERSION} or higher required. Current version is: ${LLVM_PACKAGE_VERSION}.")
 endif()
 
+# copied from: llvm/tools/clang/cmake/modules/AddClang.cmake
+macro(add_clang_tool name)
+  add_executable( ${name} ${ARGN} )
+endmacro(add_clang_tool)
+
+
 
 if(WIN32)
     # First for the generic no-config case (e.g. with mingw)
@@ -815,6 +826,7 @@ endif()
 
 message(STATUS "")
 message(STATUS "[ Build summary ]")
+message(STATUS "BUILD_INSIGHTS_OUTSIDE_LLVM : ${BUILD_INSIGHTS_OUTSIDE_LLVM}")
 message(STATUS "CMAKE_GENERATOR       : ${CMAKE_GENERATOR}")
 message(STATUS "CMAKE_EXE_LINKER_FLAGS: ${CMAKE_EXE_LINKER_FLAGS}")
 message(STATUS "CMAKE_LINKER          : ${CMAKE_LINKER}")
diff --git a/Readme.md b/Readme.md
index d55cce8..5d60106 100644
--- a/Readme.md
+++ b/Readme.md
@@ -130,6 +130,7 @@ See https://github.com/andreasfertig/cppinsights/issues/186 for an explanation o
 
 `extra/clang` and `extra/llvm` provide `/usr/lib/{libclangAST.so,libLLVM*.a,libLLVM.so}`. `libclangAST.so` needs `libLLVM.so` and there would be a conflict if `libLLVM*.a` (instead of `libLLVM.so`) are linked. See https://bugs.archlinux.org/task/60512
 
+
 ### Building outside Clang
 
 You need to have a Clang installation in the search path.
@@ -144,23 +145,20 @@ The resulting binary (insights) can be found in the `build` folder.
 
 ### Building inside Clang
 
-For building it inside the Clang source tree, assuming you have your source tree already prepared under `llvm-project`:
+The easiest way to build C++ Insights inside the Clang source tree is using the `LLVM_EXTERNAL_PROJECTS` option.
 
 ```
-cd llvm-project/clang-tools-extra/
+git clone https://github.com/llvm/llvm-project.git
 git clone https://github.com/andreasfertig/cppinsights.git
 
-echo "add_subdirectory(cppinsights)" >> CMakeLists.txt
-```
-
-To activate the C++ Insights build you have to set `-DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra"` for `cmake`:
+mkdir build
+cd build
+cmake -G Ninja -D=CMAKE_BUILD_TYPE=Release -DLLVM_EXTERNAL_PROJECTS=cppinsights -DLLVM_EXTERNAL_CPPINSIGHTS_SOURCE_DIR=<PATH/TO/cppinsights>  [INSIGHTS CMAKE OPTIONS] ../llvm-project/llvm
 
-```
-cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra" -G "Unix Makefiles" ../llvm-project
+ninja
 ```
 
 
-Then, build Clang as you normally do.
 
 ### cmake options