diff --git a/Source/cmGlobalFastbuildGenerator.cxx b/Source/cmGlobalFastbuildGenerator.cxx
index 8dc735be1e..b4c3259259 100644
--- a/Source/cmGlobalFastbuildGenerator.cxx
+++ b/Source/cmGlobalFastbuildGenerator.cxx
@@ -91,41 +91,6 @@ static std::map<std::string, std::string> const compilerIdToFastbuildFamily = {
 static std::set<std::string> const supportedLanguages = { "C", "CXX", "CUDA",
                                                           "OBJC", "OBJCXX" };
 
-static void ReadCompilerOptions(FastbuildCompiler& compiler, cmMakefile* mf)
-{
-  if (compiler.CompilerFamily == "custom") {
-    return;
-  }
-
-  if (cmIsOn(mf->GetSafeDefinition(FASTBUILD_USE_LIGHTCACHE))) {
-    compiler.UseLightCache = true;
-  }
-  if (cmIsOn(mf->GetSafeDefinition(FASTBUILD_USE_RELATIVE_PATHS))) {
-    compiler.UseRelativePaths = true;
-  }
-  if (cmIsOn(mf->GetSafeDefinition(FASTBUILD_USE_DETERMINISTIC_PATHS))) {
-    compiler.UseDeterministicPaths = true;
-  }
-  std::string sourceMapping = mf->GetSafeDefinition(FASTBUILD_SOURCE_MAPPING);
-  if (!sourceMapping.empty()) {
-    compiler.SourceMapping = std::move(sourceMapping);
-  }
-  auto const clangRewriteIncludesDef =
-    mf->GetDefinition(FASTBUILD_CLANG_REWRITE_INCLUDES);
-  if (clangRewriteIncludesDef.IsSet() && clangRewriteIncludesDef.IsOff()) {
-    compiler.ClangRewriteIncludes = false;
-  }
-  if (cmIsOn(mf->GetSafeDefinition(FASTBUILD_CLANG_GCC_UPDATE_XLANG_ARG))) {
-    compiler.ClangGCCUpdateXLanguageArg = true;
-  }
-  if (cmIsOn(mf->GetSafeDefinition(FASTBUILD_ALLOW_RESPONSE_FILE))) {
-    compiler.AllowResponseFile = true;
-  }
-  if (cmIsOn(mf->GetSafeDefinition(FASTBUILD_FORCE_RESPONSE_FILE))) {
-    compiler.ForceResponseFile = true;
-  }
-}
-
 template <class T>
 FastbuildAliasNode generateAlias(std::string const& name, char const* postfix,
                                  T const& nodes)
@@ -232,6 +197,43 @@ cmGlobalFastbuildGenerator::cmGlobalFastbuildGenerator(cmake* cm)
   cm->GetState()->SetIsGeneratorMultiConfig(false);
 }
 
+void cmGlobalFastbuildGenerator::ReadCompilerOptions(
+  FastbuildCompiler& compiler, cmMakefile* mf)
+{
+  if (compiler.CompilerFamily == "custom") {
+    return;
+  }
+
+  if (cmIsOn(mf->GetSafeDefinition(FASTBUILD_USE_LIGHTCACHE))) {
+    compiler.UseLightCache = true;
+  }
+  if (cmIsOn(mf->GetSafeDefinition(FASTBUILD_USE_RELATIVE_PATHS))) {
+    compiler.UseRelativePaths = true;
+    UsingRelativePaths = true;
+  }
+  if (cmIsOn(mf->GetSafeDefinition(FASTBUILD_USE_DETERMINISTIC_PATHS))) {
+    compiler.UseDeterministicPaths = true;
+  }
+  std::string sourceMapping = mf->GetSafeDefinition(FASTBUILD_SOURCE_MAPPING);
+  if (!sourceMapping.empty()) {
+    compiler.SourceMapping = std::move(sourceMapping);
+  }
+  auto const clangRewriteIncludesDef =
+    mf->GetDefinition(FASTBUILD_CLANG_REWRITE_INCLUDES);
+  if (clangRewriteIncludesDef.IsSet() && clangRewriteIncludesDef.IsOff()) {
+    compiler.ClangRewriteIncludes = false;
+  }
+  if (cmIsOn(mf->GetSafeDefinition(FASTBUILD_CLANG_GCC_UPDATE_XLANG_ARG))) {
+    compiler.ClangGCCUpdateXLanguageArg = true;
+  }
+  if (cmIsOn(mf->GetSafeDefinition(FASTBUILD_ALLOW_RESPONSE_FILE))) {
+    compiler.AllowResponseFile = true;
+  }
+  if (cmIsOn(mf->GetSafeDefinition(FASTBUILD_FORCE_RESPONSE_FILE))) {
+    compiler.ForceResponseFile = true;
+  }
+}
+
 void cmGlobalFastbuildGenerator::ProcessEnvironment()
 {
   bool const CaptureSystemEnv =
@@ -1178,6 +1180,9 @@ void cmGlobalFastbuildGenerator::WriteUnity(FastbuildUnityNode const& Unity)
       WriteArray("UnityInputIsolatedFiles",
                  Wrap(Unity.UnityInputIsolatedFiles), 2);
     }
+    if (UsingRelativePaths) {
+      WriteVariable("UseRelativePaths_Experimental", "true", 2);
+    }
   }
   Indent(1);
   *BuildFileStream << "}\n";
diff --git a/Source/cmGlobalFastbuildGenerator.h b/Source/cmGlobalFastbuildGenerator.h
index 280a283331..a456ce4fcf 100644
--- a/Source/cmGlobalFastbuildGenerator.h
+++ b/Source/cmGlobalFastbuildGenerator.h
@@ -347,6 +347,7 @@ class cmGlobalFastbuildGenerator : public cmGlobalCommonGenerator
 public:
   cmGlobalFastbuildGenerator(cmake* cm);
 
+  void ReadCompilerOptions(FastbuildCompiler& compiler, cmMakefile* mf);
   void ProcessEnvironment();
 
   static std::unique_ptr<cmGlobalGeneratorFactory> NewFactory();
@@ -614,6 +615,7 @@ public:
   // configuration (like .objs files used to create module definition from
   // objects).
   std::unordered_set<std::string> AllFilesToKeep;
+  bool UsingRelativePaths = false;
 
 private:
   std::unordered_set<std::string> AllFilesToClean;
diff --git a/Source/cmLocalFastbuildGenerator.cxx b/Source/cmLocalFastbuildGenerator.cxx
index fc4916051b..fcfa040f0a 100644
--- a/Source/cmLocalFastbuildGenerator.cxx
+++ b/Source/cmLocalFastbuildGenerator.cxx
@@ -16,6 +16,7 @@
 #include "cmObjectLocation.h"
 #include "cmSystemTools.h"
 #include "cmValue.h"
+#include "cmake.h"
 
 class cmGlobalGenerator;
 
@@ -96,3 +97,16 @@ void cmLocalFastbuildGenerator::AdditionalCleanFiles(std::string const& config)
     }
   }
 }
+
+std::string cmLocalFastbuildGenerator::ConvertToIncludeReference(
+  std::string const& path, cmOutputConverter::OutputFormat format)
+{
+  std::string converted = this->ConvertToOutputForExisting(path, format);
+  cmGlobalFastbuildGenerator const* GG = this->GetGlobalFastbuildGenerator();
+  if (GG->UsingRelativePaths && cmSystemTools::FileIsFullPath(path)) {
+    std::string const& binDir =
+      GG->GetCMakeInstance()->GetHomeOutputDirectory();
+    return cmSystemTools::RelativePath(binDir, converted);
+  }
+  return converted;
+}
diff --git a/Source/cmLocalFastbuildGenerator.h b/Source/cmLocalFastbuildGenerator.h
index 8c9c27da51..3af7573dfd 100644
--- a/Source/cmLocalFastbuildGenerator.h
+++ b/Source/cmLocalFastbuildGenerator.h
@@ -6,6 +6,7 @@
 #include <string>
 
 #include "cmLocalCommonGenerator.h"
+#include "cmOutputConverter.h"
 
 class cmGeneratorTarget;
 class cmGlobalFastbuildGenerator;
@@ -32,4 +33,8 @@ public:
   cmGlobalFastbuildGenerator* GetGlobalFastbuildGenerator();
 
   void AdditionalCleanFiles(std::string const& config);
+
+private:
+  std::string ConvertToIncludeReference(
+    std::string const& path, cmOutputConverter::OutputFormat format) override;
 };