diff --git a/doc/manual/src/contributing/hacking.md b/doc/manual/src/contributing/hacking.md
index 4d3d66397..421ac981c 100644
--- a/doc/manual/src/contributing/hacking.md
+++ b/doc/manual/src/contributing/hacking.md
@@ -64,6 +64,15 @@ $ nix build
 
 You can also build Nix for one of the [supported platforms](#platforms).
 
+## Makefile variables
+
+- `ENABLE_BUILD=yes` to enable building the C++ code.
+- `ENABLE_TESTS=yes` to enable building the tests.
+- `OPTIMIZE=1` to enable optimizations.
+- `doc_generate=yes` to enable building the documentation (manual, man pages, etc.).
+
+  The docs can take a while to build, so you may want to disable this for local development.
+
 ## Building Nix
 
 To build all dependencies and start a shell in which all environment variables are set up so that those dependencies can be found: