diff --git a/docs/user/builds.rst b/docs/user/builds.rst
index 00c1e9cc..a207ee12 100644
--- a/docs/user/builds.rst
+++ b/docs/user/builds.rst
@@ -47,3 +47,17 @@ will also call ``make clean`` on the OpenWRT tree, and
     make dirclean
 
 will do all this, and call ``make dirclean`` on the OpenWRT tree.
+
+Environment variables
+---------------------
+
+Gluon's build process can be controlled by various environment variables.
+
+GLUON_SITEDIR
+  Path to the site configuration. Defaults to ``site/``.
+
+GLUON_IMAGEDIR
+  Path where images will be stored. Defaults to ``images/``.
+
+GLUON_BUILDDIR ?= $(GLUONDIR)/build
+  Working directory during build. Defaults to ``build/``.
diff --git a/include/gluon.mk b/include/gluon.mk
index 9a96f045..9fac5f1c 100644
--- a/include/gluon.mk
+++ b/include/gluon.mk
@@ -1,13 +1,13 @@
 ifneq ($(__gluon_inc),1)
 __gluon_inc=1
 
-GLUON_ORIGOPENWRTDIR := $(GLUONDIR)/openwrt
-GLUON_SITEDIR := $(GLUONDIR)/site
-GLUON_SITE_CONFIG := $(GLUON_SITEDIR)/site.conf
-
+GLUON_SITEDIR ?= $(GLUONDIR)/site
 GLUON_IMAGEDIR ?= $(GLUONDIR)/images
 GLUON_BUILDDIR ?= $(GLUONDIR)/build
 
+GLUON_ORIGOPENWRTDIR := $(GLUONDIR)/openwrt
+GLUON_SITE_CONFIG := $(GLUON_SITEDIR)/site.conf
+
 GLUON_OPENWRTDIR = $(GLUON_BUILDDIR)/$(GLUON_TARGET)/openwrt
 
 BOARD_BUILDDIR = $(GLUON_BUILDDIR)/$(BOARD)$(if $(SUBTARGET),-$(SUBTARGET))