diff --git a/test/Makefile.guess b/test/Makefile.guess
new file mode 100644
index 00000000000..1916d09a6cd
--- /dev/null
+++ b/test/Makefile.guess
@@ -0,0 +1,14 @@
+# Try to guess the build directory:
+# we look for subdirectories of ../.. that look like ninja build dirs.
+
+ifeq ($(BUILD_DIR),)
+        dirs = $(dir $(wildcard ../../*/.ninja_log))
+        ifeq ($(dirs),)
+                $(error Cannot guess build dir, set BUILD_DIR)
+        endif
+        ifneq ($(firstword $(dirs)),$(dirs))
+                $(warning Candidates: $(dirs))
+                $(error Too many build dirs to pick from, set BUILD_DIR)
+        endif
+        BUILD_DIR=$(dirs)
+endif
diff --git a/test/TEST-01-BASIC/Makefile b/test/TEST-01-BASIC/Makefile
index c29fd067639..b895de8bcb5 100644
--- a/test/TEST-01-BASIC/Makefile
+++ b/test/TEST-01-BASIC/Makefile
@@ -1,2 +1,4 @@
+include ../Makefile.guess
+
 all setup clean run:
-	@basedir=../.. TEST_BASE_DIR=../ ./test.sh --$@
+	@basedir=../.. TEST_BASE_DIR=../ BUILD_DIR=$(BUILD_DIR) ./test.sh --$@
diff --git a/test/TEST-13-NSPAWN-SMOKE/Makefile b/test/TEST-13-NSPAWN-SMOKE/Makefile
index e58c0d909b0..41cca23c7fb 100644
--- a/test/TEST-13-NSPAWN-SMOKE/Makefile
+++ b/test/TEST-13-NSPAWN-SMOKE/Makefile
@@ -1,5 +1,7 @@
+include ../Makefile.guess
+
 all setup run:
-	@basedir=../.. TEST_BASE_DIR=../ ./test.sh --$@
+	@basedir=../.. TEST_BASE_DIR=../ BUILD_DIR=$(BUILD_DIR) ./test.sh --$@
 clean:
 	@basedir=../.. TEST_BASE_DIR=../ ./test.sh --clean
 	@rm -f has-overflow