diff --git a/lib/spack/spack/directory_layout.py b/lib/spack/spack/directory_layout.py
index 0ae6f765f4a..73286483efb 100644
--- a/lib/spack/spack/directory_layout.py
+++ b/lib/spack/spack/directory_layout.py
@@ -268,6 +268,13 @@ def check_installed(self, spec):
         if installed_spec == spec:
             return path
 
+        # DAG hashes currently do not include build dependencies.
+        #
+        # TODO: remove this when we do better concretization and don't
+        # ignore build-only deps in hashes.
+        elif installed_spec == spec.copy(deps=('link', 'run')):
+            return path
+
         if spec.dag_hash() == installed_spec.dag_hash():
             raise SpecHashCollisionError(spec, installed_spec)
         else: