diff --git a/kwant/builder.py b/kwant/builder.py
index 89ec7c24c68393a9916d6d314925e3783168d7b7..b31edc6beeefb0a308ed530f5d79efbf3babcd49 100644
--- a/kwant/builder.py
+++ b/kwant/builder.py
@@ -140,6 +140,10 @@ class SiteFamily(metaclass=abc.ABCMeta):
         self.canonical_repr = canonical_repr
         self.hash = hash(canonical_repr)
         self.name = name
+        if norbs is None:
+            warnings.warn("Not specfying norbs is deprecated. Always specify "
+                          "norbs when creating site families.",
+                          KwantDeprecationWarning, stacklevel=3)
         if norbs is not None:
             if int(norbs) != norbs or norbs <= 0:
                 raise ValueError('The norbs parameter must be an integer > 0.')