ogra@anubis:~/datengrab/devel/branches/ubuntu-image$ WIGGLEROOM=500 ./ubuntu-image -c edge -o u-image-amd64.img ~/datengrab/images/snappy/amd64-generic-model.assertion 
836.00 KB / 836.00 KB [=========================================================================================] 100.00 % 5.16 MB/s 
Fetching pc
...
ogra@anubis:~/datengrab/devel/branches/ubuntu-image$ git diff
diff --git a/ubuntu_image/builder.py b/ubuntu_image/builder.py
index 4d3b84a..8e24065 100644
--- a/ubuntu_image/builder.py
+++ b/ubuntu_image/builder.py
@@ -185,6 +185,8 @@ class ModelAssertionBuilder(State):
         # On a 100MiB filesystem, ext4 takes a little over 7MiB for the
         # metadata.  Use 8MiB as a minimum padding here.
         self.rootfs_size = self._calculate_dirsize(self.rootfs) + MiB(8)
+        if os.environ.get('WIGGLEROOM'):
+            self.rootfs_size += MiB(int(os.environ.get('WIGGLEROOM')))
         self._next.append(self.pre_populate_bootfs_contents)
 
     def pre_populate_bootfs_contents(self):
ogra@anubis:~/datengrab/devel/branches/ubuntu-image$ ls -lh u-image-amd64.img 
-rw-rw-r-- 1 ogra ogra 1,1G Sep 15 12:11 u-image-amd64.img
ogra@anubis:~/datengrab/devel/branches/ubuntu-image$