In order to use the interior decorator tool, you must be a co-owner.
You can only be co-owner of 1 house - which is a total pain. I currently have 3 accounts open simply so I can co-owner to different people's houses to work on deco for them.
Like Sarsmi, many of us have dreamed of an "architect" security level, to allow customization. Such access would have to block access to the moving crate because an unscrupulous architect could customize in such a way that locked down items would be forced into the moving crate and they could help themselves to rares, and such. I think we will be dreaming for a long time.
-Skylark