Seems simple enough, but I would think it through a bit and write it up properly before entering it into your contract.
Based on your above statement that the client can do 'whatever they want' with the site they could reuse the site, sell it to others, or license it however they wish - maybe you should restrict and clarify that. You mentioned that you could keep a copy, resell it, etc. but will you grant an exclusive license to the client for custom graphics and other assets that are specific to that client?
A distinction between custom front-end and the codebase can be handy. It sounds like it won't be hard to achieve what you are looking for.