Comment: Write up a simple contract.

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In reply to comment: Very, very helpful post, and (see in situ)

Write up a simple contract.

The web design company or individual that you end up dealing with will likely have you sign a contract discussing the price, the project scope, timeline, etc, etc.

You can give a general idea of the project to the designer without leaking details and specifics of the subject for the project. Once they have agreed to do the project you can just write up a short contract, signed by both parties, simply stating that the project was your idea and any attempt to copy, plagiarize, or otherwise steal it is forbidden.

Without a contract, there's no chance for protection because copyright doesn't protect ideas.