Scope Creep
Also known as: feature creep
A project quietly growing past what you agreed to, one small favour at a time.
Nobody ever asks for a second website. They ask for one more page. Then a form on it. Then could the form email two people.
Each request is tiny and reasonable, which is exactly why it works. By the end you have built something much larger than the thing you priced.
The usual advice is to be firmer, and that is only half of it. A written list of what is included does more than a strong personality, because it moves the conversation from your mood to a document you both read earlier.
A repeatable build helps too. When you know exactly what the standard site contains, the extra thing is visibly extra.