Architect demands more money

Hello,
we have a question about the architect's fees.
We planned the building application with an architect. To protect ourselves cost-wise, we agreed on a fixed price with him in advance. This is set out in a contract, but it also includes the estimated construction costs and the living area. The architect said at the time that he had to enter some values there, but they were quite irrelevant since we had a fixed price anyway. Now the architect is asking for more money because we exceeded these figures by about 25%. Does he have the right to do this?
He never informed us that we would have to pay more until we received the invoice.

Thank you for your answers.

What kind of contract do you have with the architect and what exactly is stated in it? How is the billing done according to the contract?

Construction costs and living area are fixed, as is the architect's fee for performance phases 1-4; Payment method: down payment, payment after submission of the building application, payment after building permit

The contract also states that changes/additions require written form.

In case of doubt, the HOAI applies as mandatory statutory pricing law.

In case of doubt, the HOAI applies, as it is mandatory statutory pricing law.

Theory and practice ...

Regards
Dirk Grafe