What is set in stone in the building permit application?

Unfortunately, I did not receive any notification, which is why I am only getting in touch now. Thank you for all the feedback. I have taken a closer look at the plan and there are indeed many small changes. If it were really just one, I could live with that, but actually, the draft in the BA is a rough tracing of the actual draft.

If you only submit a construction notification according to §64 HBO, you can IMHO waive the notification of all changes compliant with the development plan. It is not entirely correct, but since the designer is responsible for compliance with all building regulations and the authorities do not have to review or approve anything, it is quite justifiable. In the building permit procedure according to §65 HBO, I agree with the opinions of the previous speakers.

Are you perhaps referring to §63 and §64?

The building application states according to §64 HBO. Alternatively, you could also check the box at §63.

Just wondering: Does the architect know that you are using his plans? Or does the architect belong to the [Bauplanungsbüro]? Or have you paid the architect for his planning services?

I don't know exactly what you have planned there. But I take the liberty of raising the architect's copyright here, even if I might make myself a bit unpopular...

The architect only makes the design and knows that we forward the plans to a [Bauplanungsbüro]. So this fits here. But thanks for the note, you never know ;-)

Do you perhaps mean §63 and §64?

No. §63 are projects exempt from approval, which can be carried out outside of development plans either completely without official involvement (e.g. small garden shed) or with notification to the municipality (e.g. garage).
§64 is exemption from approval, so no building application. I would not see any need for action regarding the authority if the architect has demonstrably given his consent. Regarding possible disputes with the construction company, I would be careful to have a clear written fixation of the planning to be ultimately implemented.