Approval procedure granted, is change possible afterwards?

I just googled it, in NRW it is 2.40, he signed 2.45.. So I'm out of luck in that regard..

The drawings of the house were approved in the simplified approval procedure,

I would like to dare to doubt that. In the simplified approval procedure, the approval usually comes with the note that the planning was specifically not reviewed. Therefore, I see neither the necessity nor the point of amendments to the plans. Moreover, without knowing the floor plans, it is impossible to judge what consequences the desire for more ceiling height would have on the length of the stairs. If you want to make the steps about 8 mm higher each at the same length, you will end up with a less comfortable rise measure. That will probably make you regret the additional ceiling height easily. Especially if you have a deteriorated relationship with the architect, you should consider that the architect could sue against the modification of his work planned by you. I have already lived for ages with a ceiling height of 250 cm, even with ceiling lamps, without this seriously affecting my quality of life. More precisely: I have had a ceiling height of 251 cm for ten years, previously twenty years with 250 cm, and I have never noticed the one centimeter difference myself (it is practically only noticeable with a tape measure). Your future ceiling height is had by very, very many people — namely pretty much all who have retrofitted underfloor heating in houses from the 1970s. Nobody dies from that.

I just googled it, in NRW this is 2.40 ...

In Saxony also 2.40.

The ceiling height would be 2.44 when the floor and plaster are on, which I now find too low in hindsight and compared to other drawings from my buddy. It is a new building, everything should be according to the latest standards and not like 20 years ago.. With the additional 11.5 cm, it would be 2.55. Unfortunately, I am a layman in this field and did not notice this immediately.. It would only be one more step up to the attic, otherwise nothing would happen.. The division would be ok.. Instead of 14 steps at 26 cm and 15 risers at 18.4 cm, it would be 16 risers each 18.4 / 25 cm tread.. That fits everything and poses no problem... The main question is, if I simply build the house 11.5 cm higher but deviate from the clearance application (drawing) while still being within the development plan, there should be no major penalties if in the future someone complains and an inspection is carried out... I would submit the development plan or perhaps exceed it by 2-3 cm.. I think that would be ok since other tolerances apply in construction than in mechanical engineering..

It is a new building, so everything should be up to the latest standards and not like 20 years ago.

I am not an expert in this regard, but my impression is that especially in new apartments and new houses, the ceilings are lower to save energy (heating costs)? I basically find this a very unpleasant development, as well as bathrooms without windows. I do not know this from (also inexpensive) old buildings.

Instead of 14 treads at 26cm and 15 risers at 18.4cm, it would be 16 risers each 18.4 / 25cm tread.. That all fits and is not a problem...

Yes it does. The stairwell will be longer, and you didn’t want to touch the statics anyway. Screw the "Kumpels Competition" and build the house as planned.