Kniestock lowered afterwards

My parents have 40cm knee walls. But officially it’s a bungalow with an attic upstairs used as storage. That just doesn’t work. At least not if you don’t have 150sqm of floor space and can accept the corresponding loss on the sides upstairs. But then you still have those unused peaks, which are almost unusable even as storage space. We have that at my parents’ place: wooden walls were built in at about 120cm height and the peak behind the wooden wall, all the way to the end of the house, is used as storage; or rather: that is really the last storage option that you can use for things that are hardly ever used. You can only crawl in there. So really just wasted space...

Regarding the mistake of your contractual partner: not only did he make a huge blunder, you also made a mistake. [...] In this respect, I see the share of negligence evenly distributed.

The purchase from the developer actually relieves the buyer of the responsibilities of the builder. You buy a house just like a car. Who asks whether everything is done properly? (A little jab, it's no better with cars than in construction ...) It may be that in real life you have to help intensively to achieve a reasonable result (construction supervision ...). But that rather says something about the real existing developers. Therefore, I see the matter somewhat one-sidedly. The developer has sold a house with certain features and now cannot deliver. He should think about that.

In practice, however, our buyer suffers the damage; I would be very surprised if the contract does not leave a back door regarding possible official requirements or objections.

Now it would be interesting to know what is stated in the development plan and what is stated in the contract. Only after that (!) can one consider the further course of action. I would also first seek a conversation with the developer after clarifying these boundary conditions.

The purchase from the property developer actually relieves the buyer from the developer's obligations.

Depends on the contract; I also don't think that a classic property developer is the contracting party here.

Practically, our buyer suffers the damage, I would be very surprised if the contract does not leave a loophole regarding possible conditions or objections from the authorities.

I see it the same way.

It would now be interesting to know what the development plan says and what the contract says. Only after that can one consider the further course of action. I would also only seek a conversation with the property developer after clarifying these framework conditions.

However, this presupposes that the OP can read and understand both.

Rhenish greetings

Depends on the contract; I also don't think that a classic developer is the contracting party here.

A developer is a developer. But since a "down payment" is mentioned above, it might not be a developer at all in this case. I don't know who signed or will sign the building application. Then it would be clear to me who is responsible.

However, this presupposes that the OP can read and understand both things

If not, no problem. Many here, myself included, have been preaching for years that you can also buy expertise. You just have to do it then.