Assessment of building regulations

Regarding the foundation slab and roof structures, I have no idea

That's the problem. You’re driving yourselves crazy but don’t even know the exact heights yet. Just ask your general contractor about it. Or have them give you the construction service description; it often also includes ceiling structures, etc.

According to the building authority, the land-use plan dated 28.12.1960 applies there

But that doesn’t apply to the hinterland, right? Hinterland is hinterland. That has nothing to do with the front. You should be glad that building is even allowed there.

Apparently, the building authority is fixated on the 4m eaves height and the single-story nature. These must be observed. What use is it to you if you find out that there is no written confirmation of this, but that these measurements have been used as guidelines for hinterland development for years? You can jump naked in circles, you won’t get a stamp on your building application if that doesn’t fit. Then you’d have to sue, which brings us back to the 5 years and a lot of nerves. Because of 30cm more knee wall height, that’s nonsense. Then I’d simply make the house deeper, the ground floor bigger, plan anew, and put in a knee wall (box in the top).

Then there are simply different conditions. You quoted something here with Max. the 4 times (in m) the permitted stories. That is 4m for one permitted story.

Then there are simply different conditions. You quoted something here, with max. four times (in m) the permitted floors. That is 4m for a permitted floor.

We are allowed to build 1.5 stories at the back. So ground floor and converted attic. The four times the permitted floors doesn’t work out at the front either. According to that, it should only be 8m, but it is 8.51m. Or rather, it seems that back then a different building code applied for the building permit than the one she told me. We just get different information and plans from all sides. For one, the knee wall is 0cm-50cm, for another 75cm, etc. That I’m supposed to find a building partner and sign a contract based on that is somehow repugnant to me.

We get different information and plans from all sides. One says from 0cm-50cm knee wall, the next 75cm etc. etc. The idea that we are supposed to find a building partner and enter into a contract based on that is somehow repulsive to me.

Yes, sure, one plans 30cm ceilings, another only 20. The next one digs the slab in a bit, another doesn't bother with such a price calculation - way too much effort. It's all just preliminaries. No one is breaking their head over it yet or calculating the details for you. You pick the general contractor you feel best about. Together you then plan the house. They get the maximum out of the heights, depending on structural engineering, official requirements, and your wishes, the matter is optimized until you like it. Assume that every local general contractor can build anything. And if the knee wall is so important to you, you ask about it in the initial conversation. It's quite simple: you want at least 1m real knee wall + dormer with the given specifications - whether that's possible and if so how?! And ultimately: yes, with strict requirements you lose quite a bit of area under the slope in the upper floor. That's normal. That's why the building price rises with every row of bricks you add to raise the exterior wall. So if you can't go further upstairs, then invest the money downstairs in more space and cut back upstairs.

We are allowed to build 1.5 stories at the back.

I’m telling you. Building regulations don’t really recognize half stories.

Just for understanding: a development plan applies to the property, which does not allow construction in the hinterland. Right? If yes, your house property is initially not approvable. Now, an authority has discretion in the administrative procedure (building application). The authority has now indicated to you under which conditions it will deviate from the development plan and approve your application. Such things are determined internally. There is no statute, regulation, or anything like that, but it lies within the discretion of the responsible authority. You can then file a lawsuit against the rejection notice in a second step. Then, among other things, it will be examined whether the discretion was exercised correctly. You can definitely win that. But: you should hire an architect and take money. You commission the architect for an approvable design. And pay for it. If you do not like all this, it is simply the wrong property. Memorandum: an authority has discretion within the scope of its responsibility and powers.