Single-family city villa with tent roof - visual single-story appearance

Spinach is yuck

Without a development plan, it always applies that a house must fit into the surroundings.

That’s why I’m confused...in the initial post it does say something about a development plan.

First, the property was inherited, and as a layperson, I cannot be expected to know the laws regarding a building application. Since there is a development plan, we were told that this is how it works. We did not know that there is a §34 and that it can be applied, and the experts did not tell us either.

My grandparents' house was an architect-designed house with a one-sided shed roof extending to the ground, a swimming pool, and very futuristic, although unfortunately dilapidated. Therefore, I simply did not expect that a conventional city villa would pose a problem.

Yes, there is a development plan from 1954 in which only a building envelope is defined, nothing else.

That's why I'm confused... the original post mentions a development plan

Yes, from 1954. That was before the Federal Building Act (today Building Code) came into force. The OP writes that it only specifies the building envelope. Thus, it is a development plan according to §30 (3) Building Code
Within the scope of a development plan that does not meet the requirements of paragraph 1 (simple development plan), the permissibility of projects is otherwise governed by § 34 or § 35.