Multigenerational house (two parties) on existing plot

The basic idea is to fully renovate the larger apartment to make it ready for occupancy, while keeping the smaller apartment in reserve. To what extent renovation is necessary there to preserve the building fabric is not yet known to me. However, definitely no bathroom etc. will be installed there - except for necessary connections, of course.

Then make sure a bank agrees to that. They want finished properties. If you want KFW funding, it won't work without a bathroom either. Just a thought.

Three development plans intersect at my building plot, and about 150 meters of a fourth development plan. This means that the next building plot south of me is developed with 2.5 floors, the other two sides with my development plan and a former agricultural operation (house with two full floors). In the line of sight (150 meters away from me), cube houses with flat roofs and three full floors have already been built.

The problem could be that someone in the municipality has thought about the boundaries of the development plans and decided why up to one corner 2 full floors are allowed and then only 1+ attic floor. Your advantage will be that the plan is probably quite old and the "decision-makers" from back then are no longer in office. Exceptions with arguments like densification, neighboring development, different roof shape, etc. are then more likely. To upgrade the apartment with an upper floor portion (i.e., yours), a construction method with a recessed top floor might then also be possible, resulting in a "penthouse feeling."

: Thanks for the hint regarding the bathroom and KfW - the building financer only told me about two residential units visible on the plan for the subsidy - I definitely need to do some research, as that is exactly what's intended. Regarding financing, it's relatively simple, as the building plot significantly leverages the total value of the house, i.e. for a hypothetical 500k house, the building plot adds 250k ground value on top, which is basically equity. I also have quite a bit of equity myself, so the financing requirement will be somewhere around 300-350k based on a first rough calculation.

: Does it make sense to sound something like this out with the municipality? Personally, I don't know the current decision-makers, but my parents of course still have "connections" to them, or should something like this be done directly with the district office? As I said, I would try to keep the deviations as low as possible - a hip roof is already planned per se, but not the house height or two full floors. However, this currently represents the minimum in our region due to chronic shortage of building land. The municipality is fully built up except for the last building plot - there are only isolated building gaps in private ownership. Therefore, I assume a good argument basis for two residential units. Possibly I might have to be persuaded to rent it out...

I’m not planning to build a high-rise there, but currently a hip roof with two full floors is my favorite.

My current favorite for apartment 1 on the ground floor includes a utility room, guest WC, kitchen, and living/dining area, and the remaining rooms on the first floor. Apartment 2 on the ground floor has its own utility room and two well-designed rooms around the corner.

Your favorite conflicts with the development plan.

Does it make sense to get legal construction advice here?

Of course it is quite old, but the development plan does not significantly restrict building a house with the generous floor area ratio/building coverage ratio. You are allowed to build over 260 sqm, which can easily accommodate a decent bungalow, even with a ground-level granny flat. Just because there are other development plans doesn’t mean the existing one does not apply to this property. The purpose of a development plan is to ensure a recognizable structure within an area. Yes, at some point there is a border and a different area adjoins… If it were so absurd that comfortable living space could not be built, it would be worth challenging it. But if you only want a mainstream city villa, standard or nice-to-have, that won’t work.

Are the directly adjacent 2.5-story buildings an advantage

Nope

May I link prefabricated houses or post the floor plans here?

Links no, description and pictures gladly :) I’m rather seeing a bungalow, with an adjoining annex for a granny flat, if the building envelope allows. In the attic you can fit a home office with a hobby room or the children’s rooms. The floor area ratio is a dream! You would have to build your city villa on another plot, but then a ground-level granny flat might be difficult.

All well and good, but building living space half-finished that might maybe be needed in 15 years is simply a crazy idea. It has to be solved differently.
For example, buying a ground floor apartment nearby and renting it out until it is needed.
As currently planned, nothing good can come of it.

I took a look at the mentioned draft of Massa Haus.

The ground floor could work. However, the granny flat is certainly not barrier-free.

A hipped roof complicates the lighting. Two floors are not feasible anyway. But with a gable roof, you can accommodate the usual bedroom, 2 kids' rooms, and bathroom.

I cannot imagine that the building authority would approve a city villa.

Create a solid concept first, then a preliminary building inquiry if you want to deviate from the development plan.

Post the plot plan.