Build a house on an old barn/farmyard

Hello,
I have a farm that I gave up, now there is an old uninhabited house and the farm in a row, the whole building is easily 50 meters long. I want to tear down the house at the front and a small part of the barn. In the end, I want to demolish 18mx10m. The building is very tall, ground floor, 1st floor, and 2nd floor, about 11 meters to the top edge of the roof.
The new house must of course be the same again because I want to attach it to the old barn. The plan is to then make apartments in it and rent them out. We ourselves have a "good" house next door in which there is also an apartment rented out, since we could rent the apartment about 10 times over, there are that many inquiries in our area, so this idea makes a lot of sense. We have been thinking about it for a long time and have already compared and looked at many options. Nevertheless, I like to get further opinions.
The current house is totally wrecked, the plaster has almost completely come off, completely wet, 300 years old and mostly "empty," so I think demolition won't be cheap; maybe someone has experience?

How many apartments can one expect with 18x10 external dimensions and 3 floors? I don't want a cellar. Preferably 3 large apartments or 6 households of 1-2 people? I lean more towards 6 smaller ones, as these are more in demand in the area (there are many big companies around here); of course, this has the disadvantage that the interior finishing becomes more expensive since you already need 6 bathrooms alone. Is 6 apartments really possible on the living area?

What still makes me think is that I would actually like it to be solidly masonry-built; what timeframe should one expect from start to finish? I usually don't have the patience when construction sites never seem to end.
Recently, I looked at a construction site of a solid wood house! I like it very much too, and it probably goes faster with less dirt?!?

A new firewall would of course have to be newly built, that is clear. Another consideration was to do without a staircase and make the individual apartments accessible by an outdoor staircase -> allowed? I guess not.

These are just thoughts I have considered so far. I am open to any suggestions. I will take a picture of the location so that one can get an idea. I am aware that some do not find it sensible to build a house just for renting, but that is not the issue for now. Cost estimates are very welcome; I can do a lot of the interior finishing myself, I have already done that (attic conversion in the current house).
Go ahead, type away!

Hmm, difficult. Farms are often located in outlying areas and using [Bestandsschutz] as a standard for new developments can be problematic when changing the use. What does the community say, how far are the nearest neighbors, what do the local building regulations require?

Is the farm on the sewage canal, or would such a multi-family house be compatible with the pit?

Talk to the community about what you are allowed to do and what they are willing to support.

In our case, the requirement was that we should orient ourselves to the neighborhood. Regardless of which one. The houses have only one thing in common: 2 full floors. Otherwise, medieval, 50s, 80s and very modern with a leaky roof. We do not have to adhere to the old development. So much for theory; the building application will be created within the next 4 weeks after demolition is completed.

Demolition is not cheap. Have an architect inquire, that will be cheaper - at least that is our experience. Costs depend on - level of difficulty - amount of rubble - pollutant class - disposal fee - distance to the landfill

We are building with solid wood walls. Google "wir leben Haus".

The key point is the use or what is structurally permitted. Without an architect, this cannot be assessed from a distance.

Best regards
Dirk Grafe

Regarding your questions. Demolition. Budget around 50 thousand. Be happy if cheaper. Ideal rental size. Two-room apartment, 60 sqm living space. Access only from the outside. Would be allowed in SH. I bet in Bavaria too. Strangely, in legislation and otherwise, the two federal states are more similar than you think. Only handball the Bavarian can't do. Conversion of commercial buildings into apartments. I guess no problem. Agriculture really only makes sense from 1000 ha. Many small businesses are struggling on. But the villages are to be kept alive, so everything is done to give farms a chance of survival, with apartments, holiday apartments, wind power, etc. This is no different in Bavaria than in SH. For your basic dimensions 18 by 10 with six small apartments, I don’t see that with two floors, rather four. With three floors it gets difficult without a staircase. If there is a lot of industry around, plan for two mechanic apartments. Good return. Karsten

So much for theory Building application will be created within the next 4 weeks after demolition has been completed.

At least I would always submit the preliminary inquiry before the demolition - so that you still have the alternative "gutting," in case preserving the outer shell is the legally simpler way. Of course, this is ruled out with completely rotten material.