Buy and use undeveloped land

First of all, the question is whether you are offered it at a normal price?

Would you take up such an offer? Would you buy it and then possibly build semi-detached houses on it and then rent them out?

No, extremely poor returns. In the real estate sector, there is no worse ongoing return from rented single-family houses.

Does the speculation tax rule also apply if I buy an undeveloped plot, build on it, and then sell the houses, or does the 10-year period also apply here?

a) Speculation tax must be paid.
b) Additionally, trade tax applies for four semi-detached houses.
You can only optimize tax-wise through an SPV. (Project company, go to the notary)

With financing, I would first have to convince the bank of this idea (to build and rent).

As a layperson, you can only do this with sufficient equity – four properties en bloc are valued as a multi-family house according to the income approach. So you should expect more like 30-40% equity to get a 100% loan. Unless a separate financing is taken out for each individual house (of course on separate plots).

Because in the first rank would be the land and then the loan for the house construction.

Forget it. One bank, one financing.

I look forward to your comments/tips/ideas.

This is a project for professional developers. Not for laypersons.

Right, I just looked up the Income Tax Act again. For me, there have only been the cases "vermietet" and "nicht vermietet" so far. I also think that EFHs as an investment/for renting out are rather unattractive.