Approaches to solutions after unmarried separation

With Airbnb rentals, you have to check whether there is a [Zweckentfremdungsverbot]. Then a complete rental of a property would be difficult. With rooms in a self-used property, this is usually not a problem. But beyond a certain limit, you enter the commercial sector, which has different consequences.

Since then she has apparently been renting out a room via Airbnb

Complete rental through Airbnb was not suggested here.

Already understood. But it does not have to be the same with the OP, that's why I warned.

I can understand that. But it’s wrong to cling to it. And then start a shared flat to keep it? It’s already madness to build a house and then fall back to the status of a student shared flat. Strange people in my house? In my kitchen? On my couch? Pooping in my toilet? Never!

I lived in a shared flat for years, I don’t find it that bad. Besides, eventually a partner might come along and move in...

I had a similar situation and rented out the main apartment as a holiday rental for a few years, which unbelievably worked wonderfully… but with a lot of work! In the end, I sold it. I also think you’re just postponing something.

Even if I eventually have to sell it, at least I have the option to build up some equity again. I also considered Airbnb, but then I actually have constantly changing "tenants" and no secured rent. With a "shared flat," you could rely on relatively regular rental income. I just don’t know if I have to declare the rent in my tax return or if it is taxed from €400 upwards, even though I am still paying off the house. How is it actually if I/we sell the house at "cost price," i.e., without loss (including transfer expenses) and maybe even make a little "profit"? Don’t I have to pay tax on that since it almost resembles speculation? If afterwards a big bill from the state comes, I would be pretty much ruined. Many greetings! Jan

Make a clean break without any detours. The next house you buy alone, if financially possible. Then you won't have this problem.

Don't worry, tax is only on the profit that remains after deducting all costs. But even if not, you should have lived there yourself. Your rental income must of course also be properly declared on the tax return. But here too, you can deduct interest costs. A visit to a tax advisor is recommended. You need proper ancillary cost statements, etc. for the tenants. It's not rocket science, but you have to take care of it. As everyone else says: better sell the thing.