Rented room in a new building, what can be deducted?

Good evening,
a friend of mine occasionally looks for accommodation in the area where I want to build, so we came up with the idea to rent out a room in the house I want to build. Now the question arises, what can I deduct from the house construction for tax purposes. What does the tax office need as "proof"? Are a rental contract and proof of payment sufficient? What disadvantages would this entail?
In the event of a possible sale of the house and a possible profit, something would have to be handed over to the tax office. How much would that be? How does the statute of limitations work in this case? Was it not 10 or 20 years?

Best regards
Tobias

Lay opinion: How do you come to the conclusion that you could deduct anything? You are not additionally building an independent second residential unit. A second residential unit would have to be permitted in the development plan. I would rather consider that you would have to declare "income from rental" (provided everything is legal) and pay taxes on it.

Well, I can rent out a room in my own house. And therefore, one should be able to deduct at least a small part of the construction costs or loan interest from taxes. Or not?

Hello HG,

but you have to explain to the tax office exactly which costs relate to this one room, i.e. you need a loan that is split (into rented and non-rented), construction costs must be allocated... which construction costs relate exactly to this one room, was there a separate bathroom etc. Then you have to declare the whole thing as rental income in your tax return and can deduct the related costs accordingly (loan interest, shares of property tax, shares of insurance etc...) Have you also thought about how you want to separate the whole thing structurally? Separate electricity meters, water meters... you should consider all of that....

Best regards

For a sporadically sublet room, there’s nothing! Only for an independent living unit that is billed completely separately, as Naddl already says.

Ok, thanks! I think that will unfortunately be too complicated.

Tobi