New building with granny flat: Photovoltaic - Electricity - Heating

We have even agreed with the tenant that they can set the additional costs particularly high, which gives them tax advantages. We do not receive any additional payment or refund in return. For the electricity, we have a separate meter.

As a tenant, you can only deduct something if you have a second residence or are self-employed, and then it doesn’t matter whether it is rent or additional costs. The tax office wants to see a statement of additional costs from you.

By law, consumption-based ancillary costs are mandatory, which is certainly not a problem with the mother-in-law, but it is with the later rental. Therefore, separate electricity, cold and hot water meters, and heat meters. The second problem is what the tax office recognizes in the billing. What electricity price can be applied, which investments, etc.?

statutory consumption-based additional costs are prescribed,

Basically yes (I had written that too hastily), but with a granny flat or a two-family house (in which the owner also lives) you can do without it. The "only" problem remaining is for the tax declaration.

There remains "only" the problem with the tax return.

I have always allocated based on m2 WF. That is how my tax software prescribes it. There has never been a problem.

I have always allocated based on m2 living space. That is how my tax software requires it. There has never been a problem.

I am well aware of the options you can enter. Whether they are correct for a warm rent without billing is another question. You can assume that the tax office will not come to check your receipts and ask why you allocate by square meters. But as I said, that is a question only a tax advisor can answer.

And in the case of allocation by square meters, the critic will be less likely the tax office, but possibly the tenant. Because, if in a two-family house it is not an inclusive rent, then the billing must also be done according to actual consumption, if I am not mistaken. But, there are also pragmatic tenants.