Einliegerwohnung - double subsidy and tax benefits

Hello!

We are about to buy a house and are considering renovating the attic as a granny flat.
A complete renovation with high energy-efficient refurbishment (heating, roof, windows, exterior wall insulation) is planned.

Spatial separation for the granny flat with a separate kitchen and bathroom is given. So the technical requirements are met.

We are still not sure whether we want to rent out the apartment or just use it ourselves later when we are old, or rent it out to our aging parents.
That means the apartment would probably be empty for a few years and at most occasionally rented out as a holiday apartment.

Questions about this:
1) Can we deduct the additional costs for loan interest and renovation of the granny flat from the income tax we receive as employees, or can this only be offset against the rental income?

2) Since there are two separate residential units, would we get the subsidy twice if we renovate accordingly expensively?
Especially the roof I would almost completely attribute to the granny flat in the attic.

1) Can we deduct the additional costs for loan interest and renovation of the granny flat from the income tax we receive as employees, or can this only be offset against rental income?

Only with the intention to rent out (and actual implementation). However, no renovation costs directly, but rather via depreciation (AfA).

Since there are two separate residential units, if we renovate accordingly expensively, would we also receive the subsidy twice? Especially the roof, I would almost completely allocate to the granny flat in the attic.

Let's put it this way. Either you renovate a single-family house with costs of €100k ==> then subsidy on renovation costs of €100k. Or you renovate a two-family house with renovation costs of €100k ==> then subsidy on renovation costs of €100k. It only becomes interesting if you renovate to an efficiency house standard as of 01.07. Then a maximum of €150k per residential unit can be granted as a loan, or €60k as a loan for individual measures (or the subsidies, if renovated with equity capital).

You need to explain your second point a bit more precisely. Assuming I have an energy renovation for the house that exceeds the maximum eligible amount for funding, then splitting it into two residential units would be better, right?

2) Since there are two separate residential units, if we renovate at a correspondingly high cost, would we also receive the subsidy twice?
I would almost fully allocate the roof to the granny flat in the attic.

You don't have to assign individual measures to the residential units. The entire house will, for example, be renovated to KFW 70 EE standard from 1st July, then the maximum eligible funding amount increases to up to 2x 150,000 = 300,000 EUR.
Of course, you only benefit from this with correspondingly high investments. You can’t simply invoice / have the same costs subsidized twice ;)

That goes without saying. The fact is that I will definitely energetically renovate for over €150K. Therefore, the two residential units then make sense.

Planning elderly people as residents of the attic is rather a stupid idea without an elevator.