Residential unit in the basement

Assuming one only considers the room in the bottom left as a living unit, there would still be sufficient window areas. And below the bathroom, connections for a mini kitchenette could be provided, so it would be an apartment, albeit a very small one. Like, for example, in a student dormitory.

I think with the cellar stairs and all the measures that will become necessary, you will no longer have any financial advantages from the second KfW loan but rather lose money instead

KfW loan
100,000€ (30 years term, 20 years fixed interest at 1.5% nominal interest) 18,000€ interest (outstanding balance 26,000€)

Bank
100,000€ (20 years fixed interest at 2% nominal interest) 29,000€ (outstanding balance 44,000€)

In addition, with Kfw 40 there is an 11,000€ repayment subsidy (in BW) or with Kfw55 6,000€

This definitely makes a difference to me.

Costs basement stairs?
Costs additional front door?
Costs plumbing?
Costs electrical?
Costs interior doors?
Costs interior finishing?
...
...
...
etc.

I will also take advantage of the second KFW loan, but I will then also rent out the apartment.

But you do know that you can only apply for a promotional loan up to the value of the residential unit to be constructed, right?

I'll put it another way: if it were profitable, everyone would do it.

Best regards
Dirk Grafe