Is the Wohnflächenordnung (WoFIV) to be considered during property valuation?

In short: (AA) wants a partition auction, where due to an indefinite lease with a relative at the lowest rent the price is rock bottom.

That is not consistent. The owner-occupier simply buys the apartment and kicks out the relative. An investor will increase the rent by 70% over 6 years (20% today, 20% in three years, and another 20% three years later).
You cannot see a price difference of €100,000 here.

P.S. You cannot prevent a partition auction. You can only drive the price up.
And with reasonable apartments, you don’t get any special bargain there either. If you want to sell with profit, capital gains tax also applies.

This doesn't add up. The owner-occupier simply buys the apartment and throws the relative out. An investor increases the rent by 70% over 6 years (20% today, 20% in three years, and another 20% three years later).
You can't see a price difference of €100,000 in that.

P.S. You can't prevent a partition auction. You can only push the price up.
And you don't get a special bargain even with reasonable apartments. If you want to sell with a profit, speculation tax will also apply.

Thanks for the suggestions. I will discuss this with my lawyer. I've always been told that "throwing out" is not that easy if the relative with a partner, two children, two dogs, and little money would otherwise end up on the street.

that the "eviction" is not that easy,

It can be associated with stress. With the right lawyer, however, it is hardly preventable. Especially if you can play both sides. First rent increase with the verbal hint that another increase will come in three years (then the rent almost doubles in three years) and then the termination due to personal use.

The purchaser at auction can terminate a rental agreement for personal use.

That’s true. But what if he cannot prove it?

Then he does not bid on the apartment?

Investors will know exactly what is allowed and calculate precisely what is worthwhile and what is not. They have no personal need. As mentioned, they will increase the rent to the maximum and otherwise ensure that it is no longer really livable. There are also great documentaries on this topic.

All other private buyers have a genuine personal need. Here too, the buyers will know what is legally possible and enforce it accordingly through a lawyer.

( Are you AA or BB)