Kfw and subsequent energy deterioration due to renovation

Assuming you build a house with a KfW-55 or equivalent L-Bank loan. The house has a basement, which is not heated, is thermally decoupled from the rest of the house, and therefore was not included in the KfW calculation.

Assuming you then convert part of the basement into living space 10 or 20 years later, which is then also heated. Furthermore, assuming that the basement does not meet the KfW standards like the house does, or that the basement and the house as a whole no longer meet the KfW requirements.

What consequences would this have regarding the KfW loan if

a) it is still running
b) it has already been paid off

Regardless of KFW, you should first clarify whether this is even possible according to the [Energieeinsparverordnung].

Assuming the house would fall from, say, KfW 55 to KfW 70 due to the measures. Then it would still be within the energy saving ordinance, right?

It is probably just as unimportant as the income status in financing when an income disappears or changes. In my opinion, there is no obligation to disclose.

The same applies to the expansion of the basement or attic. Living space means habitable rooms, and certain factors must be met for that. Windows, ceiling height, ventilation options, heating, escape route?
It only becomes a concern when something happens – then negligence can become an issue. Even when selling, one might have to justify oneself. But as long as you can arrange the renovation with yourself, no one cares.
My naive [emoji57] opinion.

Brief regards

something like that ...

KfW conducts random on-site inspections ... if something comes up, that is subsidy fraud ... that is also stated in every KfW loan agreement.

I believe they reserve the right of audit for 10 years ...

I would expect that something about this is stated in the funding conditions.