Floor plan - 135 sqm, 1.5 storeys, gable roof

And, don’t always write BT (Bauträger) ... you have a GU (Generalunternehmer).

And, don’t always write BT

She already renounced it yesterday evening in #113

So the DIN 277 doesn't help here, correct?

Basically, you have to renegotiate completely. Your two-story building cannot be constructed. That would make the contract contestable for me and ultimately void.
However, both parties are interested in cooperating. For that, I would have a completely new offer made instead of trying to patch things up. Besides the lack of living space, for gable roofs the roofs are generally cheaper. With us, it was a difference of 10K. Do you have that to spare?

Yes, with the basic idea of "different house type, adapted construction contract" I am completely with you - however, a desire that is, in my perception, equally important for the TE is not having to renegotiate the financing again - that would have to be taken into account.

The financing has been ongoing since January...

Yes, with the basic idea "different house type, adapted construction contract" I fully agree with you - however, in my perception, an equally important wish for the OP is not to have to renegotiate the financing again - that would have to be taken into account.

As a bank, I would all the more not finance a house that does not correspond to the loan at all but is worth less.