KfW 55 or 40+ for new single-family house construction in Würzburg?

For KFW55 you need an energy consultant anyway. You pay them exactly to answer such questions. Otherwise, you can create a large Excel sheet yourself and make estimates, but without knowledge of thermal bridge details it is quite inaccurate.

In principle, it might just be possible, but with solid construction KFW40 is often not really cost-efficient.

By the way, you should install the ventilation anyway. If it is missing, you will regret it...

Currently, we do not have an energy consultant, hence the question. We thought about switching to 40 plus, since the photovoltaics + storage are already planned and the higher funding is quite attractive. As mentioned, it is only important to us that we do not need any additional insulation or filled stone etc. - otherwise that would be a direct deal-breaker. Regarding the ventilation system, there are probably 10,000 different opinions. If Kfw 40 plus, then we understand that one will be installed.

Energy storage makes no sense without subsidies. Central controlled residential ventilation not only reduces moisture and saves energy but also provides comfort. Whether more insulation is needed, you have to calculate (or have it calculated).

There are also 49cm bricks, which will definitely work. Or insulated bricks with lambda 0.07. With 42cm it can work if the rest is insulated more extremely accordingly.

Yes, with the storage the biggest pointless cost factor for 40 plus is already fulfilled, so financially it could work out. And maybe with 40 you can also use a smaller heating system, which is marginally cheaper. But with 40 the BAFA subsidy will be harder to get...

We did not want to go with the 49er brick, as we would lose a considerable amount of living space here. We also do not want the insulated brick. It either works with the 42.5 or it doesn't. Thank you for the note about the [BAFA Förderung] - we had definitely planned for this. I need to ask our plumber about it again.

Problem with BAFA regarding LWWP and kfw40 is the lower heating limit temperature in the annual performance factor calculation, so that the annual performance factor typically drops below 4.5. No idea how exactly BAFA will check the calculations.

And at this point the note that with a bit of trickery the entire costs for the [Kontrollierte-Wohnraumlüftung] can also be funded at 35%.