Alternative to central/decentralized ventilation

Ventilation does not have to be applied for.

No, the ventilation itself not, at least not at the building authority. But it does appear in the construction plans for the building authority, accordingly the utility room must be larger, the pipes must be taken into account in the planning, etc.

And OP is building KfW55. For applying for the subsidy, you need the ventilation proof or the information on how ventilation is to be done.

Take a look at the Aereco system. We plan to install these for wet rooms. It is only exhaust air when the humidity rises. Supply air through slots in roller shutter boxes, which can be closed if necessary. Not quite optimal in terms of energy, but cost-effective and does what it is supposed to do. The pipes are also needed, but in our case only 4 (kitchen, bathroom, guest WC, and utility room).

No, the ventilation as such not, at least not at the building authority. But it is included in the building plans for the building authority, accordingly the utility room has to be larger, the pipes have to be taken into account in the planning, etc. And OP is building Kfw55. For applying for the subsidy you need the ventilation certificate or the information on how ventilation is to be done.

Kfw55 is possible without controlled residential ventilation

And TE is constructing Kfw55. For applying for the grant, the ventilation proof or the details on how ventilation should be done are required.

Only the calculation of energy consumption... although the calculation is only a simplified one (I seem to remember). Upgrading is always possible anyway. The signature of an authorized person (architect) basically seals what has been applied for.

Kfw55 is possible without controlled residential ventilation

Yes. I also didn’t say that it is required. But you have to have decided how you want to implement it when you submit the KFW application. And if you want to build in April, the application should already have been submitted by now.

Only the calculation of the energy consumption... although the calculation is also only a simplified one (I believe I remember). Upgrading is always possible anyway. The signature of an authorized person (architect) basically seals what has been applied for.

So in our Kfw application from January of this year, the type of ventilation was included. This was done by the mandatory energy consultant. He also provided documents for the building application, which also included the type of ventilation.