Can a gas condensing boiler be built to KFW-55 standard?

One can still consider a conversion afterwards.

However, this is often no longer as effective as with direct planning. The requirements for underfloor heating and hot water often make different concepts sensible. A design for gas now may make a later conversion to a heat pump uneconomical for a long time.

However, this is often no longer as effective as with direct planning. The requirements for underfloor heating and hot water often make different concepts sensible there. Designing for gas now may make a later switch to a heat pump uneconomical for a longer time.

That used to be the case when underfloor heating systems were still designed for high flow temperatures and, accordingly, loops that were far too large for low temperature were laid.
There is nothing against running the same flow temperatures with a modern gas boiler as with a heat pump. Accordingly, the underfloor heating can also be designed appropriately.

Regarding the environmental issue: it simply has no place here. As already mentioned above, a new building is an environmental disaster of the highest order. Compared to that, I am an angel with my old oil heating in a 70s house.

That used to be the case when underfloor heating systems were still designed for high supply temperatures and accordingly had loops that were far too large for low temperatures.
There is nothing against operating with a modern gas boiler at the same supply temperatures as with a heat pump. Accordingly, the underfloor heating can also be designed appropriately.

Is the water from the solar thermal system (which is necessary for condensing boilers for KfW55) then mixed with cold water? Or is the flow rate in the underfloor heating simply reduced extremely?

The water from the solar thermal system (which is necessary for condensing boilers for KfW55) is then mixed with cold water? Or do you simply reduce the volume flow in the underfloor heating extremely?

As far as I know, only as much hot water is mixed into the returning (cold) water to keep the flow temperature correspondingly low. We have a flow temperature of 29 degrees in the underfloor heating. That is sufficient even on very cold winter days. However, the drinking water is heated to 55 degrees due to the risk of Legionella.

We also decided on gas + solar + ventilation to achieve KFW 55. Regardless of KfW, we would have chosen this combination anyway. Our municipality has had gas connected in this new development area. In addition, we did not want an external heat pump exposed to the weather. Deep drilling is not allowed for us. Therefore, the only solution left was gas. Whether heat pump or gas, opinions differ. Everyone has their own opinion. However, gas should not be fundamentally demonized here.

We also decided on gas + solar + ventilation to achieve KfW 55. Regardless of the KfW, we would have chosen this combination anyway. Our community had gas connected in this new development area. Additionally, we did not want an external heat pump that is exposed to the weather. Deep drilling is not allowed here. Therefore, the only remaining solution was gas. Opinions differ on whether to choose heat pump or gas. Everyone has their own opinion. However, gas should not be demonized here in principle.

Are you satisfied? At our place, the gas connection was offered as a special deal for 300 €. We are especially surprised by the low costs and by the fact that during the warm half of the year the gas heating practically does not have to turn on.