Do gas calorific values significantly worsen the KfW standard?

Hello everyone,

we have a two-family house (2 full stories) with a fully basement in solid construction from the 60s. The exterior walls are made of 24 cm pumice stones from the ground floor upwards.

The major renovation concerns the following:
The gable roof will be completely removed and rebuilt. All windows and doors will be replaced. Most windows face south or west. Thermal insulation of between 12-16 cm should also be added. (including the basement exterior walls). A balcony overhang and the entrance canopy would need to be completely wrapped with the insulation. The basement ceiling as well. Underfloor heating is also planned.

1. We were unsure about the heating. As soon as the energy consultant heard "gas condensing boiler," he said that at most KfW 100 would be possible. Does a gas condensing boiler deteriorate the energy standard that much, or can you hardly achieve improvements in an old building with the measures described above?

Regarding the heating, of course, a heat pump would also be an alternative. Likewise, collectors to heat the domestic hot water. One floor will have a wood stove, which could potentially also heat water-bearing lines. Whether a ventilation system or heat recovery is an option, we would have to clarify. Especially whether this is acceptable in terms of cost or even technically feasible.

2. Shouldn’t KfW 55 or 70 actually be achievable without problems given this?

Does a gas condensing boiler worsen the energy standard that much, or is it already difficult to achieve much in an old building through the measures (roughly described above)?

Getting anything better than KfW-100 with a gas condensing boiler is not that easy.
What helps a lot is solar thermal hot water preparation, and with enough roof area and the right orientation, heating support is also worthwhile. Especially since you are planning underfloor heating, that makes it economical as well. There are additional subsidies from BAFA (€2500), and the primary energy demand decreases significantly. Furthermore, if you are renewing the roof anyway, it can be implemented as an in-roof system. This reduces heat losses from the system and also (minimally) lowers the costs for the roofing.
If you already have gas in the house, I would stick with it and not choose the heat pump. With that, you should at least be able to reach KfW70, but it always needs to be individually balanced. KfW55 then requires room ventilation and heat recovery, but it probably no longer pays off financially.