Question: Is the house planning and heating okay?

Dear forum,

after having browsed here for quite some time, I am all the more impressed by how much competent help/advice there is here. Therefore, I hope that one or the other of you can evaluate our current status of house planning, as I have already heard very different opinions from various sides.

We are planning a Mediterranean house with about 140m^2 of living space without a basement. It would be built by the construction company as follows: exterior walls made of Poroton masonry with a thickness of 36.5 cm and plastered in two layers. As heating, there would be a gas condensing boiler with an approx. 5 m^2 solar system. The entire building will be heated with underfloor heating. Since we also own a piece of forest, we would additionally heat with a tiled stove, which, however, is not connected to the normal heating system. Without considering the tiled stove, the house meets the KFW 100 standard. We are now not really sure whether the additional investment and thus the higher interest rates will pay off in the long term to bring the house up to the Kfw70 standard. What do you think?

.... As heating, a gas condensing boiler with an approx. 5 m^2 solar system would be added.

Solar thermal systems are usually not economical in single-family homes.

.... Since we also own a piece of forest, we would additionally heat with a tiled stove, which, however, is not connected to the regular heating system.

At least it’s a tiled stove and not a wood-burning stove. :)

.... We’re now not really sure whether the additional investment and thus the higher interest rates will pay off in the long term to bring the house up to the KfW70 standard.

You can only be certain after an exact fundamental assessment/calculation, i.e., the actual demand (capacity, energy) for heating and hot water must be known. Everything else would be guessing, speculating, and assuming or a look into the cloudy crystal ball. ;)
With a conventional heat generator, consumption is higher than demand, meaning here it would probably appear rather positive. With a heat pump, consumption is lower than demand, here only a fraction of the investment actually becomes effective. Or in other words, the economic requirement for insulation is lower here.
With gas, better to insulate well and dispense with the solar thermal system. With a heat pump, insulate more sparingly and alternatively put photovoltaic panels on the roof. Sometimes this results in heating and hot water at zero cost, or finances this expenditure with the capital service for the photovoltaic system.
Saving energy costs usually begins with the choice of the plot, continues through building planning, and ends with proper plant dimensioning and selection. ;)
v.g.

Thank you for the quick and insightful response! I would also prefer a ground source heat pump and to retrofit photovoltaic systems later, but due to the rocky ground, the additional costs for drilling or surface installation are too high for me. Besides, there is not enough long-term experience with it for me. An air source heat pump is also not an option so far. On the one hand, I think the noise would bother me because of the lack of a basement, and on the other hand, the efficiency is too low for me in case electricity prices suddenly rise sharply. The company that would build turnkey for us has already estimated the heating costs at about €1400 per year, without taking the tiled stove into account.

..... in case electricity prices suddenly rise sharply.

The specific costs of all energy sources will increase progressively. Sometimes one will increase more, the next year then the other(s).

..... The company that would build turnkey for us has already estimated heating costs at around €1400 per year, without considering the tile stove.

Well, estimating is a tricky matter. Quite a few have significantly underestimated. I would not rely on that for a major investment decision.

regards