Is the total budget for the construction project realistic?

A brine heat pump requires at least twice the area to be heated. With a 500 m2 plot, 160 m2 living space, and an assumed 150 to 200 m2 sealed area, it will already be borderline. Keep in mind that heat will be extracted from the ground, so the snow will melt later than at the neighbor's or the cherry tree will finish weeks later. The more area you have available, the less this effect occurs.

BAFA funding is currently as follows: (Quote from the BAFA homepage) Heat source earth and water The innovation funding amounts to 100 euros per kilowatt of installed nominal heat output, but at least:

[*]4,500 euros per system for all heat pumps with the heat source geothermal energy, with simultaneous geothermal probe drilling [*]4,000 euros per system for all heat pumps with the heat sources geothermal energy or water

Is the bonus for smartgrid-ready already included? Or is it no longer available?

what you write does not apply to deep drilling, but only to surface collectors (and even there, it seems exaggerated to me, based on what I have heard from actual users). But with the plot size, I wouldn’t build a surface collector either.

By the way, deep drilling is not possible everywhere, that should be clarified before getting too enthusiastic. And depending on the area and soil, it does not have to cost as much as writes. Our offer (from the general contractor, not even directly from the drilling company) includes about 6,000 € surcharge for a brine heat pump compared to an air-water heat pump. But for us, there is only sand underneath.

A brine heat pump requires at least twice the area to be heated. With a 500 m2 plot, 160 m2 living space, and assumed 150 to 200 m2 sealed surface, it will already be borderline. Keep in mind that heat will be extracted from the ground, so the snow will melt later than at the neighbor’s, or the cherry tree will finish weeks later. The more area you have available, the smaller this effect will be.

This is not absolutely correct: There are also brine heat pumps with deep drilling: and in that case, the problem of shallow temperature differences is absent or almost absent.

Is the bonus for Smartgrid-ready already included? Or is it no longer available? But here we also have only sand underneath.

No idea what else is still available - I'm just starting out here and am still reading up on the topic of "funding"....

Sand directly at the ground or a layer of sand is not the problem. The question is what comes out during drilling.

Sand is not a problem at all but on the contrary cheap to drill. Our sand "layer" is several 100 m thick

For 500m², the drilling must be well planned, but I do not see this as unsolvable. One should also always expect that sometimes 3 more drillings might have to be done quickly because the drill might not go deep enough. This is not a problem, as it is usually charged by meters. Whether 1x 60m or 2x 30m, it has cost us the same.