That is all on the BAFA website. You get EUR 4,000 for a ground-source heat pump (basic funding) with an annual performance factor of 4.5 or better and can still "grab" a few EUR-500 units if certain additional conditions are met (extra storage, deep drilling, etc.).
To increase the basic funding, you have to massively oversize the heating system for a single-family house, which certainly doesn't make sense.
At least a few months ago, there was a PDF document online that detailed this – by now, the homepage is also quite tidy and clear.
If you buy the heat pump directly from the heating installer and not from the general contractor/general planner/trade contractor in the construction contract, it is naturally a bit more relaxed in terms of timing, as the contract will probably only be concluded during the construction phase.