"According to information available at the authority, the groundwater level distance to the surface is between 5 and 10 m for the location. Capillary water in the upper soil horizons cannot be ruled out. To determine the exact distance, it is recommended to prepare a geotechnical report, within the scope of which the permeability of the soil in connection with rainfall infiltration could also be determined."
In terms of layperson readability of official language, Germany is truly a developing country. It says that groundwater "stands" between five and ten meters below the ground surface and can then practically gather into underground puddles. The authority wants to fulfill its duty to inform that you should have a soil report made – in this case not because of suspicion of rock or similar, but specifically to clarify the facts ;-) how quickly rainwater drains through the soil. I do not see any concerns here that the subsoil would not support a house, but certainly a note for very careful basement waterproofing.
Some interested parties apparently want to build a multi-family house, but the owner does not want a multi-family house there and prefers interested parties for a single-family house.
That does not matter; building (planning law) is not a wish concert even in the middle of nowhere. Multi-family houses require significantly higher floor area ratios and especially floor space indices and building heights. Where those are allowed, conversely, land development with single-family houses has poorer chances. But: from the authority’s side, I do not expect a commitment to a multi-family house here (which would also require a development plan; in §34 areas they cannot be "prescribed"). So if both could be built, then rather "up to multi-family house" than "only multi-family house." Single-family houses are always allowed by authorities. THEN the owner's wish is decisive, and I can predict it for you: outside of boomtowns, a more attractive neighboring multi-family house would empty his apartments faster than the groundwater infiltrates ;-)
However, if the municipality gets the idea, either formally through planning law with a restriction to single-family houses (or their conditions) or tactically by lacking a firm "yes!" to keep investors at bay, then the owner can wish for whatever he wants until he turns black, and practically only single-family house interested parties have a chance. For investors, the map is arranged somewhat differently than for the average person: where small money also turns slowly, they won’t even go there. Many public administrations therefore keep investors away through their sluggishness alone and do not even need legal (building planning) obstacles. A restrictive development plan or a §34 alone may not yet suffice, but in combination with a sluggish mayor, the single-family house builder can be sure that no investor will bid. So the "some interested parties for multi-family houses" will hardly be anything more than village money aristocrats.