Cooling via underfloor heating with brine heat pump

I unfortunately did not do it, but if possible, I would definitely do it with the [Betonkernaktivierung]. It not only makes cooling more efficient but also heating, as the necessary supply temperature can be lowered further. The effort is very low, "simply" a pipe in the suspended ceiling and to the heating circuit distributor, it doesn’t get much easier. You just have to remember.

with the same principles as with underfloor heating (maximum circuit length 100 m, avoid small circuits) etc.?

How does it work with the precast concrete slab, the pipes for the BKA + electrical lines + controlled residential ventilation pipes etc. are laid there before the concrete is poured, right? And then the concrete is poured on top. Is there not something to consider regarding insulation or something?

Phew, I can't help with the detailed questions regarding BKA; I did all my trench collector and heating planning through the purple forum, where the experts on the topic are; however, I never really discussed BKA in detail, so I don't know how to dimension them precisely. But I believe it's similar to what you said, ... similarly long loops as in the rest of the heating system.

As a heating engineer, I never install cooling in the underfloor heating! The system can only cool the floor by about 2 to 3 degrees - consequence: cold feet! The room temperature is still too warm - so a separate air conditioning unit in the living and bedrooms, and then you can sleep cool and possibly cool the living room at head height without having cold feet. And what I build for ourselves, I also recommend to our customers.