Sell the property completely or partial sale with investment?

I haven't seen that in new buildings for a long time

We would install underfloor heating. But then consumption measurement would have to be done for each control circuit

There are companies that rent out the devices and take care of the billing, measurement, and maintenance of the things, which are installed in front of the distributor in the residential unit. Likewise, they have a meter for cold and hot water in every room with a water connection. The costs for all this are then passed on to the tenants.

I would not recommend instantaneous water heaters or boilers. That is outdated. Better to install water meters for hot and cold water.

For the underfloor heating, you can install a heat meter in every apartment. These are also available with wireless reading. However, they are relatively expensive and must also be regularly replaced (calibrated) (even without wireless). The costs can, however, be passed on through additional charges.

The usual suspects for heating and water metering and billing are Brunata, Ista, and Techem. Just to name the big players.

Instant water heaters are completely common for domestic hot water when the tapping point is about 10 meters away or so (I don't know the exact distance) and hot water has to travel a long way. Especially nowadays, when "electricity costs" apparently no longer matter. This is not something from "the past."

Instant water heaters ... are not something from "yesterday".

Yes, long distances would be expected. The building alone is 40m long and has 4 floors...

That is nothing from "the day before yesterday".

Ok, the new electric ones are not bad. But you also have to watch the connection power, so the cables don’t glow... but I know that more from renovations in old buildings. New electricity is easier to install than additional hot water lines.

Hot water is insulated and operated with a circulation pump and timer control. You can also argue well about what is better/more cost-effective in the long run.

You can also have a good argument about what is better/more cost-effective in the long run.

I can imagine that. Constantly circulating through the long pipes up to the 4th floor, a small circulation pump is no longer sufficient. An instant water heater is of course easy in comparison and also easy for the tenant to understand. Unfortunately, in our case they only switched on when the water pressure reached a minimum level. So you could never run a small amount of warm water. That used to annoy me.