How much buffer storage is needed for approximately 300 m²? Is cooling worthwhile?

What are you planning that you need 500l of hot water? Several residential units? The amount of water also needs to be kept at temperature.

What are you planning that you need 500l of hot water? Several residential units? The amount of water also needs to be maintained at temperature.

Yes, it is one large apartment and four small apartments.

What are you planning that you need 500 liters of hot water? Multiple residential units?

Five apartments - 500 liters could already be very small (what is the temperature inside, how is it heated...). If only heated to 55 degrees and five bathtubs are filled with it, it will be a cool bath!

I would rather consider designing the water tank smaller and only for the large apartment, and using tankless water heaters for the small apartments.

I am relatively sure that from more than 4 residential units onwards, different conditions apply for hot water preparation (legionella prevention / thermal disinfection, etc.) Leads to high energy consumption with a large hot water buffer tank for the reason mentioned above. Therefore, it may really be necessary to consider DEH or fresh water stations (or have a qualified MEP planner consider it). With long pipe runs, no more than 2 liters? should remain in the pipe (I am not sure about the numbers, ask an MEP planner), pipe diameters, circulation, etc.

Cooling is worth it in my opinion if the heat pump can do it "anyway," for example a brine-water heat pump (Sole-Wasser-Wärmepumpe) with integrated "PassiveCooling." Then it brings a 2-3 degree delta which can take the edge off heatwave peaks. My heat pump does that, I have had positive experiences with it. However, I live in a passive house where a lot of effort has been put into summer heat protection.