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

For an energy efficiency house 70, a heat pump is planned. The living area is approximately 300 m2 spread over 2 floors. It is a large apartment and 4 small apartments. Underfloor heating will be installed everywhere.

1.) How large should the buffer tank be? Is 400 liters sufficient?
2.) Is the extra cost for a cooling function worthwhile? By how many degrees can room temperature be lowered approximately with it?

1.) How high should the buffer tank be? Is 400 liters enough?

Why have a buffer tank at all?

2.) Is the extra cost for a cooling function worth it? By how many degrees can room temperature be lowered approximately?

No, it's not worth it. Maybe about 3 degrees?! It's not just the cooling function you need, but also humidity sensors in the apartments. And cooling the air of course leads to an increase in humidity and it can get very muggy in the rooms (for example, 27 degrees at 70% relative humidity becomes 24 degrees with 83% relative humidity).

Why have a buffer storage at all?

Isn't that more sensible in combination with a photovoltaic system, e.g. for water heating?

Isn't that more sensible in combination with a photovoltaic system, for example in water heating?

Buffer tank or hot water tank? They are different things. Or combination tanks? A classic buffer tank stores the heating system's hot water so that the heat pump does not run as often/long. In fact, a buffer tank masks poor heating system planning and results in increased energy consumption. A hot water tank either stores heated drinking water or ensures through an integrated heat exchanger that flowing drinking water is appropriately heated when drawn. Have you already had heating system planning done by a professional planner?

Buffer tank or hot water tank? They are different things.

My question about whether 400 liters is enough for us refers to the heating water buffer tank. Additionally, a 500-liter hot water tank is planned.

My question about whether 400 liters is sufficient for us concerns the heating water buffer tank.

Then my first answer applies. Avoid buffer tanks if possible, as they not only generate acquisition costs but can also increase operating costs.
If for whatever hydraulic reasons it is still necessary, the specialist planner will tell you. They will also specify the size.

P.S. Photovoltaics play no role in this consideration.