Extremely high heating costs with underfloor heating. Incorrect setting?

Good day.
We have noticed that our neighbor in the multi-family house (6 units, Kfw 70 built in 2016) has extremely high heating costs.
We all have underfloor heating (normal windows with controlled ventilation), pellet heating.
We suspect that his heating behavior is not entirely correct.

Can anyone say what might be done wrong?
90m2, upper floor, single household (retiree). Consumption in October about 400 kWh for 90m2 (2 weeks quite warm (15-10 degrees) in October, 2 weeks about 10-0 degrees Bavaria)
Rooms living room, bathroom, hallway, bedroom are heated to about 21 degrees using a room thermostat.
The office is not heated at all.
Generally, he likes it warmer.
But he does not heat the office at all.
Ventilation is done normally through the window (occasionally tilted).

Who can say how to correctly heat with underfloor heating for a single person (retiree).

Thank you very much

Is hot water included in the consumption? How was the consumption determined?

What is the reference value, e.g. from you, or where does the assumption that it is a lot come from?
400 kWh of gas costs about €24, I think pellet fuel costs are slightly lower. Where is the problem?

Is hot water included in the consumption? How was the consumption determined?

How high is the comparison value, for example from you, or where does the assumption that it is a lot come from?
400 kWh gas costs about €24, I think pellets are still slightly lower in fuel costs. Where is the problem?

Good day.
No, hot water is not included. Pure kWh for the underfloor heating.
The same apartment (however east, south) has about 60 kWh in comparison (to 400 kWh).
Same household (1 person). Here, however, all rooms are heated between 18 and 20 degrees.
Ventilation is done by airing (instead of leaving the window partially open).

However, all rooms are heated between 18 and 20 degrees here

The neighbors are heating the apartment as well. For 18 degrees, I can almost leave the heating off all year round in the multi-family house if the neighbors are heating.

Partially leave windows open).

One does not do that either.

The orientation, especially during the transitional period, also plays a role, including three degrees less desired temperature, different ventilation behavior, etc.

Keep observing this, especially during the heating season. Currently, there is no reason to be alarmed.

At 18°C I wouldn't have to turn on my heating at all, 4° more make a huge difference in consumption... not to mention leaving the window open.