Zinspoker - Long term or short term?

The heating system is a new oil heating from 2011 and the lower floor is heated by a wood stove. The owners showed me their utility bills and with the current oil price you can get through the winter well with €350.

Either the villa is insulated, mostly heated with wood (which is also not free), or some rooms are not sufficiently heated. Which one is it? For an uninsulated old villa of this size, I would estimate a well-maintained 50,000 kWh p.a. That wouldn’t kill you either, but if I were you, I would take a closer look at this point...

A new energy certificate is available, the indicated consumption is 350 kWh Heating is provided for 230 sqm, there are two storage rooms in the basement that are not heated.

New roof and insulated attic make a big difference, and I think that with new windows (current windows from 1947), we can save another 40-50%.

Insulated attic according to the last statement? With the new windows, you will definitely not achieve the desired savings. The uninsulated shell is too large for that.

The attic was insulated 10 years ago. The current windows have a U-value of 6. What savings can be expected with windows having a U-value of 1 in such a house (sandstone)?

the stated consumption is 350 kWh

Per sqm and year? That would be 80,500 kWh and almost 8,000 liters of oil

That is correct, however, as mentioned, only the upper floor and hot water are heated with oil, the entire basement is heated with the wood fireplace. The consumption in this case also refers to 5 people, we are two.