Zinspoker - Long term or short term?

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)?

Probably around 250 kWh of heating energy per year per square meter of window area.

the entire basement is heated with the wood stove.

But that doesn't change the energy consumption, whether I then only spend 2,300 liters of oil and additionally a few thousand for wood. Who knows how much longer heating like this will still be allowed. Bans are already being discussed. I just wanted to make you aware that the place is a disaster in terms of heating (which is not bad, you just have to take it into account and be able to pay for it).

Great, thank you very much! If necessary, one would also have to consider facade insulation? Would an additional €20,000 be sufficient?

Facade insulation, windows, front door. I assume the cost will be well into the six figures for the size of the house.

P.S.: If you replace the windows, the facade must also be insulated, otherwise the dew point will shift into the walls and the house will rot away.

If necessary, would one also have to consider facade insulation? Would an additional €20,000 be enough?

With Art Nouveau and thus a significantly more elaborate facade than new houses, as well as possibly adjusting the roof overhang (just a guess), I would estimate your price to double, possibly even more.

All in with the windows, it can actually quickly reach six figures.

This is/will be a money pit in terms of heating technology, which is why the "hut" is relatively inexpensive. Have I overlooked the location and plot size?