Thank you for your answers. The decision is made, we will go ahead with the renovation. Whether it will then be 270,000 euros remains to be seen. Now, various detailed questions arise, but I prefer to open new topics as needed for that.
By the way, we want to leave the basement (too low anyway for living) unchanged and outside of the thermal insulation. The roof should be insulated in order to possibly convert the attic into living space later.
The established garden, which has grown a bit wild thanks to somewhat negligent tenants, is already nice. We are looking forward to being able to use it next summer - unfortunately only then.