Floor plan optimization single-family house 140m2 on narrow plot

If wet, then yes. Ceiling height 235, so room height probably about 210, would be the same for the jam jars as well as for the one going to them. Except for laughing, no enjoyable thing is done in the basement. A basement is a basement substitute room substitute. You don't put the trampoline there, just set it down.

You can use a basement for so many useful things: hobby, party, workshop, laundry, storage.

The basement is unfortunately wet, but I will have pictures of this shortly.
First the exterior view (the facade is deceptive)

It already looks unlovingly maintained in several places, but a professional inspection could still be worthwhile (and scaffolding and covering the roof as well as further dismantling). Simply knocking it down outright and undervaluing the destroyed assets is quick, and a demolition contractor will gladly make a "cheap" offer that can be carried out with cheap labor. I stick to this:

I wouldn't have time before October (except for a preliminary discussion), but I am definitely interested in the project.

A basement can be used for so many useful things: hobby, party, workshop, laundry, storage.

I agree with you! Especially since I would like to set up my hobby workshop in the basement. Also, it would be worth considering making the access to the new basement open and heated and possibly converting another room into living space. But of course, everything ultimately depends on the budget.

In several places, it already looks neglected, but a professional inspection could still be worthwhile (and scaffolding and covering the roof as well as further dismantling). Quickly knocked away and undervalued destroyed assets is easy, and a demolition contractor is happy to make a "cheap" offer that can be carried out with cheap labor. I stand by that:

The neighbor has already been there and said that in the event of a demolition, he would take the roof panels.

I agree with you! Especially since I would like to set up my hobby workshop in the basement. Also, it would be worth considering keeping the access to the new basement open and heated and possibly finishing another room to be livable. But of course, everything ultimately depends on the budget.

The neighbor was already there and said that in case of a demolition, he would take the roof panels.

You can get rid of almost everything that is still usable through classified ads or neighbors. Saves costs and resources.

I would definitely heat the basement. A basement is far too valuable to be misused as just storage space. And therefore, I would not waste a thought on preserving this wet basement.

How old is the roof then? It doesn't look like from [BJ 1955].

How old is the roof? Doesn't look like it was built in 1955.

As young as this roofing is in years, someone must still have valued the house at this investment. Let us assume it was not a madman, and derive a suspicion from that that a hopeful look at the entire substance is still worthwhile.

The neighbor was already there and said that in case of demolition he would take the roof tiles.

A usual demolition turns everything into rubble. For reusability, a roofer must come here, so dismantling (probably best also with scaffolding) instead of demolition.

I would definitely heat the basement. A basement is far too valuable to misuse it just as storage space. And therefore, I would not waste any consideration on preserving this damp basement.

I consider it worth checking whether the basement is objectively as shabby as it appears superficially; and in that case, I would dedicate it only to utilitarian purposes.