Outbuilding (stable) in residential building (including renovation and extension)

Practically speaking, I have already responded too much to you considering the enormous amount of illustrated facts. Basically, if a plot of land is available, a 140 sqm house can be built for 400,000 EUR, even on a 9 x 11 m footprint. But credibly achieving that as an "extension" to a 5 x 11 m shed, I do not see yet.

Thank you. I will provide pictures or sketches tomorrow evening. It is indeed a proper house that was used as a stable back then. As I said, I will best roughly sketch it tomorrow.

Attached is a sketch. The wall material is brick.

So you would have a plot of land with a house and the statement from the building authority that it might be possible to convert and extend it. That is already worth something. I would probably first look for an architect who would basically accompany such a project. Maybe a preliminary building inquiry with a building as you would like it would be enough at first; just to test the waters with the building authority. With a professional at your side, you will probably be in a better position. And then you just have to see. Partial demolition of course costs money. But you also save money if you can use as much of the old house as possible. The more, the better.

how old is the house? Condition? Square meters?