Plan living area floor plan

In-house waterworks is planned at the top right on the other side of the threshing floor in the basement.
Since only cold water and the heating are to take place in the new living area, you can plan freely there.

In the existing building, I would first clarify the connections exactly before designing wildly. How and where are water disposal, drinking water connection, possibly gas?

In the existing building, I would first clarify the connections exactly before designing wildly. How and where are waste water disposal, drinking water connection, possibly gas?

It is not an existing building; the project of the mother concerns an undeveloped barn.

Uhm, before I would start thinking about the floor plan, I would first clarify which fundamental renovation work is necessary to turn the barn into a living unit. "Low conversion effort" is something I have never encountered when converting a barn for residential purposes. It starts with excavation of the soil, which often goes below the foundation limit and requires safety measures, up to the desalination of the walls. Depending on the ceiling construction, it may also be necessary for the permit to reinforce the ceilings structurally. Converting barns for residential purposes is one of the most expensive conversion measures I know.

It is not an existing structure, the mother's project concerns an unfinished barn.

For me, that is also an existing structure, and with probably the most expensive and worst conditions imaginable - also still protected as a historical monument. I hope the account is well filled. :)

I don't know how undemanding the mother-in-law is and from what apartment size she is moving in with you, but I would skip the kitchen/guest WC combo at that point and plan a properly sized apartment for the mother-in-law and her dog. The bathroom is in no way elderly-friendly in terms of dimensions, and I find the access through the kitchen almost a bit disgusting. The bedroom is also rather small if you want to fit a proper wardrobe in there and possibly move around with a walking aid. Better to have a proper bathroom, a well-sized bedroom, and a small room for the ironing board or later on for 24-hour care.

I have also given it some thought.