Floor plan narrow house - Basement as a white bathtub

Senior-friendly is flexible. Would plan with stairlift and attic. Must move the building services into the extension

My parents, who are over 70, are supposed to move in. Since you never know, I thought we could plan ahead and make it suitable for the elderly.

There isn’t much living space on the ground floor anyway. Making it senior-friendly is not easy.

I have a plot of land on which I am allowed to build a house with a maximum size of 6.5m * 11m.

It is a plot with a house in which I live with my family. Now an extension is to be built, see the attached picture.

Wow, these two representations would still be politely described as "completely different." So you don't have a plot of land, but a leftover building window between an existing house and the building boundary, and you don't want to build a house, but an extension. You probably call it a "house" because it can or should have an independent entrance, a separate roof, and possibly an independent utility room; and the fact that it now at least partly obstructs the previously free-standing sides of the existing house seems to be insufficiently clear to you. These are completely reshuffled cards compared to your original representation!

I have a house with external dimensions of 10.5x7sqm. This results in a walled area of 60sqm. Presumably, you can plan a cozy two-room apartment there... But that is already quite cramped. In any case, I have only accommodated a living room, kitchen, WC, and stairs to the upper floor, and the HAR(7sqm). I don't see a proper bathroom and a senior-friendly bedroom there.