For cultivation at a single-family house as a senior citizen apartment, ideas?

My question is more about the technical implementation.

The technical implementation is the least of the problems.
- has it been calculated whether the plot allows for an extension in terms of area
- is there possibly a development plan that could oppose two residential units on one plot
- additional (two?) parking spaces possible on the plot
etc.

After that, think about the room program (number of rooms, sizes, etc.).

Finally, look for architects. An extension to the existing house only works with an architect. Prefabricated house builders etc. build houses, but no special extensions from the catalog.
Who ultimately carries out the technical implementation of the extension does not matter – whether solid construction, timber frame, general contractor, individual tradesmen, etc.

We have been working on this topic for almost three years. We want to build an extension or a small bungalow of a maximum of 80 sqm for the mother-in-law on our property. The first idea was to convert an outbuilding. We have the permit for that, but the renovation would not be economical due to legal requirements and would be more expensive than a new build with a difficult floor plan. The idea of demolishing and rebuilding came up this year. So far, it has failed because all affordable solutions are surprisingly expensive. We always end up with at least 2500 euros/sqm. And the region is far enough away from commuter belts; construction prices are not inflated. However, we had also planned to build wheelchair accessible if so, and then you do end up with quite a few sqm. Since the mother-in-law is likely to have a small pension, the construction would mean that she would have to invest all her savings into the build, which will be difficult in the long term when it comes to dividing the inheritance among my wife's siblings.

80-100m2 is roughly 240-300k investment. Can you manage that? Then the next step. That has already been shown. We are happy to help.

Not currently, as we do not want/cannot rely on my employer at the moment. A generational change is imminent. but theoretically yes. Therefore, we are somewhat idly planning ahead.