Price difference between ground slab and basement. Can it be estimated?

If the mother-in-law is supposed to move into the [Einliegerwohnung] because she can no longer manage on her own, then for me the question of which floor she should move to does not even arise. Basically, it can only be the ground floor, to ensure that she can always get in and out easily. Unless you want to lock her up on the upper floor with her rooftop terrace. The basement is out of the question. She would have to go down stairs and there are no windows either. Nobody wishes such a dungeon on anyone, not even the mother-in-law.

I would try to integrate the [Einliegerwohnung] into the ground floor. Everything you don't need on the ground floor I would shift upstairs, including the utility room. Having the dining and living area upstairs can be done, but I find it suboptimal in most cases. All groceries have to be carried upstairs and from there you cannot access the garden. That would only be worth considering if it’s a hillside property.

If your plot is fairly flat, I might also consider a 120m² house with an adjoining [Einliegerwohnung] as an extension. A flat roof for the rooftop terrace would be built above it, and later, in case there accidentally turn out to be two children, you could add a few more square meters.

An apartment in the basement is nonsense unless you happen to have a hillside property offering this. Building downwards is always more expensive, and if it is to be livable, excavations for daylight are needed, which also restricts you in the outdoor areas. Additionally, excavations are not cheap.

I think some kind of extension on the ground floor would be best. But with a separate entrance so that renting to outsiders is possible later.

And don’t underestimate the costs. Even if you say you are well set up budget-wise, the questions here in the forum on the subject of apartments for relatives often show a noticeable overestimation regarding one’s own financial capabilities. Recently, someone with a net income of €1500 wants a house for themselves plus their parents. They want the parents to pay below-average rent (rental income is taxable! Money under the table is not recognized as income by the bank...). Sometimes the idea is even to let them live rent-free, even though the finances of the person asking suggest renting rather than owning a home.

The apartment will easily cost you €120,000–140,000. If you really only need it for your mother-in-law, i.e., 5, 10, 15 years, then forget about it. Unless your generosity (or budget) is so large that you can leave this appendix unused once your mother-in-law has to go to a care home or is no longer around.

Thank you for the answers A basement granny flat can be nicely designed with large windows and light wells... but I just can’t imagine that it saves anything (as our developer says) The granny flat without stairs would obviously be best! If the mother only had to go up/down stairs to leave/reach it, that could still be accepted. At the moment, there are other health issues that endanger living alone. If climbing stairs is no longer possible one day, there are appropriate aids for that (I am quite familiar with such things) Simply partitioning off a quarter or so of the house would of course be the easiest option, even if a second staircase is needed. But as you already write, stairs inside the granny flat are really bad for an older person. I think the best solution is to build a “normal” single-family house and integrate the granny flat partly into the ground floor and partly as a single-story extension, which is probably best. Is it true that with single-story extensions/additions every square meter costs almost twice as much as with 2 or 1.5 stories? Maybe our developer just likes to build basements... ? Neither of us know anything about building and we rely on good advice

Being twice as expensive doesn't make sense, since for the exact same money you can build two stories. But the point isn't entirely wrong. You need a slab foundation, supply and drainage lines, foundations, soil replacement, etc. You also need a roof. Both of these are quite expensive trades compared to walls and intermediate ceilings. The complete interior work (electricity, heating, water, windows, wallpaper, floors, ceilings, etc.) costs the same. By the way, it's the same with a basement. The walls just have to be much thicker (earth pressure), you have a lot of groundwork, possibly have to waterproof against groundwater, etc. For that, you don't need an additional roof. I think in the end, both options don't differ that much. It may even be that the basement is slightly cheaper, but then you also have an underground floor and it will be larger than 45m². If you have the floor area and the development plan allows it, I would probably prefer the extension variant. But the cheapest option is definitely if you can somehow integrate the granny flat into the "normal" house. Then you could get away with 80-90k€ for the granny flat. For the extension or basement, it will probably be significantly more.

When we started to deal with the topic of house construction, we also had quite different ideas. A small prefab house of medium standard that would be paid off relatively quickly. But in life, some things just change...

It is clear that the [Einliegerwohnung] will cost quite a bit extra and the few € rent will not cover that. Also considering everything that can be deducted from taxes (which is why I am currently looking for a tax advisor).

If the apartment is there, we would indeed rent it out to others before it stands empty.

In order for you to deduct the granny flat for tax purposes, you must also receive a (sufficiently high) rent from your mother-in-law. Additionally, only the interest is deductible. I believe many are under the misconception that you can get a good tax break from renting it out. The rent counts as income again. I’ve dealt with this topic with my parents. Ultimately, it doesn’t bring much tax benefit when you offset the effort involved. Does your mother really need a granny flat, or would a larger room + bathroom in a "separate area" suffice? Then it might fit into a classic house design with an additional guest toilet on the ground floor. Renting it out would, of course, no longer be an option later. But if she really needs her own apartment, renting nearby might also be an option. Personally, I have my own home so I don't have to share it with anyone else. The thought of tenants in my house wouldn’t please me at all. Financially, it doesn’t add up anyway, as you have already noticed. But everyone has to decide that for themselves.