ᐅ Price difference between slab foundation and basement. Is it possible to estimate that?

If the mother-in-law is going to move into the granny flat because she can no longer manage on her own, then for me there is no question about which floor she should live on. It really has to be the ground floor to ensure she can get in and out at any time without difficulty. Unless you want to lock her up on the upper floor with her roof terrace. The basement is out of the question. She would have to go down stairs and there are no windows. No one deserves a dungeon like that, not even the mother-in-law.

I would try to integrate the granny flat on the ground floor. Anything you don’t need on the ground floor I would move upstairs, like the utility or mechanical room. Having the kitchen and living areas upstairs is possible but, in most cases, I find it less than ideal. All groceries have to be carried upstairs, and you can’t access the garden directly from there. This would only be worth considering if you have a sloped lot.

If your lot is fairly level, I might also consider a 120m² (1,292 sq ft) house with an adjacent granny flat as an extension. You could put a flat roof over that for a roof terrace, and later, in case you end up with two children, you could add a few more square meters.

A basement apartment is nonsense unless you happen to have a sloped plot of land. Building downwards is always more expensive, and if you want it to be livable, you need excavation for natural light, which limits your outdoor space as well. Excavation work is not cheap.

I think some kind of extension on the ground floor would be best, but with a separate entrance so that renting it out to someone else is possible later on.

Also, don’t underestimate the costs. Even if you say your budget is solid, questions here in the forum about basement apartments often reveal an overestimation of financial possibilities. Recently, someone reported a net income of €1,500 and wants a house for themselves plus their parents. They expect to charge below-average rent (rental income is taxable! Money paid informally is not recognized by banks as income…). Sometimes the idea is even to let them live rent-free, even though the asker’s finances would usually suggest renting, not owning a home.

The apartment will easily cost you €120,000–140,000 (about $130,000–150,000). If you really only need it for your mother-in-law, i.e., for 5, 10, or 15 years, then I’d advise against it. Unless your generosity (or budget) is large enough that you can afford to leave this addition unused once your mother-in-law needs a nursing home or is no longer there.

Thanks for the responses.
A basement apartment can be nicely designed with large windows and light wells... but I just can’t imagine it actually saving money (as our builder claims).

The apartment without stairs would obviously be the best! If the mother only had to use stairs to leave or access it, that might still be acceptable. At the moment, there are other health issues that make living alone risky. If stairs ever become impossible, there are appropriate aids for that (I’m quite familiar with these kinds of solutions).

Simply separating about a quarter of the house would of course be the easiest option, even if a second staircase is needed. But as you already mentioned, stairs inside the apartment are really difficult for an older person.

I think the best solution might be to build a “normal” single-family house and integrate the apartment partly on the ground floor and partly as a single-story extension.
Is it true that with a single-story extension/addition, each square meter is almost twice as expensive as with a two- or one-and-a-half-story build?

Maybe our builder just prefers building basements... ? Neither of us knows much about construction, so we rely heavily on good advice.

Doubling the cost doesn’t make much sense, since for the same amount of money you could build a two-story house. However, the point isn’t entirely wrong. You need a concrete slab, supply and drainage lines, foundations, soil replacement, etc. You also need a roof. Both of these are relatively expensive trades compared to walls and intermediate ceilings. The complete interior finish (electricity, heating, water, windows, wallpaper, flooring, ceilings, etc.) costs about the same. By the way, this is also true for basements. The walls just need to be much thicker (to withstand earth pressure), there’s a lot of groundwork, and you may have to waterproof against groundwater, etc. But you don’t need an additional roof. In the end, I think both options are quite similar in cost. It might even be that the basement is slightly cheaper, but then you have a floor below ground level, and that area will be larger than 45m² (480 sq ft). If you have the building footprint available and the zoning or planning permission allows it, I would probably prefer the extension option. The cheapest solution is definitely if you can somehow integrate the accessory dwelling unit into the “main” house. Then you might get away with €80,000–90,000 (around $90,000–100,000) for the accessory unit. For an extension or basement, it will probably cost significantly more.

When we first started looking into building a house, we had very different ideas. A small prefabricated home with mid-level standards that could be paid off relatively quickly. But life changes some things...

It’s clear that the granny flat will add significant extra costs, and the few dollars of rent won’t cover that. Even considering everything that can be deducted from taxes (which is why we are currently looking for an accountant).

Once the flat is built, we would consider renting it out to others rather than leaving it empty.

In order to claim the granny flat for tax purposes, you need to charge (a sufficiently high) rent to your mother-in-law. Also, only the interest portion is tax-deductible. I believe many people mistakenly think there are significant tax benefits from renting out a property. However, the rent counts as income again. I've dealt with this topic with my parents. In the end, it doesn’t bring much tax advantage when you balance it against the effort involved.

Does your mother really need a separate granny flat, or would a larger room plus a bathroom in a “separated area” be enough? In that case, it might fit into a classic house design with an additional guest toilet on the ground floor. Renting it out wouldn’t be an option then, of course.

But if she really requires her own apartment, renting nearby could be an option to consider. Personally, I also own my home so I don’t have to share it with strangers. The idea of tenants in my house wouldn’t appeal to me at all. Financially, it usually doesn’t add up anyway, as you have already realized. But everyone has to make that decision for themselves.