Calculation for a single-family house with 175m² living area, basement, and double garage

Why do you need a basement there?

"Need" is always relative. I find 80m² of additional usable space practical at first. On the other hand, it is of course also a "buffer" that could potentially be converted into additional living space at a later date. Of course, this is associated with costs/effort, but the possibility would be there in the long run. To be honest, I would be willing to invest up to 80,000 EUR for this and rather make concessions elsewhere.

Everything is planned very generously. Smaller would mean more relaxed costs... I don’t mean small.

To be honest, one has to say that the dimensions of the rooms are based on our current apartment. The living/dining area here is also 50m², the kitchen with 10m² about 5m² smaller than planned, but for good reason. We both find the current kitchen too small. The office is currently about 8m² smaller than what we have in the apartment right now. And I don’t find the 20m² children’s room exaggerated at all. Sure, everything is planned generously, but I think we will "need" that. Last week we visited a show house with an eaves height of 80 cm, also 1.5 storeys with 140m². Just to get a feel. And afterward, we both said: The rooms upstairs (bedroom, child’s room, office, and bathroom) could all be at least 4-5m² bigger. And so the 160m² of living space come about.

Or (depending on the execution) make the attic usable. We have friends who have an additional room and office in the attic. Expanding the attic was certainly much cheaper than building a complete basement in addition.

We have the "problem" that we are only allowed to build a maximum of 1.5 storeys (eaves height 60 cm). So, according to the current plans, we actually only have the basement as a possible "expansion reserve."

Maybe I approached the topic of the basement too naively. The costs I had in mind so far and those you can find on the net meanwhile are (roughly) 40-60,000 EUR additional costs compared to "just" a slab foundation. I still consider that justifiable. You can’t simply add a basement underneath the house afterward.

As for the ground, the question was also asked, I will look into that.

40-60k is not enough. For our house (with a significantly smaller area to be basemented), completely problem-free soil and perfectly level ground, we ended up at 70k + significantly higher earthwork costs. Of course you have to decide for yourself, but for that money you can really, really nicely design the space under the roof or build it bigger.

Without wanting to start a basement discussion here...(is that even avoidable?) I see the costs for a basement depending on the equipment and ground conditions in the size also at 70k+

...buuut...

One must of course not forget that I need some replacement space. Fairly, I have to offset that against it. Of course, it never cancels out. Nevertheless, one must be aware that when planning a switch, one first has to cut off or add 10-15sqm (or more) of utility room/technical room above ground. Often an office or similar is also accommodated. Is the storage space under the roof enough and what do I even want to store there via a "folding staircase"? If I have a basement, don't I simply "get rid of" the space and the staircase under the roof? Do I then alternatively need a larger garage for some things (workshop or similar)?

These questions are of course partially individual. Still, if I don’t want to make compromises above ground, I might have to offset the 70k+ with maybe 30-40k opportunity costs.
This is not about whether a basement makes sense or not, only that one cannot simply leave out the basement in a calculation and save 70k+.

No, I see it the same way.

It would just be wrong to think that the basement is in the range of 40-60k, because then we probably would have built one too (even though only usable space would have been created there). We didn’t think the usable space was worth 70k or more, but that’s very individual.

But from the looks of it, the money question isn’t really an issue for the OP. Then I would probably just build with a basement as a reserve. The calculation should still be adjusted.

Of course, one must not forget that I need some replacement area for certain things. I have to fairly offset that. Naturally, that never balances out completely. Nevertheless, one must be aware that when switching plans, 10-15 sqm (or more) of utility room/HT above ground first has to be cut off or added.

In my opinion, this is another point in favor of the basement. I already consider 10-15 sqm to be too tight here. Washing machine, dryer, drying rack + actual "storage space" for boxes, Christmas decorations, beverage crates, etc.... and with 20 sqm plus X, we are quickly talking about 20 - 30 thousand euros more costs. AT LEAST. And then the price of the basement is relativized a bit more. Then we are talking about "only" 50 thousand euros effectively more (80 thousand euros basement minus 30 thousand euros saved area on ground/floor) and the fact that long-term additional expandable space is available.

As I said, I will push forward the (concrete) investigations regarding the plot/basement here in order to possibly get a reliable statement about the costs in advance.

I would calculate 80k for an 80sqm utility basement (at a normal prefab height of 2.5m, if you potentially want it to be expandable) + additional earthworks costs and disposal of quickly around 20-30k. If the heating is to be placed down there, to my knowledge, it must not be outside the thermal envelope either, so the basement costs will probably potentially increase.

And then you are rather in the range of 80-90k more than just on the ground slab. For that, you would get about 40 sqm of fully usable living space above ground.

For 40-60k you will never get a basement. Don't forget the earthworks and disposal costs in the calculation. A lot more is added to the pure basement price.