When is a slope a slope? Basement vs. slab

Hello everyone,

At the moment, my family and I are looking for a plot of land. In a potential new development area, where the sale of plots will soon begin, I have discovered a few spots (plots have not yet been officially surveyed) that we like very much. However, all these spots have a more or less significant slope. Attached is a screenshot from the development plan, which contains some elevation details; I have not yet received any documents concerning the soil conditions.

In the upper part, you can see that the plots will have a slope of about 4m over 20m; the further down you look, the less the slope is.

We actually want to build without a basement on a slab foundation (for cost reasons). This raises the following questions, to the answers of which I would be very grateful:

- Is this possible in the upper area with reasonable additional costs compared to a flat plot?
- In the lower area?
- So that I don’t have to open the same thread with other plots next week, how much slope can a plot have before the additional costs for a slab foundation become excessive and building a basement should be considered?
- Can the additional costs already be roughly defined (e.g., can we assume clay soil with a clear conscience)?
- Would building a basement, possibly open/semi-open into the slope, cause significant additional costs compared to a basement on flat land?
- Should I have a soil survey carried out before the purchase, I assume? Is a survey from nearby insufficient?

Thank you very much in advance!

Bo

In the upper part, you can see that the plots will have a slope of about 4m over 20m,

Assuming roughly uniformity of this slope, and a house depth of about 8 to 12 m, that would result in a height difference between the street side and the garden side of 1.60 to 2.40 m. In the event of heavy rain, you wouldn't want the front door to be below ground level, nor to have to lift wastewater into the sewer with great effort. So you will likely go rather more than less below that mentioned height from your floor slab (since the ground floor floor will be at least at the upper level). Building no usable space between these walls would be almost a waste. Conversely, this slope already provides a quite nice above-ground height on the valley side, making the basement usable for living or sleeping rooms. So the fact that one will have to be there, like every coin, has two sides.

About 6 m over a 20 meter height difference is a lot. Without a basement and retaining walls, you will not get by. A slab alone is not enough, you have to support the slope. So you easily need an extra 30,000k if not even more.

With us, there is a 3 meter difference over about 22 meters and we had to work with 4 meter long L stones on the front door side. But we deliberately built with a basement so it wouldn't become a dark hole.

If you are watching costs: As level as possible. As little slope as possible. I'll give you an example: Our ground: Topsoil, underneath clayey and gravelly. Our plot: 30 m deep at the widest point, 24 m at the narrowest. Slope from street level to the rear post, 1.2 m. Slope in the house area, house 10.5 m wide, - 1 m, house 5 m behind the street boundary. The floor slab laid so that at the front we are 0.5 m out, at the back 0.5 m into the land. Averaged out. Additional earthwork costs about 6500 euros. The conclusion: With your slope specification, there will be significantly more filling or digging. So build as level as possible. Karsten

A floor slab alone is not enough; you have to retain the slope. So you easily need an extra 30,000k if not even more.

Apart from the fact that for such an (also cost-related) effort "under the house" I would also want to have space in return: I would not even think, in the literal sense, to rely on a retaining wall to keep a half-slope under my floor slab in check and thus the floor slab "in water." Slope property and slab-on-grade house (or vice versa) do not go together. On the described property, I only see the way to build with a basement; or conversely, a slab-on-grade house only on another property. In my opinion, any effort to manage without a basement on this property will at least not pay off cost-wise. That would be a Pyrrhic victory.

Hello everyone, thank you so far. I realize that this plot with its 20% slope can only be reasonably built with a basement. The note about the ground level is also interesting, I hadn’t considered that before. By the way, I’m not completely against a basement – on the contrary, I would like one on the hillside – but I am somewhat hesitant because, according to some acquaintances, the biggest financial risks lie there.

Can someone tell me roughly (really just very roughly and generously) how much it would cost to build a residential basement there? We can assume clay soil. I’m interested in all costs excluding development so that I can continue calculating with a price for the top edge of the floor slab / basement afterwards.

By the way, if 20% slope is too much for a floor slab, how much is allowed? 10%? 5%? 5% but only with an incline from the street due to ground level?

:
If I understand you correctly, you have a slope of about 0.8 m over 10.5 m, which according to my smart calculator corresponds to about 7.6%. Is that right or did I misunderstand you?

Additional costs of €6,500 are of course always annoying, but I personally consider that still acceptable (compared to a basement). But I assume the additional costs were not shown during the planning either? May I ask what total price you came to for the floor slab?

Best regards

Bo