ᐅ Preparing the Site for a Concrete Slab on a Gentle Slope

Hello everyone,
we are planning to purchase a plot of land with a slight (?) slope.
As shown in the attachment, the plot is between 24m and 37m deep and 16m wide. However, 3m of the plot’s width is restricted by a public easement for infrastructure, so no changes should be made in that area.
I have marked the possible building area and also a preferred floor plan (dashed line) on the plan.
What we are wondering now is how much effort it would take to build on this plot with a slab foundation, that is, without a basement. As you can see, within the preferred building area there is a height difference of up to 80cm (31.5 inches). In addition, there is a height difference of about 60cm (23.6 inches) from the edge of the desired house position to the easement area. The distance between the desired house position and the easement is approximately 1.6m (5.25 feet).
A soil report is currently in progress, but generally, clay soils are predominant in our region.
We are now wondering to what extent the plot is buildable as we envision it, and what measures might be necessary (e.g., supporting the ground towards the easement).
We are also willing to consult a specialist but are unsure who the right contact would be.

Best regards and thanks in advance
Perigrin

Basically, this is possible. You just have to expect that the house will extend quite a bit at the lower corner, and that on top of the concrete slab there will be about 15cm (6 inches) of screed and flooring. So, the finished floor level will be significantly higher than you might initially expect.

If I take the 87.7m (288 feet) as a reference for the ground floor height, then you have about a 1.5m (5 feet) difference in elevation over 1.6m (5.25 feet) at the driveway. That would roughly correspond to a 45-degree slope. This situation would definitely justify a retaining wall since the house sits directly on it. With that difference, it might also be worth considering raising the ground floor a bit and building a basement underneath, as this location would allow for sufficient window area even without a light well. A study, hobby room, or similar spaces could be well accommodated there.

Thank you for your response.
Unfortunately, we cannot afford a basement financially.

Are you referring to a wall that only supports the slope or the house? I imagine a wall that supports just the slope would be significantly more affordable, and above all, we could still build it afterward and decide whether we want to raise the ground level up to the height of the ground floor.

Peregrin T. schrieb:

We are planning to buy a plot with a slight (?) slope. [...] As you can see, within the preferred building zone there is a height difference of up to 80cm (31 inches).

I would still consider that a "slight" slope when viewed on its own, and according to 11ant’s basement rule, it would correspond to an expected 40% increase in basement costs.

Peregrin T. schrieb:

Unfortunately, a basement is not financially feasible for us.

If that is the case – which we have not been able to confirm so far – then that raises a big red flag regarding your building plans. If your budget is strictly unable to tolerate any complications, that will limit you to a very small ideal plot. You might be better off considering a package deal from a developer.

Peregrin T. schrieb:

However, 3m (10 feet) of the width of the plot is "blocked" due to a development easement, so no changes should be made in that area.

That would require some clarification: An easement in favor of whom / which development? Presumably, the requested changes mean that the terrain there should remain unchanged (?)

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https://www.linkedin.com/company/bauen-jetzt/

11ant schrieb:

... and according to the 11ant basement rule, this corresponds to an expected 40% basement cost.

How exactly should I understand that?

11ant schrieb:

If that is the case – which we haven’t been able to verify so far – then there is a serious red flag regarding your building plans. If your budget is categorically intolerant of complications, this would basically limit you to a very small ideal plot. You would probably be better off with a developer’s offer in that case.

I should probably avoid being too biased here; a basement is not categorically excluded. The budget includes a buffer in several areas, which we want to maintain for as long as possible. We expect total costs for the house including the foundation slab of up to approximately €400,000. If we build smaller with a basement, that would likely reduce the cost increase somewhat. We have calculated €35,000 to €40,000 to prepare the land to a buildable state with a foundation slab.

11ant schrieb:

That would require further explanation: building burden in favor of whom / which utility connection? The changes probably mean leaving the terrain there unchanged (?)

The building burden is for a paved path that leads to the property behind. Since it needs to be drivable, we can’t simply cover it with earth, for example.

Peregrin T. schrieb:

How exactly should I understand this?

My rule of thumb (which surprisingly often applies accurately, even to myself) is that a basement, where it is considered necessary based on the property’s conditions,
1. generally incurs similar avoidance costs as construction costs at the same height, and
2. this principle is approximately valid proportionally, following the formula: "10% per 20 cm (8 inches) of elevation difference within the building plot."
You are referring to 80 cm (31.5 inches), so according to the formula, for earthworks, "L-shaped retaining walls" and similar, you would account for about 40% of the cost of a built basement even if the basement itself is not constructed.

Peregrin T. schrieb:

Here I probably shouldn't be too biased; a basement is not categorically excluded. [...] If we build smaller with a basement, that would likely reduce the cost increase somewhat.

However, in this case, the potential to shift usable space into the basement is limited. You will probably only relocate the above-ground basement replacement area but won’t be able to create economically viable living space in the basement.

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https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/