ᐅ Costs of Raft Foundations – Experiences?

Created on: 10 Feb 2023 20:10
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Narnulf1368
Hello everyone,

We are currently planning our house, and according to the soil report, it looks like we will need a well foundation. Initial research shows this could add at least €10,000 (around $11,000) in additional costs.

Has anyone had experience with this? Especially the pricing would be interesting.

We plan for a 120 m² (1,290 sq ft) footprint plus 60 m² (645 sq ft). For the garage, we have a quote of €15,000 (about $16,500) for the related concrete slab and another €15,000 for the garage itself (excluding the door).

According to the report, a slab foundation would be possible if we include a basement. Now I’m considering possibly building a basement to avoid the well foundation.

The idea is, instead of paying €30,000 (around $33,000) for the garage plus at least €10,000 for the well foundation, to invest the money in a basement. I have requested a quote for that and expect it early next week. But I wanted to ask here beforehand if this is a completely dumb idea.

Good luck and thanks in advance!
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Narnulf1368
11 Feb 2023 21:09
Update:

Here is the geotechnical survey report.

It states the foundation type as follows: in the basement section of the building, a raft foundation on an elastically supported slab; in the non-basement section, shallow foundations on pad foundations or drilled piles. Recommended: combined foundation
Geology table with layers 1–4, soil groups and depth information.

Technical geology page with text on groundwater, foundation and protective measures.

Document with DIN standard on capillary break layer, slopes and trenches

Document page with headings on reuse of excavated soil and topsoil (layer 1).

Document page on geotechnical properties with layers 1–3 and key values.

Technical page with tables on soil properties (layers 3 and 4).

Technical construction documents page with sections on waterproofing, water management, frost, seismic activity.
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xMisterDx
13 Feb 2023 20:31
Well, it also states that infiltration is hardly possible in layers 2 and 4. This means your basement will be submerged in water for a significant part of the year.
You definitely won’t get that basement for 60,000 EUR; it will cost six figures.
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Narnulf1368
13 Feb 2023 20:40
xMisterDx schrieb:

Well, it also says that infiltration is hardly possible in layers 2 and 4. That means your basement will be submerged almost completely in water for part of the year.
You definitely won’t get that basement for 60,000 EUR, it will be a six-figure amount.

I was hoping to receive the offer today; as soon as I have it, I will report back.

In the meantime, I am also trying to get quotes for a well foundation.
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Cronos86
13 Feb 2023 22:01
Besides the fact that the report is already several years old, based on the soil layers I don’t initially see any real need for a deep foundation. To make a final assessment, I would of course need more information about the plot, or from the soil report. DPL and borehole profiles would be helpful.

I would suggest getting a new report done that is specifically tailored to your house.

Best regards, Tobi
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Narnulf1368
14 Feb 2023 08:31
Cronos86 schrieb:

Aside from the fact that the report is already several years old, based on the soil layers alone I don’t see a real need for a deep foundation.
To make a final assessment, of course I would need more information about the site, or from the soil report. Drill logs and borehole profiles would be helpful.

I would consider having a new report done, tailored specifically to your house.

Regards, Tobi


Hello Tobi,

thank you for your reply!

Here are the remaining pages of the geotechnical soil report. Is this enough to assess the situation, or do you need anything else?

We will commission another report once we have the structural calculation for the house. But as I understand it, it makes the most sense to know exactly what kind of house and where it will be built first.
We are just trying to roughly estimate how high the costs for a shallow foundation could be and to compare that with the costs of a basement. We have to decide that already when submitting the building permit / planning permission application.
Layer directory: tabular soil report with drilling KRB 1.

Geological report: layer directory with tables for soil investigation.

Layer directory - soil investigation with tables and drilling data (KRB 2.1)

Scan of a soil investigation form: tables with soil types, layers and measurement parameters.

Soil investigation sheet with depth profile, diagram and tables, handwritten notes in green.

Geotechnical soil profile with depth list, penetration depth graph and marked topsoil

Geotechnical borehole data: depth list and layer diagram

Technical soil profile sketch with depth information, table on left, diagram with notes on right.

Site plan of a building plot with green marked boundary, building outline and measurement points.
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hanse987
14 Feb 2023 08:43
Narnulf1368 schrieb:

We will commission another expert report as soon as we have the structural engineering calculation for the house.
Why only after the structural calculation and not before?