Floor slab with strip foundation on topsoil?

Hello everyone, first of all, thank you for reading my question. My partner and I are currently having a house construction company calculate a quote for a 155m2 city villa. For the calculation of the foundation work, we submitted a soil report which shows that about 50-60cm of topsoil lies above the load-bearing soil. Today, the company called me and offered two options:

Option 1: - Remove the topsoil and store it on the property - Fill the pit with filler sand and compact it in layers - Excavate trenches for strip foundations about 80 cm deep and pour them - Pour the slab on the strip foundations - Dispose of the excavation or spread it on the remaining property

Option 2: - Only remove the turf - Excavate trenches for strip foundations about 80 cm deep and pour them - Pour a load-bearing slab (according to the statement significantly thicker and with more steel than in option 1)

Option 2 is supposed to be about 2.5 thousand euros cheaper than option 1 + the lower cost for the disposal of the excavation because significantly less material is removed.

Has anyone worked with such a foundation or has experience with it? I have never heard of building a foundation on topsoil. On the other hand, the weight of the slab does not rest on the topsoil but on the strip foundations which reach into the load-bearing soil layer. In the end, I do not want to risk damage to the entire house for a few thousand euros in savings.

Unfortunately, Google could not help me. Here I always only find the option of strip foundation with non-load-bearing slab or the load-bearing slab alone. Both, of course, on load-bearing soil.

Thank you very much!

Hi,

you basically already wrote it yourself. Saving about €2500 on the foundation with construction costs of >€500,000 doesn’t sound smart. I would prefer option 1, so the slab lies everywhere and permanently (organic matter can disappear) on solid ground. If moisture should ever get under the slab, sand is also better than topsoil.