ᐅ Filling or backfilling for the concrete slab foundation

Timo12345 schrieb:
I also believe that even if we have to add fill material, a slab-on-grade foundation will be far less expensive than a full basement.

You won’t achieve cost parity. That would mean that refilling the ground costs the same as building and waterproofing a basement. That’s obviously not realistic. However, the basement as an alternative to filling might not seem as expensive as commonly assumed (“€50,000 for costly storage space”).
Approximate cost parity can (nearly) occur if above-ground living space can be saved, for example by downsizing large utility or equipment rooms, or by relocating an office to the basement.

Gartenfreund schrieb:
So that adds up to about €1300. But all done by yourself.

With €1300 you can water your garden here for 30 years. The prerequisite is a water meter on the outdoor water connection so that only fresh water—not sewage water—is charged. This is not allowed everywhere and should be clarified with the local authority.
Rainwater tanks also don’t stay full all the time, so municipal water usually will still be needed. They may require occasional maintenance and will have to be replaced eventually.
€1300 really seems cheap, probably due to the DIY efforts. Others have reported significantly higher figures. The return on investment might only be realized after the lifetime of the current young homeowner—or rather never—since the rainwater tank will need replacement or at least servicing before then.

Nordlys schrieb:
Backfilling can’t be that bad.

I think it largely depends on the geotechnical report and the condition of the existing fill material. Does it need complete removal and disposal (expensive; definitely consider basement construction), can it possibly be excavated and compacted in layers and reused (somewhat less expensive), or can it simply be further filled and compacted to support a slab-on-grade foundation?

Thanks already for all the help.

On Saturday, by chance, the earthmoving contractor for the neighboring property was there, and they have already moved quite a bit of soil on that site. (They also demolished the old house, which was basically responsible for the crater.)
They were fairly optimistic that, due to a slight slope from the back of the garden upwards, the ground would still need some leveling, and the soil could apparently be reused well, along with the excavation material the neighbors will have left over (they will receive their soil report in June).
This means we probably won’t need that much additional material after all. We’ll see, but for now, we can definitely approach the issue of extra costs a bit more calmly.

Perhaps first a strip footing with a vertical wall, then a slab-on-grade on top.

Under the house, before pouring the concrete, I would fill with sand.

Alex85 schrieb:
For €1300, I can irrigate my garden here for 30 years. The prerequisite is a water meter on the outdoor water connection, so that only fresh water is billed and not wastewater.

Yes, Alex, you are right. However, the cistern was installed solely to avoid the rainwater fee. So, it basically pays for itself here after about 5 years.

Nordlys schrieb:
Filling sand costs 13 per ton. Concrete recycling 20 per ton. One hour of excavator work 80,- All plus tax. So backfilling shouldn't be that expensive. Karsten

Then your sand is very expensive. Where I am, a ton costs around 5.50 €.

Gartenfreund schrieb:
Hello Timo,

So you’ve already received quite a bit of information about the basement. I will step back from further discussions about the basement since I’m not very experienced in that area, and I’ve already shared my opinion with you.

Regarding the cistern, here’s what I can tell you.
I installed a plastic tank buried underground with a volume of 4 m³ (4,000 liters / about 1,057 US gallons) according to the manufacturer. I once filled it using a non-calibrated water meter and noticed that the meter registered significantly less than 4,000 liters. Unfortunately, I don’t have the exact difference anymore. However, it was a noticeable discrepancy. I can’t say whether this means the tank’s actual capacity is smaller than stated or if the meter was inaccurate. I wouldn’t be surprised if manufacturers quote the external volume rather than the internal capacity as the tank size, but that is just a guess.

As mentioned above, this tank is buried. There are shallow tanks and deep tanks (I have a deep one). Shallow tanks are more expensive but require a less deep excavation. The downside is that any remaining water might freeze in winter. So, a tank buried deeper is preferable.

I retrofitted an existing property accordingly. This means all the roofs and the driveway were connected to the tank, and the existing connections to the sewer were decommissioned.
In detail, I completely rewired all the pipes but left the old ones in place—just in case the municipality changes its mind and demands fees again, I can easily revert the system, and then the municipality can deal with the rainwater.

What I am about to say applies to my situation based on my understanding.

When the tank is full, one option is to channel the excess water into the sewer, but this incurs fees again.

Another option is disposal via an infiltration basin (soakaway). For this, I would have needed permission from the local water authority (lower water authority), which would have cost about €100. On top of that, there would have been the cost of the infiltration unit.

This was a bit too expensive for me, plus I didn’t know where to place the infiltration unit in my garden, as there are many trees.

My alternative:
Using a simple sewage pump to ensure the water level stays below a maximum value, pumping any excess water into a wastewater pipe that I laid above ground in the garden. I cut several cross-slits in the pipe so that by turning it, I can somewhat control where more or less water will discharge if necessary.
In terms of appearance, you can more or less hide this by planting appropriate vegetation.

Regarding the tank size, check terms like “rainwater tank calculation” or similar. You can find guidelines on the size you might need. Be careful, though—I found that different websites can give quite different estimates based on the same data.
I can only say, my tank was partially emptied multiple times by the pump in spring 2016, and I could have really used this water in summer. So, it’s better to build about 5 m³ (5,000 liters / about 1,320 US gallons) more capacity than 1 m³ (1,000 liters / about 264 US gallons) less, to put it simply.

I can’t give exact material costs, but roughly:

Tank €750
Pipes, bends, etc. €100
Two pumps at €40 each €80
Water level switch €50
Drainage channel €250 (cheaper options are possible)
Miscellaneous €100

That adds up to about €1,300, all done as DIY work.

Ultimately, you will have to calculate what applies in your case.

Sorry this got a bit long, but I hope I have answered your questions about the cistern as well as possible.

If you decide on a cistern, then definitely choose a concrete one.
Regarding the capacity, supposedly about 50 liters (about 13 US gallons) per square meter of roof area is a good guideline.
You can’t just say the bigger, the better. It’s important to have a regular “exchange” of water (not just when cleaning) so that water doesn’t stagnate in the tank for years.
At least that was the advice from our (known to us) civil engineer and a neighbor who works at the water utility.