ᐅ Filling or backfilling for the concrete slab foundation

Hello, I’m new to the forum and we are currently busy planning our first house. For cost-benefit reasons, we decided against a basement and expect the building’s exterior dimensions to be about 11 x 9 meters (36 x 30 feet). We are still in the initial planning and decision phase and have not chosen a company yet, but we have already received some house offers.

Now to the problem:

On the plot where we want to build, there used to be a house with a basement that was demolished. As a result, there is quite a large “crater” with depths of about 1 to 1.5 meters (3 to 5 feet), although the ground is very uneven. The soil mainly consists of sand and some mineral concrete.

The question that comes to mind is: since we want to build with a slab foundation, a lot will need to be done with the hole first—backfilling, compaction, etc. I just can’t imagine what extra costs this might involve, whether it will stay within a reasonable range or quickly become a five-figure amount.

One construction company even said (they know the plot) why build without a basement if much of the soil is already removed.

We are now a bit uncertain.

Hope you can help me out a little.

Greetings from Franconia

First of all, hello to everyone in the forum.

I hope you will receive many useful tips here. I also hope that my contributions will help you at least a little.

Now to your question

I have to agree with the builder. Removing the remaining soil is really not a big deal. Someone else has asked about this before. They mentioned amounts up to 25,000 euros. Ask what it would cost in your area. It could be that this amount would almost cover half the cost of a basement.

I am someone who cannot imagine living without a basement. Probably because I am used to it. You can perfectly accommodate the heating system and washing machine in the basement, store Christmas decorations and similar items, set up a hobby room, and also have a storage room. Since it stays around 13°C (55°F) there year-round, we also keep a chest freezer down there.

If money is tight, I recommend considering whether you can do some tasks yourself that you would otherwise hire a company for. I’m thinking of wallpapering, gardening, and similar things. Or perhaps postponing the attic finishing (if planned) a bit.

I would seriously reconsider spending a lot of money just to fill in the hole instead of building a basement.

One more tip.
How is your rainwater drainage? I ask because in our area, every square meter of sealed surface connected to the sewer system costs money, so it might be worthwhile to think about a rainwater cistern right from the start.

A few years ago, I installed such a cistern myself. After about five years, it has paid off.

But that’s just a side note.

Thank you very much for all the information. Basically, you are right if the costs really amount to 25,000 euros (I hope that includes the foundation slab). Excavation and removal of soil usually make up a large part of the costs, depending on the type of soil (which, as mentioned, should mostly be sand in our case). But the cellar itself will probably add another 30,000 euros if that’s enough.

Of course, you can only say more for sure if you get a quote for what it would roughly cost.

Some work is planned as self-performed labor, especially since many construction companies do not include tasks like painting and flooring (at least in the living and sleeping areas), so the house looks cheaper overall.

Regarding rainwater, thanks again for the tip. We were already advised by the seller of the land (who will also be our new neighbor soon) to consider a soakaway pit because, as you mentioned, each square meter of sealed roof area costs a certain amount to discharge into the sewer system.

A cistern would of course be another option. This would then probably be buried inside the house or on the garden side in the ground? How many liters should such a system have, what happens when it is full, and what additional costs did it involve for you?

Best regards,
Timo12345

What just came to mind is that the neighboring lot has also been sold and construction is underway there as well, but without a basement. The ground there appears to be level. Additionally, a trench several meters long will need to be dug for the water line, which still has to be laid from the back of both properties to the respective connection rooms. As I understand it, the new neighbor would give us all the excavation material generated from his foundation slab. He is also building a standard house with an estimated floor area of 100 sqm (1,076 sq ft).

Of course, he would leave it to you because removal is expensive. However, this excavation is of no use to you as it is not load-bearing.

Not even for partial filling? I mean, it does need to be compacted as well, that’s clear.