Looking for information on basements with a white tank

We want to build a utility basement with light wells (approx. 1-1.5m deep for windows) under a prefabricated house.
The soil report for the plot itself is not yet available (-> commissioned), but for the entire new development area there is one, and there were also cone penetration tests almost directly next to the plot. (See file)
Due to occasionally pressing water, a watertight concrete shell ("white tank") must be constructed.

Without a white tank, we have so far received offers from prefabricated house suppliers for a size of approx. 9x10m with prices including foundation, 4 light wells, and 2 windows ranging from €70-80k
and earthworks costing €20-25k.

We have several questions:
1. How exactly does a light well in a white tank with drainage option work (we have not found a photo) and what additional costs must be expected here?
2. What additional costs are approximately to be expected for the above-mentioned basement if built as a white tank?
3. At neighboring plots, certain earthworks have already been planned where gravel or similar has to be added if building without a basement. Can it already be seen from the attached file that this is no longer necessary if building a basement, since the foundation comes very close to the firmer soil?

I look forward to all information and inquiries.

Best regards


"6.2.6 General assessment of moisture protection According to DIN 18533 or the execution provisions of the German Committee for Reinforced Concrete (DAfStB guideline Watertight Structures made of Concrete) the load case of soil moisture and non-pressing water may only be applied if a permeability coefficient of kf 10-4 m/s can be assumed for the standing soil. For the clayey soils likely present in the area of the working slab, it must be expected that this requirement will largely not be met. The following options for moisture protection are therefore considered: 1. External waterproofing according to DIN 18533 based on water exposure class W2.1-E (moderate exposure to pressing water) 2. Execution of all earth-contacting structural parts according to the DAfStB guideline Watertight Structures made of Concrete. Design is based on stress class 1. It applies to pressing, non-pressing, and temporarily ponding water. All structural joints and penetrations must be made watertight with coordinated systems (joint tapes, joint sheets, injection hoses, etc.)."

Hello, this is my first post here and I hope it is the right category and the right information on my part. We want to build a utility basement with light wells (about 1-1.5m deep for windows) under a prefab house. The soil report for the property itself is not yet available [...] Due to occasionally pressing water, it would be necessary to build with a waterproof concrete shell.

This is not only your first post, but you have only been here for two hours. You should first take the time to read around here. Afterwards, you will probably take a few steps back because you will realize that, measured against your knowledge, you have planned far too much already. A utility basement with light wells sounds like quite nonsense, because house connections, supplies, and junk do not demand a view. Conversely, room uses that require a view are no longer utility basement rooms. You only tell about qualitative soil conditions regarding your property, but nothing about the aspect of terrain heights – and these are at least equally important. I can well imagine, therefore, that your property does not even speak in favor of a basement; and the further question of how to keep it dry accordingly does not arise. If your soil survey is still pending, you cannot reasonably proceed to design phase 2 with an architect, ergo you have not yet made the decision about the construction method on a factual basis. So what do you want now already with the decision for a "prefab" house? Just searching the forum with the term "prefab house" alone will give you several days of reading material that will bring you back down to the ground of your still insufficient knowledge. So better replace myths & co. with insights first before you let your planning dreams soar high (unless you want to pay a lot of learning money later on the bill).

1. How exactly does a light well in a white tank with drainage option work

Definitely not with drainage, with pressing water it runs backward and fills your light wells, and without submarine windows also the basement. If the level is below the sewer, this means a lifting station.

The light wells of the usual suspects can be installed watertight against groundwater pressure – the additional effort is minimal (perhaps 1 hour of extra work per light well, if at all), but they must be mounted directly on the WU concrete, which is not so good thermally (provided your basement is to be included in the thermal envelope). There are also solutions to realize this completely free of thermal bridges; however, this is somewhat more expensive because special prefabricated insulation elements must then be used as the mounting base.

I see no problems here with drainage – the light well itself is "watertight" from below, left, and right and is simply connected watertight to your drainage system as well. Normally, no drainage is done with WU basements, which certainly results in additional costs compared to the offers mentioned above.

It is complicated if the light wells and basement windows must be floodproof. However, this requires either expecting water from above (flowing into the light well) or backwater from the drainage system – for the latter problem, a backwater valve can be installed at the drainage connection of the well (which, of course, quickly becomes problematic with prolonged rainfall...).

Additional costs for WU execution are actually manageable – did they offer you prefabricated elements? If so, then WU execution should actually be included in the price. Additional costs arise only compared to masonry basements, which, I think, no prefabricated house manufacturer will offer.

However, 20-25 thousand euros for earthworks is a very optimistic estimate. You really have to be careful there. Did they also offer disposal, etc.?

We want to build a utility basement with light wells (about 1-1.5m deep for windows) under a prefabricated house.

I would first consider whether it might be better to build the house on top of the basement. Otherwise, wait for the soil survey first. If the basement is absolutely necessary, then your planner should know what he is doing. The spoiler should be especially observed!

Here is another picture of LS with drainage