ᐅ Septic Tank or Watertight Concrete Basement? What Was Recommended to You and Why?

In our municipality, it is no longer allowed to connect a drainage system directly to the sewage system without a retention option. A home builder is therefore insisting that we absolutely need a waterproof concrete basement (“white tank”) because of potential groundwater backflow. However, a salesperson from another construction company says this is unnecessary; they simply install a soakaway pit between the drainage system and the sewer connection. This way, water from the drainage enters the sewer through the soakaway pit when it becomes full.

So, who is right? A waterproof concrete basement costs roughly €8,000–12,000 more. Is it possible to save this expense? Or would that be too risky?

I see a potential problem mainly if the water doesn’t come from above but from the sewer system itself, for example during heavy rainfall. But whether that actually happens, I don’t know.

As long as both sides are just guessing, neither is right. What is necessary mainly depends on the soil’s infiltration capacity and the groundwater level—in other words, whether there is permanent or temporary, pressurized water.

You can find this out through a geotechnical report, in which the geologist also provides recommendations regarding the type of waterproofing.

However, nowadays quite a few construction companies build exclusively with a "white tank" (watertight concrete structure). Quote: "No geologist will provide a 100% guarantee in writing that water will never accumulate, and we have no interest in arguments if it does end up becoming damp."

In our case, according to the report, a simple waterproofing plus drainage directly into the sewer would be sufficient. If it doesn't cause a significant financial burden, however, we will still have a white tank installed.

Constructing a rainwater retention system is likely to be significantly cheaper than allowing the water to infiltrate the ground, which creates a load on the building and requires corresponding countermeasures. Whether this is actually the case, as mentioned by Fuchur, your geologist can advise you.

A soakaway with an overflow is certainly inexpensive to build, but it might also affect the building, and at least in some cases, soakaways are often not approved by the authorities.

What about a cistern with an overflow? That is truly just retention, and the water can be used for the garden with a pump. In my opinion, cisterns are not economical, but if you need to build something for retention anyway, then a cistern is probably the best choice. At least you get some return in the form of saved tap water.

The soakaway pit is also likely to be approved; the company has already obtained approval for this in our street.

The idea of a cistern or well is actually a good one. I need to find out what the costs would be.

The soil survey results are still pending. However, it is known that the area mainly consists of clay soil, on which almost no water infiltrates. In my opinion, the soakaway pit is mostly symbolic for the approval process and doesn’t provide any real technical benefit. If the water does not soak away and the pit stays permanently full, the drainage will simply run into the sewer.

Therefore, I actually believe we don’t need a waterproof concrete shell (basement box). The question is how safe that is, or whether it makes sense to build a waterproof basement just in case the sewer system backs up and water accumulates.

No question, never skimp on that. Definitely a waterproof concrete tank!