Septic tank or white tank? What were you recommended? Why?

In our municipality, unfortunately, one is no longer allowed to connect a drainage directly to the wastewater system, only with a retention option. A house provider therefore says that we absolutely need the "Weisse Wanne" because of backflowing water. The seller of another construction company says this is not necessary; we simply make a soakaway pit between the drainage and the sewer connection. So that the water from the drainage runs through the soakaway pit into the sewer when the soakaway pit is full.

So who is right? A WU basement costs about €8,000-12,000 more. Can one save that?
Or would that be too unsafe?
I actually only see a problem in the case that the water does not come from above but from the sewer, for example, during heavy rain. But of course, I do not know whether that happens.

As long as both only guess, neither of them is right. What is necessary depends primarily on the infiltration capacity of the soil and the level of the groundwater, in other words: whether there is permanent or occasional, pressing water.

You find this out with a geotechnical report, and the geologist also gives recommendations on the type of waterproofing.

However, there are now quite a few construction companies that only build [weiße Wanne]. Quote: "No geologist gives a 100% guarantee in writing for never rising water, and we have no desire to argue if it should ever get damp."

For us, for example, a simple waterproofing plus drainage directly into the sewer would be sufficient according to the report. If it does not mean a financial disaster, we will still have [weiße Wanne] built.

Constructing a rainwater retention system is likely to be significantly cheaper than letting the water infiltrate and thus creating a load case on the building and having to take corresponding countermeasures there. Whether that is actually the case, see Fuchur, your geologist can tell you.

An infiltration shaft with overflow is certainly cheap to manufacture, but it may also have an impact on the building, and at least here the infiltration shaft is often not approved.

What about a cistern with overflow? That is "really" only retention and the water can be used for the garden with a pump. I am of the opinion that cisterns are not economical, but if you have to build something for retention anyway, then a cistern is probably the best option. At least you get something back in the form of saved tap water.

So the soakaway pit is actually approvable, the company has already gotten it approved like this on our street.

The thing with the cistern or well is actually a good idea. I have to find out what that costs.

The soil survey is still pending. But it is known that the soil here mainly consists of clay, which hardly absorbs any water. In my assessment, the soakaway pit only has a symbolic character for the approval process and no technical added value. If the water does not seep away in it and the thing is permanently full, the drainage simply flows into the sewer.

That's why I actually think we don't need the white tank. The only question is how safe that is. Or whether it might still make sense to build a waterproof basement in case the sewer system ever backs up and the water accumulates.

No question about it, never save on that. Definitely [Weisse Wanne]!