Rainwater infiltration

The thread actually starts with the statement that according to the soil survey, infiltration is not possible. A retention shaft certainly helps, but wouldn't the soil survey then have suggested infiltration with pre-installed retention right away? Are there any fellow sufferers (soil not suitable for infiltration, but infiltration installed)?

We also currently have the problem that the development plan states that rainwater should be allowed to infiltrate on the property, which according to the soil report is not possible. My architect is now requesting the costs for an infiltration system. I am curious to see how much this will cost. Even if you always plan with a buffer, it is money that will ultimately be missing somewhere else.

What I would generally be interested in:

What on earth is this kind of "policy" that generally prohibits the introduction of rainwater into the sewer system, but at the same time complains that the sewer system cannot be flushed enough because people pay attention to low water consumption?

I can understand that municipalities do not allow general introduction, as with the increasing number of construction areas, the sewer system might quickly become overloaded. = higher effort for maintenance and sewage treatment plant
However, if one allows an overflow, on the one hand, you would not have to handle the massive amounts of water, but on the other hand, in the event of heavy rain, there would still be a helpful "flushing" of the sewer system.
Furthermore, I fear that on properties where the soil survey says that infiltration is not possible, with frequent heavy rain (like this year), even a soakaway reaches its limits and cannot absorb the rainwater. Then the owner has, in the worst case, two problems at once – 1. a flooded property and 2. the municipality admonishing him that the rainwater must infiltrate on his property.

I wonder who is actually liable if water rushes over one's own property, for example into public space. Or then continues to the neighbor and causes damage there. Am I as the property owner responsible if water rushes over my area?

Why the rainwater should not be discharged ... maybe someone from the experts will comment on that, but I want to speculate. Because as far as I know, discharging rainwater is comparatively expensive. You have to maintain sewage treatment plants and pumps with extreme peak loads to dispose of rainwater. Wastewater from households is quite predictable and exposed to few extreme situations. So you constantly maintain excess capacities that are rarely used.
In addition, there are also separate systems, so rainwater and wastewater are led separately in the sewer. The wastewater sewer therefore has a smaller cross-section and is thus cheaper. If I also save the rainwater sewer when developing, I've done the city treasury a favor again.

... What kind of "policy" is this that generally forbids the discharge of rainwater into the sewage system generally, but at the same time complains that the sewage system cannot be flushed enough because people pay attention to low water consumption?

I can understand that municipalities do not allow general discharge because with the increasing number of new developments the sewage system may quickly become overloaded. = higher maintenance effort and sewage treatment plant cost
However, if an overflow is allowed, on the one hand you would not have to deal with the huge volumes of water, but on the other hand during heavy rain you would still have a helpful "flushing" of the sewage system ...

It is actually quite simple.
During heavy rain events, or when the treatment plant is overloaded, the wastewater is "only" mechanically pretreated and diluted wastewater is discharged into the body of water.
This lowers the water quality of the body of water, which is prohibited by the river basin communities because otherwise the EU would penalize them.

Rainwater, however, can be discharged almost directly because it usually "only" transports manure from the farmland.
This is also the reason why more and more municipalities are switching to a separate system.
To attenuate and discharge a multiple of wastewater during precipitation compared to dry weather flows, storm overflow basins or storage sewers are often constructed, and homeowners are forced to install rainwater cisterns.

I would refuse to install an infiltration system. What's the point of doing soil surveys if the findings are ultimately just ignored? In the end, you are the one who suffers if your garden turns into a swampy area. But this is often the problem with new housing developments. Municipalities/towns have difficulties connecting completely new areas to the existing system because capacities are limited or because of height issues. Then a soil survey is conducted, which is interpreted in such a way that the water might possibly be allowed to infiltrate, and suddenly it is included in the building regulations. Problem solved, and when after development and at the start of construction detailed surveys are made by the builders and the soil does not actually permit infiltration, the initial surveyor is suddenly no longer reachable. I've experienced this several times.

Usually, a storage trench combined with a cistern is used to introduce the water with a delay. Part of the cistern is used as an "intermediate storage" to then slowly feed the water into the storage trench via a throttle. This, of course, reduces the usable volume of the cistern, or the cistern must be enlarged by the retention volume. Typically, this is in the range of 1.5–3 m³, depending on the roof area, size of the storage trench, and soil conditions. For me, regardless of the costs, this would still not be a solution because no one guarantees that sufficient water can infiltrate, or the soil survey even contradicts this. Of course, it’s also useless if you get rid of your own water "somehow" differently and you get washed away by the infiltrated water of the neighbors; but at least in that case, you can still take legal action.

If a sewer connection is technically not feasible, it naturally becomes difficult because the municipality/town will then block you, and you will be left alone with the problem. Then, besides looking for another building plot, the only option is to let the water infiltrate and hope that nothing happens. However, with groundwater at 3.5 m, the question also arises how deep a storage trench may be so that the infiltrated water does not go directly into the groundwater.