1. First the terms: what is what?
Four terms are regularly confused. The distinction determines what you actually need:
The four terms
- 1Soakaway (old usage): colloquial for a pit in which water "disappears". The term conflates two different things — foul-water soakaways (no longer permitted for domestic sewage today; see below) and rainwater infiltration.
- 2Infiltration shaft, swale, soakaway trench: the modern, dimensioned types of rainwater infiltration. Which one fits follows from the calculation — not from habit.
- 3Geotechnical / soil report: prepared by a geotechnical engineer; provides, among other things, the kf-value (how well your soil lets water infiltrate).
- 4Infiltration verification: the calculation built on that per DWA-A 138-1 — which system, how large, with which rainfall data (KOSTRA-DWD). That is the document the authority wants to see.
2. When does infiltration require a permit?
In NRW, rainwater must be infiltrated locally, trickle-discharged, or routed into a water body without mixing with foul water, unless regulations or water-management concerns prevent this (§ 55 (2) WHG, concretised by § 44 LWG NRW).
At the same time, infiltration is in many cases a permit-requiring use of a water body: the use of a water body generally requires a permit or authorisation (§ 8 WHG). Whether your project falls under this depends on the individual case — location, area, subsoil, groundwater clearance, and the practice of the local water authority. The authority decides on the basis of the verification.
In practice this means: the soakaway is not the problem — the missing reviewable document is. Whoever infiltrates "just like that" or additionally seals a surface often learns of the permit requirement only through the authority.
3. Important: rainwater is not foul water
Different rules apply to domestic foul water than to rainwater. Permeable "soakaways" in which sewage seeps untreated into the subsoil do not meet the generally accepted rules of technology (§ 60 (1) WHG) and are not permitted for new installations. Where there is no sewer connection, sealed collection tanks or (small) treatment plants are the permitted route.
For existing legacy installations, time-limited adaptation or refurbishment rules may apply (§ 60 (2) WHG). What applies in your case varies by region — the responsible authority sets it.
4. The typical process
Four steps from the first question to submission — not every case needs all of them to the same depth:
From question to submission
- 1Clarify what your case needs — not every project needs the same documents (free initial consultation).
- 2Geotechnical soil report (if not already available) — from a geotechnical engineer, with the kf-value.
- 3Infiltration verification per DWA-A 138-1 — calculation, site plan, conformity declaration, signed under responsibility.
- 4Submission — by you, your architect, or on request we accompany the procedure. The water authority decides on the permit.
5. Common misconceptions
Three assumptions that regularly lead to queries or rejection:
What often goes wrong
- 1"I can calculate that myself." The authority as a rule does not expect an informal calculation, but a reviewable design for which an engineer takes responsibility.
- 2"The calculation is a formality." The calculation determines what gets built — size and type follow from the verification. A system assumed by rule of thumb can cost several times what a precisely dimensioned one would have.
- 3"Infiltration is always allowed because it is ecological." Whether and how infiltration is permitted is decided by the water authority — among other reasons for groundwater protection and clearances.
6. What we do — and what we do not
We prepare the infiltration verification per DWA-A 138-1 — reviewable, signed under responsibility, with a conformity declaration. On request we accompany the procedure at the water authority.
We plan and sign off under professional responsibility — the construction is carried out by your firm, your landscaper or your builder. Responsibility for the design stays with us, execution with your trades.
Infiltration verifications from €700 net (€833 incl. VAT) as a fixed price; the plausibility check of an existing verification €600 net (€714 incl. VAT). Promptly after the initial consultation you receive the binding offer — or the specific query about which detail is still missing.