1. Legal basis — why a certificate is required at all
At the federal level, § 55 (2) of the German Water Resources Act (WHG) requires rainwater to be infiltrated locally, trickle-discharged or routed directly into a surface water body, unless legal, technical, geological or topographical reasons prevent this. Connection to the combined sewer is explicitly subordinate.
In NRW, § 44 LWG concretises this requirement: rainwater shall be infiltrated locally to the extent that this is technically feasible and economically reasonable. Exemption from this obligation is only granted in justified individual cases — and must be substantiated.
The water-rights permit per § 8 WHG is issued on the basis of this certificate. Without it, the water authority does not approve — and therefore the building permit cannot be issued.
2. What the water authority actually checks in 2026
The review by NRW's regional water authorities follows a largely standardised pattern. Case handlers check six core points — and exactly these six are what a standard infiltration certificate must deliver:
Water authority review grid
- 1Location-specific KOSTRA-DWD 2020 rainfall data extract (T = 30 a, durations from 5 min to 72 h)
- 2kf-value from a geotechnical soil report per DIN 18130 — with multiple boreholes in heterogeneous subsoils
- 3Mean highest groundwater level (MHGW) with at least 1.5 m clearance to the infiltration system base
- 4Dimensioned site plan showing infiltration system, connection lines, and elevations
- 5Dimensioning calculation including storage-to-discharge ratio and emergency overflow concept
- 6Conformity declaration with DWA-A 138-1, plus signature and stamp of a registered engineer
3. The five most common rejection reasons
In over 90 % of cases where the water authority requests revisions, the issue is one of these five:
Where certificates fail
- 1Generic kf-value without a soil report — e.g. 'kf = 1·10⁻⁵ assumed' instead of a measured value. Not accepted.
- 2Missing KOSTRA data — dimensioning against generic figures ('15 l/(s·ha)') instead of KOSTRA statistics for the specific location.
- 3No groundwater level — without the MHGW, the required clearance to the system base cannot be substantiated. Always re-requested.
- 4Missing emission assessment for discharge — for sealed surfaces with elevated pollution loads (traffic areas, industrial) or direct discharge into water bodies, the AFS63 certificate per DWA-A 102-2 is often missing.
- 5Sketch instead of calculation — a contractor's site sketch without hydraulic dimensioning does not meet the formal requirements. The authority expects a calculated design.
4. Pre-submission checklist
Before submitting the infiltration certificate, verify these eight points. Each one has caused a rejection in practice:
Pre-submission check
- 1Geotechnical soil report on hand, explicitly stating the kf-value with measurement method (DIN 18130-1 or field test)?
- 2KOSTRA-DWD 2020 data extract for the exact location (8×8 km grid) attached?
- 3Highest groundwater level substantiated — either in the soil report or via the LANUV database?
- 4Site plan shows system geometry, connection lines, emergency overflow, and elevation marks?
- 5Dimensioning includes storage-to-discharge ratio and infiltration time as a function of layer thickness?
- 6Conformity declaration with DWA-A 138-1 attached and signed?
- 7Above 800 m² sealed surface or for discharge: additional pollutant-load certificate per DWA-A 102-2?
- 8Calculation carries the stamp and signature of an engineer registered with the Chamber of Engineers NRW?
5. What good engineering delivers beyond the minimum
A code-compliant certificate is the obligation — sound engineering is the value-add. Three points we include in every project, beyond what DWA-A 138-1 strictly demands:
Climate adaptation surcharge: The standard requires a safety factor for climate change — we document it explicitly with the applied value and rationale, so the authority can trace the robustness of the design.
Hybrid systems for borderline soils: When the kf-value is at the lower limit (around 5·10⁻⁶ m/s), we combine an infiltration swale with retention storage rather than operating the system right at its edge.
Emergency overflow with destination proof: Where the design storm is exceeded, we substantiate the ordered overflow into a defined receiving feature — not just 'overflow provided'.