In this article (4)
Not in every case. The technical report on rainwater and wastewater management is required when the provisions of the Water Code (Code de l’eau) applicable to your project call for it. This depends on the project, on the plot’s wastewater treatment regime and on the rules that apply to it. New buildings are covered, but so are extensions, when the obligation to infiltrate rainwater or treated wastewater applies (SPW FAQ, questions 10 and 35).
What the report is for
The report justifies the solution chosen to manage the project’s water:
- infiltration, with its sizing (a permeability test and a calculation table are often useful);
- or the impossibility of infiltration (poorly permeable soil, site constraints), with the alternative solution.
So it does not only concern projects where infiltration will be carried out. For paper submissions, the SPW FAQ asks for at least 4 copies.
If your project is not concerned
Airplan recommends that you attach a short note explaining how water is managed on the project. This is advice, not a legal obligation: some municipalities ask for it, and it often avoids a request for additional information.
💡 The ministerial circular of 16 July 2026 on rainwater management clarifies how the existing rules should be read. It does not create any new obligation.
Individual wastewater treatment system (SEI)
| SEI situation | Environmental regime |
|---|---|
| Individual treatment zone, fewer than 100 population equivalents | Generally, a class 3 declaration |
| Individual treatment zone, 100 population equivalents or more | Class 2 environmental permit |
| Collective treatment zone, by derogation from the obligation to connect to the sewer | Class 2 environmental permit |
A single permit (permis unique) is only required if the project needs both a planning permit and an environmental permit. Source: Installing an individual wastewater treatment system (wallonie.be).
In Airplan
The “Contexte” (context) column shows the plot’s wastewater treatment regime (Code de l’eau theme). When the data justify it, Airplan recommends the technical infiltration report in section 6 of Annex 4. Always check this recommendation against the rules applicable to your project.