Rainwater in Wallonia, sizing your tank and infiltration correctly

This summer, the Walloon Government put rainwater management back at the heart of planning applications. For those of you building, renovating or developing a property project, the right question is no longer whether to manage rainwater, but how. Here is the method Optibim applies, from sizing the tank to the CertIBEau check.

What Walloon regulations say in 2026

First, a clarification. In Wallonia there is currently no general and systematic requirement to install a rainwater tank in new builds, unlike in Flanders and the Brussels Capital Region. Beware however, some municipalities do require a rainwater harvesting system to be installed. Always check the local council regulations before you start designing.

Infiltration, on the other hand, is governed by the Water Code (Code de l'Eau). Under article R.277 of the General Sanitation Regulations, rainwater follows an order of priority:

• infiltration on the plot;
• where technically impossible or where there is insufficient available land, into an artificial drainage channel or ordinary surface water;
• the sewer, only as a last resort.

Another point often overlooked, for new builds, even when the new dwelling has a tank, its overflow cannot be sent to the sewer.

The new element for 2026 is a circular. On 16 July 2026, Minister Desquesnes adopted a circular on rainwater management. According to the Union des Villes et Communes de Wallonie (Walloon association of towns and municipalities), it provides indicative sizing guidance for rainwater tanks and applies to all projects subject to planning permission that create or increase impermeable surfaces. It does not create a new standard. In practice, though, it gives municipalities a common reading grid, and your plans will be assessed against it.

Sizing your tank without oversizing it

A tank is sized from two quantities, the water your roof can collect and the water you need. On the rainfall side, the RMI (IRM) points out that annual averages across Belgium can double, from 700 mm a year in the northern part of Hesbaye to almost 1400 mm a year in the High Fens region. A project in Gembloux therefore cannot be calculated the same way as one in the Ardennes.

The simplified method from the Sustainable Building Guide of Bruxelles Environnement is a good starting point. You take the smaller of the two annual volumes (rain collected or need), multiply it by a desired autonomy [days] (between 2 and 5 weeks, so 14 to 35 days), then divide by 365.

A worked example, for a new house in Walloon Brabant:

• roof of 120 m² in horizontal projection, around 800 mm of rain a year;
• with a roof coefficient and filtration losses of around 0.75, that gives roughly 72 m³ a year collected;
• four occupants, toilet and washing machine connected, some garden watering, needs come to around 55 to 60 m³ a year;
• for three to four weeks of autonomy, that gives a tank of 4,000 to 5,000 litres.

Bigger is not necessarily better. An oversized tank never fills completely. It will not overflow enough to flush out floating particles. The water stagnates and degrades. Conversely, an undersized tank overflows constantly and empties as soon as the first dry summer arrives. If your municipality requires an additional buffer volume (a part of the tank that empties slowly through a calibrated outlet), this comes on top of the reuse volume.

Separating the networks, the CertIBEau check point

The CertIBEau is the condition for having the water meter switched on. Made fully mandatory for new connection requests since 1 June 2021, it is the essential precondition for obtaining the final activation of a water meter. According to the Embuild Wallonie federation, two notable elements, among others, will be checked, the complete separation of wastewater and rainwater circuits, and the presence of approved non-return valves.

In practice, two separations need to be planned from the drawing stage.

• Rainwater and mains water. Creating a bridge, for example, between the mains water network and the rainwater network to make up for a lack of rainfall is therefore prohibited. If you want top-up supply, it must be done through a free overflow into the tank or a management module with an air gap in compliance with standard NBN EN 1717.
• Rainwater and wastewater. The Water Code requires that grey water and black water cannot be mixed with rainwater, they must be separated using different pipework.

The pitfalls we most often encounter during certification visits:

• outdoor tap connected to the wrong pipe;
• rainwater pipework neither marked nor labelled;
• missing or non-approved non-return valve after the meter;
• downpipe from an extension connected out of convenience to the wastewater inspection chamber.

Correcting a buried pipe after the screed has been laid costs far more than planning it in from the outset.

Infiltrating the surplus, swales and structures suited to the site

The tank overflow and the water from terraces and driveways must return to the ground. The most common solutions:

• the swale, a wide, shallow, grassed or planted ditch that stores then infiltrates water;
• the rain garden, a planted depression near the house;
• infiltration trenches or crate soakaways, where space is limited;
• permeable surfaces, drainage paving or cellular tiles, which reduce the volume to be managed.

It all depends on the soil. Hesbaye loam infiltrates poorly, Brussels sand much better, clay almost not at all. Before sizing anything, an on site permeability test is needed, at the depth of the planned structure. You also need to check the water table level, the slope and the distance to foundations and plot boundaries. For municipalities, the UVCW states that when infiltration of rainwater or of water from an individual treatment system cannot be ruled out from the outset, a hydrogeological study should be requested. A file without this study therefore risks being declared incomplete.

Risk areas, planning ahead for flood risk

In a flood risk area or along a runoff path, requirements become stricter. The minister reiterated this in an interview with L'Avenir, a planning permission can be refused or made conditional on this type of work if the application concerns a property exposed to flood risk. In these situations, we recommend that you:

• check the SPW flood risk mapping from the sketch design stage;
• limit hard surfacing to what is strictly necessary;
• combine a tank with buffer volume, a swale and a limited discharge rate;
• position structures following the natural topography.

For a developer, this consideration affects siting and therefore the possible density. It is better to think this through before buying the land than at the point of submitting the planning application.

The role of Optibim in your project

Optibim gets involved at several stages, working directly with the architects at STARC:

• hydrological study, permeability tests and sizing of the tank, buffer and infiltration structures;
• technical note included with the planning application;
• design of the separate networks together with special engineering services;
• CertIBEau certification, as an accredited certifier, at the end of the works.

The same firm designs the installation and then checks it. Non-conformities are therefore identified on the drawings, before construction begins.

Are you preparing a new build, an extension or a housing project in Wallonia or Brussels? Contact Optibim or STARC in Gembloux to review the rainwater management for your site. An initial analysis of the site and the local regulations is often enough to point you towards the right choices.

Frédéric Odent