Rainwater on your plot, the hierarchy you need to follow

A hierarchy set out in the Water Code

Roof, terrace, driveway: every square metre you seal off sends water somewhere the ground used to absorb it. In Wallonia, where that water must go has been set by law for several years, and a new circular has clarified since this summer how councils should assess it. Here's what you need to know before submitting your planning application.

According to SPW Environnement (the Walloon public environment service), since 1 January 2017 the Walloon Water Code (Code de l'Eau) has required rainwater to be managed on the plot in line with a hierarchy, which must be respected in planning and subdivision projects. In practice, the order is as follows:

• first, infiltration into the soil of your plot;
• if this is impossible or insufficient, rainwater should preferably be discharged to surface water and, failing that, to the sewer network;
• in the latter two cases, this discharge must comply with the conditions set by the watercourse manager or the intercommunale, particularly regarding maximum discharge rates.

Attenuation is therefore not a separate step: it almost always goes hand in hand with discharge. In most cases, if infiltration cannot be the sole solution, an on-site storage system will need to be provided to meet the imposed discharge rate. And before all that, the simplest reflex remains the first one: according to SPW, the very first measure is to limit sealed surfaces to what is strictly necessary. A car park in stabilised gravel or open-jointed paving means that much less volume you will have to store.

A useful reminder: managing rainwater separately means strictly separating wastewater and rainwater pipework inside buildings, which the Water Code has required since 20 July 2003.

What the circular of 16 July 2026 changes

On that date, Minister Desquesnes adopted a circular on rainwater management, notified to local councils on 27 July 2026. The Walloon Government makes clear that it creates no new legal obligation, but provides authorities and project designers with a shared framework for interpretation. It follows on from the 2021 floods and draws on the Walloon Reference Framework for Sustainable Rainwater Management (Référentiel wallon de gestion durable des eaux pluviales), available since 2023.

According to the Union des Villes et Communes de Wallonie (the Union of Walloon Towns and Municipalities), the key points are as follows:

• it applies to all projects subject to planning or subdivision permission that create or increase sealed surfaces;
• its strategy is to combine various measures for capturing and making use of rainfall, storing and attenuating it, and infiltrating the surplus;
• it focuses in particular on rainwater tanks, giving indicative sizing guidance and examples of standard conditions.

In practice, expect council services to scrutinise the water management report more closely, and for numerical conditions to appear more often in permits. A vague file on this point means a request for further information, and weeks lost.

Practical solutions and rough costs

There is no one-size-fits-all solution. The right choice depends on soil permeability, slope, available space and the presence of a water table. Here are the systems we most often come across, with indicative ranges observed on projects for houses and small buildings (excluding VAT, to be refined case by case):

• The rainwater tank: it makes use of the water (toilet, washing machine, garden) and can act as a buffer if it includes an attenuation volume with a flow-restricting outlet. Generally allow a few thousand euros for a 10,000-litre concrete tank installed, connections included.
• The swale: a wide, shallow, grassed channel that stores water before letting it infiltrate. This is often the most economical solution where space allows, in the order of a few tens of euros per linear metre, excluding major earthworks.
• The dry basin or rain garden: a landscaped depression that only fills during storms. It integrates well into a garden or a shared space within a building.
• Underground infiltration crates or blocks: useful when surface area is limited, more expensive, often between 150 and 400 euros per cubic metre of storage depending on the system and site access.
• The green roof: it retains part of the rainfall and spreads out the runoff, without replacing a storage system.

To get a sense of the volumes involved: a house with 120 m² of roof and 60 m² of terrace and driveway, that is 180 m² of sealed surface, receives 5.4 m³ of water during a 30 mm downpour. This is an order of magnitude, not a sizing method: the regulatory calculation takes into account the design rainfall event, the runoff coefficient of each surface, and the measured infiltration rate. SPW also provides the sizing tool developed by the Groupe Transversal Inondation (Cross-Departmental Flood Group), for infiltration and/or attenuation systems.

The role of the hydrological study in your application

This is what demonstrates, with supporting evidence, that your project complies with the hierarchy. The planning application must show whether or not it is possible to infiltrate water on site, in particular through infiltration tests and a rainwater management study. At Optibim, a typical study follows these steps:

• analysis of the plot: slope, flood risk maps and runoff pathways, possible discharge points;
• infiltration tests at the depth planned for the structures;
• calculation of sealed surfaces and volumes to be managed;
• selection and sizing of the systems, with a layout plan;
• if infiltration is not sufficient, justification of the discharge and calculation of the attenuation volume based on the imposed discharge rate.

The pitfalls we see most often:

• testing the soil at the surface when the structure will sit at a depth of 1.5 m;
• placing an infiltration system too close to foundations or a boundary line;
• forgetting the terrace or driveway in the calculation, counting only the roof;
• designing a tank that is permanently full and presenting it as a buffer volume, when a full tank buffers nothing.

For a house, the study and tests generally cost from a few hundred euros up to around 1,500 euros depending on complexity. That is little compared with the cost of a refusal, or of a poorly sized structure that has to be redone.

And in Brussels

The logic is similar, but the tools differ. According to the Guide Bâtiment Durable (Sustainable Building Guide), the Brussels Regional Planning Regulations (Règlement régional d'urbanisme, RRU) require, for new construction, a rainwater tank of at least 33 litres per m² of roof in horizontal projection. Bruxelles Environnement (Brussels Environment) also points out that, where works increase sealed surfaces, the environmental permit sets out the obligations to be met for reusing rainwater or managing it on the plot. Some municipal regulations and PPAS (Plans Particuliers d'Affectation du Sol, local land-use plans) contain stricter requirements, which should be checked at the design stage.

Let's talk about your project

Are you planning a new build, an extension or a housing project in Wallonia or Brussels? Optibim carries out infiltration tests, hydrological studies and the sizing of structures, working alongside STARC's architects or your own project designer. Get in touch as early as the sketch design stage, since that is when a well-placed swale or tank costs the least.

Frédéric Odent