What is the water test (watertoets)?

The water test is the assessment that the authority must carry out before it grants a permit or approves a plan: could the project harm the water system, for example because room for water is lost or because less rainwater can soak into the ground? If so, it imposes conditions, such as compensation or a sufficiently high floor level (vloerpeil), meaning the height of the ground floor. If that cannot offset the harm, it refuses the permit. You do not apply for the water test separately: it is part of the decision on the permit.

When and by whom

The water test takes place in any case for:

an environmental permit (omgevingsvergunning) for planning works, such as building or renovating;
an environmental permit to subdivide land into building plots (verkavelen);
a spatial implementation plan (ruimtelijk uitvoeringsplan, RUP) or a land-use plan (plan van aanleg);
an urban planning certificate (stedenbouwkundig attest), which says in advance whether a project can reasonably be permitted.

The authority that decides, the permitting authority (vergunningverlener), carries out the water test; usually this is the municipality. It gives its reasons in the decision, in a section called the water paragraph (waterparagraaf).

Other things often matter on the same plot too, such as the soil certificate and the orientation soil investigation and the P-score and the G-score. You can see how a report brings them together in the sample report.

When the water manager gives advice

Sometimes the permitting authority must first ask the water manager (waterbeheerder), the body that manages the watercourse, for advice. This is required, among other cases:

if the project lies wholly or partly on the advice map (advieskaart) of the water test;
if it lies in the bed of a watercourse, or closer to the top of the bank than 50 m (navigable watercourse), 20 m (first category), 10 m (second) or 5 m (third category or public ditch);
if it lies in a designated riverbank zone (afgebakende oeverzone), a defined strip along a watercourse;
for structures or paved surfaces of more than 0.1 ha (1,000 m²), or more than 1 ha if the water drains into a navigable or first-category watercourse.

The category of a watercourse and its manager are shown in the Flemish Hydrographic Atlas (Vlaamse Hydrografische Atlas) on Waterinfo. The water manager normally advises within thirty calendar days; the advice does not bind the permitting authority, but if it departs from it, it must give its reasons.

What the outcome can be

Without a harmful effect, the water test is positive. With an effect, a permit follows with conditions that avoid, limit, repair or compensate for it, that is, make up for it elsewhere. If no acceptable alternative or repair is possible, the authority refuses the permit, according to vlaanderen.be in exceptional cases.

For flood-prone areas along watercourses, a circular (omzendbrief) from the Flemish government lists conditions such as these:

a floor level above the flood level where the probability of flooding is medium;
no loss of room for water, in area or in volume;
no basement or other underground space, unless exceptional and adequately protected, and no underground heating oil tank;
non-return valves (terugslagkleppen) on the sewer pipes, so that sewage cannot flow back;
compensation if the ground has to be raised.

For flooding from heavy rain, the circular qualifies these rules: where water collects in the street, a sufficiently high floor level is usually enough. These are guidelines, not a legal rule for every plot.

The maps and the signal areas

Since 1 January 2023, the water test has used the advice map, which shows for each plot whether advice must be sought and from whom, and three maps of flood-prone areas: from the sea, from watercourses and from heavy rain. These three maps also give the P-score (P for ‘perceel’, the plot) and the G-score (G for ‘gebouw’, the building), from A (no flooding modelled) to D (a medium probability in today’s climate). The maps do not decide on the permit: the permitting authority does.

The rules are stricter in a signal area (signaalgebied): an area not yet developed whose zoning allows building, such as a residential area, where that zoning may clash with what the water system needs. If it has a no-build objective (bouwvrije opgave), meaning the area is to remain unbuilt, permits there are as a rule refused. In the other signal areas a stricter water test applies: building is possible, on the conditions the Flemish Government set for each area.

What does this mean if you are buying a house?

The purchase itself involves no water test. But the advertisement and the deed must state the P-score and the G-score, and say whether the property lies in a designated flood area (afgebakend overstromingsgebied) or a designated riverbank zone: that is the disclosure duty (informatieplicht), a separate rule. It does not cover a signal area or the risk zone (risicozone) for natural disaster insurance; the Waterinfo flood report (overstromingsrapport) does show them.

The water test only comes into play when you apply for a permit, for example to renovate, extend or rebuild; in a flood-prone area or near a watercourse, conditions may then apply, such as a higher floor level or no basement. A good P-score is no guarantee: the water test also looks at how rainwater soaks into the ground, groundwater, the distance to a watercourse and signal areas.

The online water test tool of the Coördinatiecommissie Integraal Waterbeleid (CIW) gives a first estimate, but it is an aid, not a decision: for a specific project, ask the municipality.

What the Perceelscan shows

For each address, the Perceelscan shows the P-score and the G-score, and whether the plot lies in a designated flood area or riverbank zone, a signal area or a risk zone. It also shows the plan that applies, the regulations and the environmental permits on the plot and next door.

The Perceelscan does not carry out a water test and does not say how a permitting authority will decide. It is not a certificate and not advice from a notary.

In the other regions

In Brussels: How do you check the flood risk in Brussels?
In Wallonia: What does the flood hazard map of Wallonia say?

Sources

Check it for your address

Updated on 24 October 2026