You have an idea of the pool you want. Now ask yourself the opposite question: does your garden want this pool? The feasibility of a project doesn’t just depend on the budget or the choice of pool installer – it starts under your feet, with the composition of the soil, the orientation of your plot, and your local town planning regulations. A thorough assessment at this stage will save you costly surprises once excavation begins.
Soil testing: the diagnosis nobody thinks to ask for
This is the most frequent and costly surprise in swimming pool projects. The nature of the soil directly dictates the type of structure to be built, the excavation techniques to be used, and ultimately, the final price of the works. However, the majority of owners sign a quote without having had their soil analysed beforehand.
Stability: what the ground can support
An inground swimming pool, once filled with water, represents a considerable load. A pool measuring 8 × 4 m with an average depth of 1.50 m contains approximately 48,000 litres of water, which is nearly 48 tonnes – to which are added the weight of the structure, the surrounding decking, and the equipment. Soft ground, sandy or backfilled, can progressively settle under this load, causing cracks in the pool or deformations of the decking.
A clay soil is even more problematic: clay expands when wet and shrinks during dry spells (swelling-shrinking phenomenon, well-documented in France and classified as a natural hazard). A concrete pool built on clay without specific precautions can crack as early as the first dry summer. The geotechnical survey — carried out by a specialist design office for €300 to €800 — is the only way to precisely know the composition and load-bearing capacity of your soil before any commitment.
Water management: groundwater and drainage
The presence of a high water table is one of the most underestimated constraints by prospective swimming pool owners. When emptying the pool for winterisation or maintenance, the pressure exerted by groundwater on the empty shell or walls can cause stripping The swimming pool is literally rising out of the ground, like a plug. This phenomenon is irreversible and catastrophic.
If your land is in a wetland area, at the bottom of a slope, or near a watercourse, always ask your pool builder how they take this risk into account in their design. Solutions do exist (reinforced anchoring, decompression drain, anti-buoyancy valve) but they incur costs and must be planned from the outset, not added during construction.
A rocky subsoil presents another challenge: groundwork becomes significantly longer and more expensive, sometimes requiring drilling equipment or even blasting. Additional costs of €5,000 to €15,000 are not uncommon in granite or limestone areas. Without prior surveys, this item is entirely unpredictable.
How to get a soil survey done?
Two options are available to you. Some reputable pool installers include a Reconnaissance survey in their quotation process — this is a good sign of professionalism. Otherwise, you can directly instruct an independent geotechnical engineering firm. Mission G1 (preliminary site study) is the most suitable at this stage: it identifies the main risks without going as far as a complete foundation study.
If a pool installer gives you a detailed quote without having seen your floor, ask them directly: «On what ground assumption did you base this quote?» The answer will tell you a lot about how serious they are.
Sunlight and wind: positioning the pool for maximum comfort
The location of your swimming pool in your garden is not just an aesthetic question. Poor orientation can cost you several hundred euros in additional heating each year, and turn a sunny swimming pool into a cold, underused pool in reality.
The solar orientation: maximising natural heating
L'eau d'une piscine se réchauffe essentiellement par le soleil. direct solar radiation. A south to south-westerly orientation is ideal: the pool benefits from sunshine from midday until late afternoon, which is the warmest part of the day. A north or north-easterly orientation can reduce the water temperature by 3 to 5°C compared to an optimal orientation – that's several weeks less season without heating.
Observe your garden at different times of the day before deciding on the final location. The shaded areas cast by the house, garage or large trees move throughout the hours and seasons. An area that is sunny in July at 2pm may be in shade by 4pm in September. If possible, carry out this observation over several weeks and at different times of the year.
The wind: the silent enemy of water temperature
Evaporation accounts for 70 to 80 per cent of a swimming pool’s heat loss. However, wind significantly accelerates evaporation by constantly renewing the humid air above the water’s surface. A swimming pool exposed to prevailing winds can lose up to 2°C more per night than a sheltered pool under the same weather conditions.
Identify the prevailing wind direction on your land. In France and Luxembourg, winds generally come from the south-west to the west. A hedge, a low wall, a pergola or a green windbreak on the exposed side can significantly reduce this heat loss, without obstructing light. Investing in a windbreak is often much more cost-effective than oversizing a heating system.
One thermal blanket (bubble cover or solar slatted cover) fitted each evening is the most effective measure for limiting overnight losses, regardless of wind exposure. It can maintain the water temperature 4 to 6°C higher than it would be without protection.
The surrounding vegetation: asset or constraint?
Nearby trees create shade (negative for heating) but also a natural windbreak effect (positive for limiting evaporation) and an undeniable aesthetic setting. The real problem is organic pollution: dead leaves, pollen, seeds, insects - everything the trees disperse ends up in the pond. This overloads the filter, consumes treatment products and multiplies maintenance time.
The rule of thumb: maintain a distance of at least 5 metres between the edge of the pool and the canopy of mature trees. For conifers and poplars, whose pollens and resins are particularly problematic, allow 8 to 10 metres instead.
Regulations: what you need to check before any quote
The regulations applicable to your swimming pool project depend on several levels: national law (the Urban Planning Code), your municipality's Local Urban Plan (PLU), and, if applicable, specific easements or protections related to your land. Do not begin any commercial steps without addressing these points – an administrative refusal during a project is a situation that is as costly as it is unpleasant.
National rules: prior declaration or building permit?
French common law distinguishes three situations according to the size of the basin:
- Pool under 10 m² No administrative formalities required (unless local provisions state otherwise)
- Pool between 10m² and 100m² Prior declaration of works mandatory at the town hall — processing period: 1 month in ordinary zones, 2 months in protected zones
- Pool over 100 m², or covered by a shelter over 1.80 m in height: planning permission Mandatory — processing time: 2 to 3 months
These thresholds apply to the surface area of the water mirror, not to the total site footprint (including the shore and technical room). Integrate these administrative deadlines into your schedule: a declaration submitted in February will give you approval in March-April, which is compatible with a spring construction.
Your Local Urban Plan (PLU): local rules take precedence over national rules
The Local Urban Plan of your municipality can be more restrictive national law on several points: setback from boundary lines (the general rule is 3m, but some local urban plans impose 5m or more), soil sealing coefficient (some municipalities limit the total impervious surface area on a plot), colours or materials permitted for the perimeter paving, and even a complete ban on swimming pools in certain areas (flood-prone areas, protected natural areas).
Consult the PLU directly on your town hall's website or on the Géoportail de l’Urbanisme (geoportail-urbanisme.gouv.fr). If your land is in a zone ABF (within a 500-metre radius of a historic monument), the Architecte des Bâtiments de France must give their approval – factor in additional time and potential aesthetic constraints.
Luxembourg's Case: Highly Variable Municipal Rules
In the Grand Duchy, regulations concerning private swimming pools are primarily managed at the municipal level. Each commune has its own PAG (General Development Plan) which sets the construction rules. In most Luxembourgish communes, an in-ground swimming pool requires a building permit issued by the municipality, accompanied by plans and an application form.
Consultez notre page dédiée aux Administrative procedures for building a swimming pool in Luxembourg for the specific details per municipality.
A point often overlooked: taxation
En France, une piscine enterrée est considérée comme une construction soumise à déclaration préalable de travaux ou permis de construire selon sa taille et éventuellement à une taxe foncière (taxe d'aménagement). permanent construction and is included in the property tax calculation. It is subject to a declaration to the tax authorities within 90 days of the completion of the works (H1 or IL form). The amount of the increase depends on the surface area and the municipality – generally expect between €200 and €600 in additional property tax per year. This is not a deterrent, but it is an item to be included in your recurring annual budget.
What your field diagnosis should provide you with
Before you meet your first pool installer, you should ideally have:
- One basic knowledge of your soil (ground type, presence of a water table, identified risks)
- One prime location in your garden, taking into account the sunshine and prevailing winds
- La confirmation that your project is administratively feasible (Consultation of the PLU, identification of any constraints)
- One Estimation of specific constraints liable to affect the quote (difficult terrain, nearby trees, mandatory setbacks)
These elements will allow you to approach quotes with a clear specification and compare proposals on an equal footing. A pool installer who does not ask you these questions during the first appointment is a pool installer who bases their quote on assumptions – and assumptions tend to be revised upwards once work has begun.
In the following section, we compare the three main pond technologies in detail: Polyester shell, concrete and kit — to help you choose the one that suits your land, your use, and your budget.
