What the law says.

No. Using rainwater to water a garden or wash an outdoor surface requires no declaration — neither before the 2024 reform nor after. It is the simplest configuration, and the most common one for a villa tank.

Sources: Arrêté du 12 juillet 2024 (base AIDA – INERIS)

Everything changes. As soon as a single indoor fixture — even one WC — is connected to the tank, a declaration of use at the town hall becomes mandatory. [[TO CONFIRM: the legal basis for this obligation after the 2024 order must be validated by a lawyer]]

Sources: CGCT art. R. 2224-19-4 · Me Reda Kohen, 23 Sept. 2026

No, as long as it stays fully buried. A fully buried tank creates no floor area within the meaning of the Code de l’urbanisme (France’s planning code) — so neither a déclaration préalable (prior declaration) nor a building permit, whatever its volume. 3,000 litres or 30,000, the rule is the same.

Sources: Code de l’urbanisme art. R. 421-9 à R. 421-12

None. A simple flush access hatch counts neither as floor area nor as footprint.

Sources: Code de l’urbanisme art. R. 421-9 à R. 421-12 · Construires.fr, “Récupération eau de pluie : règles et contraintes”, 4 Sept. 2026

Then the ordinary thresholds apply: more than 5 m² triggers a déclaration préalable (prior declaration), more than 20 m² a building permit. Only the visible part counts — never the buried volume.

Sources: Code de l’urbanisme art. R. 421-9 à R. 421-12

A protection perimeter, generally 500 metres around a historic monument, within which the Architecte des Bâtiments de France (the state architect for heritage sites) must be consulted. Inside it, a déclaration préalable (prior declaration) becomes systematic, whatever the size of the project. [[TO CONFIRM: application of this rule to a tank with no floor area and no visible footprint]]

Sources: Code de l’urbanisme · Construires.fr, 4 Sept. 2026

The Cannes PLU (local urban plan) regulations cap earthworks at 1.50 m from natural ground level, but expressly exempt fully underground structures from that cap — a buried tank fits within it. They also prohibit connecting rainwater to the wastewater network (article 4): a tank’s overflow goes to infiltration or the storm drain.

Sources: Règlement PLU de Cannes (updated 2 April 2026), art. 3

Because the regulations of Valbonne’s PLU (local urban plan) classify the entire territory of the commune as a site inscrit (a registered heritage site) — not just the historic bastide. Consultation of the ABF (the state architect for heritage sites) is therefore the rule, across the whole commune, with no exemption clause for buried structures as there is in Cannes.

Sources: Règlement PLU de Valbonne (updated 2 April 2026)

The old village has two listed monuments — the Notre-Dame-de-Vie chapel (1927) and the Porte Sarrasine (1942) — each with a 500-metre ABF perimeter (where the state architect for heritage sites must be consulted) covering the historic heart. The Mougins PLU (local urban plan) regulations themselves could not be read (a scanned document with no text layer): so we check each plot before answering, rather than generalising. [[TO CONFIRM: reading of the Mougins PLU regulations by OCR or with the planning department]]

Sources: Base Mérimée (historic monuments) · Règlement PLU de Mougins (scanned document, not read)

Only in Cannes. Mougins and Valbonne are inland communes and are not concerned. In Cannes, three layers are added to the PLU: the espaces remarquables (outstanding natural areas: zones AL/NL, where building is all but ruled out), the 100-metre strip (off limits outside already urbanised areas) and the areas close to the shore (which target the extension of urbanisation). A court of appeal has ruled that a 16 m² pool built against an existing house, in an already built-up neighbourhood, is not an “extension of urbanisation”. The reasoning holds by analogy for a buried tank — by analogy, not as an automatic guarantee.

Sources: Loi n° 86-2 du 3 janvier 1986 · CAA Bordeaux, 23 févr. 2021, n° 19BX03643

Yes. Stored rainwater is explicitly excluded from the restriction and suspension measures, even during a heightened alert or a crisis. The only nuance: watering is discouraged, not banned, between 9 am and 7 pm in summer, during the hours of high evaporation.

Sources: Arrêté préfectoral “Alerte Siagne aval”, 16 July 2026

Alert: no watering of green spaces between 8 am and 8 pm. Heightened alert: no watering of green spaces at all, by day or by night. Crisis: vegetable gardens on drip irrigation only, outside 8 am–8 pm; lawns and flower beds strictly banned.

Sources: Préfecture des Alpes-Maritimes, drought orders, June–September 2026

A rainwater network entirely separate from the drinking-water network, with no connection point at all; where the mains tops it up, a total-overflow disconnection with a visible air gap; upstream filtration ≤ 1 mm, an opaque and covered tank, screened vents ≤ 1 mm, an overflow able to absorb the full maximum inflow — to the storm drain or a soakaway, because the CASA and the Cannes PLU prohibit the wastewater network — and regular visual checks for leaks. “Eau non potable” (non-drinking water) signage is not required for detached single-family houses; it remains good practice.

Sources: Décret n° 2024-796 · Arrêté du 12 juillet 2024, art. 2 and 3 · ReutilisationEau.fr; Cuve-Expert.fr (summaries)

No legal text settles it — it is a choice made by the ground. Concrete neutralises the acidity of rain and keeps the water cool thanks to its thermal mass, up to about 20 m³. Polyethylene is light, supplied with filter and pump often pre-fitted, unaffected by frost, UV and soil pressure, and has no effect on pH.

Sources: CAUE Occitanie, La Maison Saint-Gobain, Test-Achats (technical sheets)

The area that counts is the footprint of the connected roof — length × width, not the sloping surface. The maximum volume is calculated as follows: annual rainfall × footprint × roof coefficient (0.9 for tiles or slates) × filter coefficient (0.9). In Cannes, Météo-France records 881 mm a year: a 150 m² tiled roof gives at most in the region of 107,000 litres a year, before real-world losses. The Gutters page details the method.

Sources: CSTB guide (2009) · ASTEE guide (2015) · Météo-France, Cannes climate sheet 1991–2020

Since 1 September 2024, greywater (showers, baths, washbasins, washing machines — never WCs, and the kitchen sink often excluded because of grease) may be used to water the garden, provided it is declared to the prefect (the State’s representative in the département) via demarches-simplifiees.fr (the government’s online procedures portal). The quality required is A for watering and outdoor washing, A+ for WCs or a decorative fountain. Monthly tests are mandatory for the first two months; after that, the owner remains responsible for quality and maintenance over the long term.

Sources: Décret n° 2024-796 · Arrêté du 12 juillet 2024 · ARS PACA

The Alpes-Maritimes Département funds 50% of the cost excluding VAT through the Green Deal 06, capped at €5,000. The Communauté d’agglomération Sophia Antipolis (CASA — Valbonne and Mougins, not Cannes) adds 10%, capped at €1,000, on top of a Green Deal grant already awarded. Conditions: a buried or integrated tank of at least 3,000 litres, one grant per tax household, no purchase before the Département’s notification. The CASA top-up also requires a main residence and installation within two years. [[TO CONFIRM: 2026 amounts and conditions — to be reconfirmed on the publication date]]

Sources: CASA, règlement récupérateur d’eau (VF3) · Département 06, Green Deal 06

Eligible for the Green Deal 06 for a rainwater tank: the Département’s rules (7 November 2025, art. 2.1.2) include second-home owners. The CASA 10% top-up, however, requires a main residence. We tell you at the survey stage, before the quote — not afterwards. [[TO CONFIRM: to be confirmed by phone with the Département (08 00 03 70 16) before publication]]

Sources: Département 06, règlement Confort Energie 06 (7 November 2025) · CASA, règlement récupérateur d’eau (VF3)

A real case analysed by a property lawyer in September 2026 describes a connected tank discovered eight years after it was installed: sale blocked, a hidden-defect (vice caché) claim possible within two years of the discovery, and a presumption of liability falling on the installer or the seller. A complete file — quote, technical data sheet, grant certificate, declaration if one was due — turns the tank into a selling point rather than a risk.

Sources: CGCT art. R. 2224-19-4 · Me Reda Kohen, lawyer, analysis of 23 September 2026