Building permit or prior declaration: which one do you need?

Two surface measures that determine the formality

The choice between a building permit (permis de construire) and a prior declaration (declaration prealable) depends on two measures: the floor area (surface de plancher, the sum of enclosed and covered surfaces with a ceiling height above 1.80 metres) and the ground footprint (emprise au sol, the vertical projection of the construction's volume). The higher of the two thresholds reached is the one that governs the procedure.

A new construction whose ground footprint and floor area both remain at or below 5 m², with a height at or below 12 metres, is in principle exempt from any formality (article R.421-2 of the French Town Planning Code), except in a protected area. Between 5 and 20 m², a prior declaration is enough. Above 20 m², a building permit becomes mandatory.

The threshold raised to 40 m² to extend in an urban zone

When the works concern an existing construction located in the urban zone of a local town planning scheme (plan local d'urbanisme, PLU), the prior declaration threshold is raised to 40 m² of floor area or ground footprint created (article R.421-14 of the French Town Planning Code). Below 40 m², a prior declaration is therefore enough, compared with 20 m² elsewhere.

One important caveat remains: works creating between 20 and 40 m² still require a building permit where they bring the construction's total floor area or total ground footprint beyond one of the thresholds set by article R.431-2, namely 150 m² for a construction used for a purpose other than agriculture (article R.421-14 b of the French Town Planning Code). That same 150 m² threshold governs the requirement to use an architect: the exemption in article R.431-2 benefits only natural persons who build or alter for themselves below that ceiling.

In summary, for the floor area or ground footprint created: up to 5 m², no formality is required, whether for a new construction or an extension in an urban zone. From 5 to 20 m², a prior declaration applies in both cases. From 20 to 40 m², a new construction requires a building permit, while an extension in the urban zone of a local town planning scheme still needs only a prior declaration. Above 40 m², a building permit is required in both cases.

Check the PLU and protected areas before deciding

These national thresholds do not remove the need to consult the municipality's local town planning scheme, which may add its own rules (height, siting, appearance, footprint). In a remarkable heritage area, near a historic monument, or on a listed or registered site, some exemptions fall away and a formality may again become mandatory even for minor works.

If you are unsure which procedure applies, requesting a town planning certificate or a visit to the town hall's planning department avoids a pointless filing or, conversely, an unlawful construction. A garden shed, a veranda or a carport follow the same grid of thresholds. A swimming pool, by contrast, has its own regime, expressed in terms of the surface of the pool: exempt up to 10 m² (article R.421-2), prior declaration from 10 to 100 m² (article R.421-9).