In short
Everything depends on the surface de plancher (floor area) or emprise au sol (footprint) created. For a new building or an extension, a déclaration préalable (prior declaration) is enough above 5 m² and up to 20 m². Above 20 m², you need a permis de construire (building permit). For an extension in an urban zone of a PLU (plan local d’urbanisme, local development plan), the permit threshold rises to 40 m², unless the extension takes the total floor area above 150 m²: the permit and an architect then become compulsory again (code de l’urbanisme, the planning code, art. R421-9, R*421-14, R*421-17 and R*431-2). The mairie (town hall) processes a prior declaration in 1 month, a permit for a detached house in 2 months, and other permits in 3 months (art. R*423-23). A permit is valid for 3 years (service-public.gouv.fr).
The thresholds: 5, 20 and 40 m²
| Project | Prior declaration | Building permit |
|---|---|---|
| New building (shed, detached garage, etc.) | over 5 m² and up to 20 m², height 12 m at most | above 20 m² |
| Extension of an existing building, general case | over 5 m² and up to 20 m² | above 20 m² |
| Extension in an urban zone of a PLU | over 5 m² and up to 40 m² | above 40 m² |
| Extension of 20 to 40 m² in an urban zone taking the total above 150 m² | no | yes |
Sources: art. R421-9 a (new buildings, version applicable to applications filed since 1 December 2024), R*421-17 f and R*421-14 a and b (works on existing buildings).
Floor area or footprint? Both count: the text refers to the creation of “floor area or footprint”. To remain under a prior declaration, both must respect the threshold (art. R*421-17 f).
Urban zone. The 40 m² threshold only applies in “urban zones of a local development plan or a planning document in lieu thereof” (art. R*421-14 b). Outside a PLU, in a zone to be developed or a natural zone, the threshold remains 20 m². Your mairie will tell you the zoning of your plot.
Protected area. The R421-9 a threshold applies outside sites patrimoniaux remarquables (designated heritage sites) and the surroundings of historic monuments; in those areas, the application file and the time limits differ (service-public.gouv.fr).
When an architect is compulsory: the 150 m² threshold
A private individual is exempt from using an architect for a non-agricultural building whose floor area does not exceed 150 m² (art. R*431-2). Above that, the plans must be drawn up by an architect. The threshold is assessed on the total floor area of the building after works: article R*431-2 requires an architect when works on an existing building take it over this threshold, for example a house going from 130 to 160 m².
Other thresholds in the same article:
- agricultural building: 800 m² of floor area and footprint;
- production greenhouses: 2,000 m², with a side wall height under 4 m.
A legal entity other than a farm (an SCI, a property-holding company, for example) must always use an architect (service-public.gouv.fr).
Processing times
| Application | Standard processing time (art. R*423-23) |
|---|---|
| Prior declaration | 1 month |
| Building permit for a detached house or its outbuildings | 2 months |
| Permis de démolir (demolition permit) | 2 months |
| Other building permits, permis d’aménager (development permit) | 3 months |
Starting point. The period runs from the filing of the complete file. If documents are missing, the mairie requests them within the month following filing; you have 3 months to supply them, failing which the application is rejected (service-public.gouv.fr).
Extensions. The period is extended by one month, in particular when the project is in a designated heritage site or near a historic monument (art. R*423-24). It is extended to 5 months for a permit concerning a building listed as a historic monument (art. R*423-28).
Silence from the mairie. If there is no reply by the end of the period, the permit is tacitly granted. The mayor can, however, withdraw an unlawful permit within 3 months of the decision (service-public.gouv.fr).
After approval: display, appeals, validity
Display. The sign, rectangular and over 80 cm, is put up on the land from notification or from the end of the processing period, legible from the public road. An extract is displayed at the mairie within 8 days and for 2 months. The period for third-party appeals is 2 months from the first day of a continuous 2-month period of display on the land (a compulsory statement citing art. R. 600-2).
Validity. The building permit is valid for 3 years from notification. It lapses if works do not begin within that period, or if they are interrupted for more than one year. It can be extended twice by one year, on an application filed at least 2 months before expiry, if planning rules have not changed. Authorisations issued between 28 May 2022 and 28 May 2024 are valid for 5 years, with no extension possible (service-public.gouv.fr).
If refused. You have 1 month for a recours gracieux (informal appeal) to the mayor and 2 months to bring the matter before the administrative court.
FAQ
I am building a 15 m² garden shed: which authorisation do I need?
A prior declaration, since the area created exceeds 5 m² without exceeding 20 m² (art. R421-9 a), outside a protected area.
Does my 35 m² extension in an urban zone need a permit?
No, a prior declaration is enough, unless the total floor area of the house exceeds 150 m² after works: in that case, a permit and an architect are compulsory.
How long does processing a prior declaration take?
One month from the complete file, extended by one month in a designated heritage site or near a historic monument.
Can I start work without a reply from the mairie?
For a permit, yes: no reply by the end of the period amounts to tacit approval.
My permit expires soon: can I extend it?
Yes, twice by one year, if you apply at least 2 months before expiry and planning rules have not changed.
Which professional to consult
The géomètre-expert (chartered land surveyor) measures the existing and created floor area and footprint, which determines the applicable threshold. Before buying a plot or a house to extend, the notaire (notary) and the agent immobilier (estate agent) can check the authorisations obtained and their validity. The courtier en crédit (mortgage broker) helps you align financing with the processing timetable.
Find a professional
One professional per trade and per area, selected and scored out of 100 from public data, with a verifiable title. The trade, its official register and the professionals selected (pages in French):
The chartered surveyor (géomètre-expert) selected in the largest cities
| City | Professional | Area | Score /100 | Status |
|---|---|---|---|---|
| Paris | Actif Géo | Paris, 14e arrondissement | 77 | Selected |
| Marseille | ATGTSM | Marseille, 9e arrondissement | 73 | Selected |
| Lille | MA-GEO | Lille | 74 | Selected |
| Cannes | Pythagore Géomètres-Experts | Cannes | 70 | Selected |
| Nîmes | Relief GE | Nîmes | 72 | Selected |
| Perpignan | Geopole - Géomètres-Experts Associés | Perpignan | 72 | Selected |
| Nancy | Geodatis Selarl Geometres Experts | Nancy | 76 | Selected |
| Annecy | GEHOM | Annecy | 74 | Selected |
| Avignon | Géo-Experts | Avignon | 70 | Selected |
| Quimper | Cornouaille Ingénierie Topographie | Quimper | 70 | Selected |
The 10 cities with the most sales (DVF 2025) where a chartered surveyor (géomètre-expert) is selected; in a city with several areas, the best-scoring profile. "Verified": title read at the register by Payotte; "Selected": title declared, verifiable on the profile.
Related guides
Sources
- Service-public.gouv.fr — Permis de construire (PC), vérifié le 13 février 2026 (checked 11 October 2026)
- Légifrance — Code de l'urbanisme, art. R*421-14 (version en vigueur depuis le 1er janvier 2016) (checked 11 October 2026)
- Légifrance — Code de l'urbanisme, art. R*421-17 (version en vigueur depuis le 22 février 2026) (checked 11 October 2026)
- Légifrance — Code de l'urbanisme, art. R421-9 (version en vigueur depuis le 16 novembre 2024) (checked 11 October 2026)
- Légifrance — Code de l'urbanisme, art. R*431-2 (version en vigueur depuis le 23 juin 2019) (checked 11 October 2026)
- Légifrance — Code de l'urbanisme, art. R*423-23 (délais d'instruction de droit commun) (checked 11 October 2026)
- Légifrance — Code de l'urbanisme, modification du délai d'instruction (art. R*423-24 et suivants) (checked 11 October 2026)
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