Which authorisation you need depends on surface and zone: roughly, ≤ 20 m² of new surface → déclaration préalable (DP) (up to 40 m² in zones urbaines of a PLU); beyond → permis de construire (PC). City-hall (mairie) instructs both.
Étapes
- Check the parcel's rules (PLU) at the mairie or on the Géoportail de l'urbanisme.
- Prepare the dossier: Cerfa (PC or DP) + plans (situation, masse, coupes, façades, insertion).
- File at the mairie or online via the commune's guichet numérique (GNAU) / the AD'AU assistant on service-public.gouv.fr.
- Instruction: DP ~1 month, PC ~2–3 months (site/zone dependent). Silence can be tacit acceptance — get the certificat.
- Display the panneau d'affichage on site (starts the 2-month third-party recourse window); declare start (DOC) and completion (DAACT).
Pièces justificatives
- Cerfa + plans; architect mandatory above 150 m² of surface de plancher.
Délais et échéances
- Recourse des tiers: 2 months from continuous display. Authorisation validity: 3 years (extendable).
Portail / Où faire la démarche
- Mairie / GNAU of the commune · AD'AU on service-public.gouv.fr. Taxes (taxe d'aménagement) come after — simulate on the official simulator.
Pièges connus
- Building at 19 m² "to avoid papers" but crossing thresholds with existing surface.
- Skipping the display panel — the recourse window never closes.
- Zone ABF (bâtiments de France): longer delays, stricter rules.
Sources
- service-public.gouv.fr — « Permis de construire » / « Déclaration préalable » (checked 2026-07)