Do you need a prior declaration or a permit to extend a house?
As a general rule, an extension of more than 5 m² and up to 20 m² of floor area or ground coverage needs a déclaration préalable (prior declaration); beyond that, a permis de construire (building permit). In the urban zone of a PLU, the permit threshold rises to 40 m², unless the house would then exceed 150 m² in total: a permit is needed, and an architect. The mairie examines the application and decides, and near a historic monument the Architecte des Bâtiments de France (ABF), the state heritage architect, is consulted.
The thresholds in one table
The thresholds are measured in new floor area or new ground coverage, whichever is larger. They apply to an extension, a conservatory, an attached garage, a mezzanine or an extra storey (arts R421-14 and R421-17 of the Urban Planning Code).
| Area created | Outside a PLU’s U zone | In a PLU’s U zone |
|---|---|---|
| Up to 5 m² | Generally no formality | Generally no formality |
| Over 5 m² up to 20 m² | Prior declaration | Prior declaration |
| Over 20 m² up to 40 m² | Building permit | Prior declaration, if the total stays at 150 m² or less |
| Over 40 m² | Building permit | Building permit |
These thresholds are the general rule. A protected sector, such as the surroundings of a historic monument or a protected heritage area, can make works that would be exempt elsewhere subject to a declaration, for example changes to the exterior.
A house’s planning rules are read together with other documents, such as the planning certificate and the PLU zones. You can see how a dossier brings them together for one address in the sample dossier.
The 150 m² limit and the architect
A private individual building for themselves can do without an architect as long as the house’s floor area stays at 150 m² or less (art. R431-2 of the Urban Planning Code). Beyond that, the permit plans must be drawn up by an architect. For an extension, the house’s area after the works is what counts.
The same limit explains the proviso on 40 m² in a U zone: an extension of 20 to 40 m² that takes the house beyond 150 m² falls outside the prior declaration and needs a building permit, with an architect.
Time limits and display
- Prior declaration: in principle 1 month from the filing of a complete application.
- Building permit for a detached house: in principle 2 months.
- These periods are longer in a protected sector, where the ABF must give an opinion.
- The mairie has one month to say that an application is incomplete.
- Once granted, the permission is displayed on the site; third parties can then challenge it.
Near a historic monument: the ABF
Around a historic monument, works that change the external appearance of a building are subject to the opinion of the Architecte des Bâtiments de France. Where no specific boundary has been set, the surroundings generally cover buildings visible from the monument or seen together with it, within 500 metres. The ABF can require materials, colours or a roof shape, and the opinion can lead to a refusal.
Before buying a house you intend to extend, check whether it lies within these surroundings. The easement appears in the PLU annexes and on the Géoportail de l’urbanisme.
Before signing: check what already exists
An extension project starts with the house as it stands. A conservatory or garage added without permission counts towards the areas, and a new application may require the existing work to be regularised. Ask the seller for the permit or non-opposition decisions and the declarations of completion. To find out whether your own project would go through, the operational planning certificate or a conversation with the mairie’s planning department are the intended routes; the decision belongs to the mairie alone.
What to check before buying a house
Frequently asked questions
A 15 m² conservatory: declaration or permit?
As a general rule a prior declaration, since it creates more than 5 m² and less than 20 m². A protected sector may add requirements; the mairie will say.
Does the 40 m² threshold apply everywhere?
No. It applies only in the urban zones of a commune covered by a PLU or an equivalent document. Elsewhere, a permit is required beyond 20 m².
When is an architect compulsory?
For a private individual, once the house’s floor area exceeds 150 m², before or after works that need a building permit.
Can you ask the mairie before buying?
Yes: an operational planning certificate describes the project and the mairie says whether it is feasible. It is information, not permission.
Sources
- Service-public.fr: prior declaration
- Service-public.fr: building permit
- Service-public.fr: when an architect is required
- Service-public.fr: mezzanine or extra storey
- Urban Planning Code, arts R421-14, R421-17 and R431-2
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