Extending a house in France: permits, architect and budget
An extension comes down to a few square metres: 20 m², 40 m² and 150 m² in total decide which authorisation you need and whether you must use an architect.
Prior declaration or permit: the thresholds
An extension, an extra storey (surélévation) or a conservatory creates floor area, and that decides the procedure.
- In the urban zone (U) of a local plan (PLU): prior declaration up to 40 m², building permit above. In a protected site the limit stays at 20 m².
- Outside zone U or with no PLU: prior declaration up to 20 m², however small, permit above.
- Important exception: in zone U, an extension of more than 20 m² that takes the house above 150 m² in total needs a building permit.
When an architect becomes compulsory
For a project that needs a permit, an architect is compulsory if the total floor area, existing included, exceeds 150 m² after works, or if the house was already over 150 m². An extension under a prior declaration never requires an architect.
| Example | Authorisation | Architect |
|---|---|---|
| 100 m² house, 35 m² extension in zone U, outside a protected site | Prior declaration | Not compulsory |
| 120 m² house, 45 m² extension in zone U (165 m²) | Building permit | Compulsory |
| 135 m² house, 25 m² extension in zone U (160 m²) | Building permit | Compulsory |
| 110 m² house, 30 m² extension outside zone U (140 m²) | Building permit | Not compulsory |
Local plan rules to check before drawing
The local plan (plan local d'urbanisme, PLU) sets, plot by plot, what is allowed: maximum footprint, height, distance from boundaries and the street, the look of façades and roofs, sometimes a share of the land to keep unbuilt. You can view your commune's PLU on the Géoportail de l'urbanisme or at the mairie: the authoritative documents are those held by the commune, the intercommunality or the prefecture.
An extra storey also raises structural questions: an architect will have an engineer check that the walls and foundations can carry it.
The fee budget
For an extension, pricing guides usually quote 10–15% of the cost of the works excl. VAT for a full mission, a little more for small projects. A permit application alone is often a fixed fee of about €1,500–3,500 excl. VAT. The fee calculator gives a range for your budget.
The steps of an extension project
- Check the local plan and the existing floor area (in the title deed or by a survey).
- Have two or three options sketched, with a first cost estimate.
- File the prior declaration (1 month) or the permit (2 months for a house).
- Post the authorisation on site and wait for the neighbours' appeal period to end (2 months); the mairie can also withdraw an unlawful permit for 3 months.
- Ask builders for quotes, sign the contracts, then start the works.
In an urban zone, outside protected sites, an extension up to 40 m² only needs a prior declaration, unless it is over 20 m² and takes the house above 150 m². Then a building permit and an architect are compulsory.
Frequently asked questions
Do I need an architect for a 40 m² extension in France?
Not if it only needs a prior declaration. In the urban zone of a local plan, an extension of more than 20 m² that takes the total above 150 m² needs a building permit, and therefore an architect.
Does a conservatory count as an extension?
Yes. A conservatory (véranda) creates footprint and floor area, so the same 20 m², 40 m² and 150 m² thresholds apply.
How much does an architect cost for an extension?
For a full mission, usually 10–15% of the cost of the works excluding VAT. A permit application alone is often €1,500–3,500 excluding VAT.