Do all property extensions in France require an architect?

Reader question: I want to add a large extension to my property. Do I have to hire an architect or can I do it without one?

Alteration work on your property in France will require you to either send a Permis de construire or Déclaration préalable de travaux (DP) to your local mairie. Rules on which applies depend on the location of the property and the size of the extension.

You must receive approval (tacit approval in the case of a DP) before you can begin work.

Generally, smaller work can be undertaken by yourself or tradespeople hired by yourself, but for projects of a certain size (see below), it is necessary to hire an architect.

Furthermore, in cases where an architect is required, they are responsible for submitting the planning permission application to the mairie so they need to be contacted well in advance.

On top of the necessity of hiring an architect in certain cases, they can also provide expert advice, as well as having good contacts for building companies and building material providers.

One thing to note is both work needing a permis de construire and work requiring a DP could potentially be subject to hiring an architect.

When do I need an architect?

Hiring an architect is mandatory when constructing a new building that will have a floor surface area greater than 150m² – a construction of this size will require a permis de construire.

It is also mandatory when any expansion work to a building or a detached house will make its total floor surface area greater than 150m², regardless of the planning permission needed or the size of the extension bringing it above this size.

If a house is already 150m² or more, however, extensions under 40m² subject to a DP do not require one.

There is also no obligation to use an architect when submitting a DP if the work will only increase the surface area by less than 20m² (unless this will bring the size of the property over 150m²).

In the case of a DP where an extension is between 20 and 40 m², as long as the extension is located in an urban area covered by a local town planning scheme (Plan local d'urbanisme or PLU) – and does not increase the total floor area above 150m² – you also are not required to hire an architect.

You can ask your local mairie if your commune has a PLU.

Any DP which will increase the surface area of the building by over 40m² will need an architect, however.

Read more: How can I oppose my neighbour’s building application in France?

Rules for other cases differ slightly

These rules do not apply, however, to agricultural buildings. In this case, an architect is required for constructions larger than, or expansions that bring a property over, 800m².

Unlike for private buildings, all expansions when an agricultural building is over 800 m² need an architect, regardless of if they only add a few extra m² to the surface area.

It is also mandatory to hire an architect if any work is being done on a building used by a company or association as opposed to by an individual.

If planning to convert one larger plot of land over 2,500m² into several smaller lots to construct on, either an architect or a landscape designer (paysagiste-concepteur) is required.

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