Floor area:
definition and calculation

Floor area is defined by article R111-22 of the French planning code, in force since 1^st January 2016. It is the sum of the floor areas of each enclosed and covered level, calculated from the inner face of the external walls, from which a closed list of areas is deducted. It replaced the former SHON and SHOB: if an old permit or a deed of sale still refers to SHON, the figure is not directly comparable.

Three expressions in this definition drive the whole calculation.

“Enclosed and covered”. Only spaces closed on their sides and covered by a roof or a floor count. A canopy, an open carport or a terrace, even a covered one, is not floor area. An enclosed conservatory is.

“Each level”. All levels are added together: ground floor, upper floors, converted attics, mezzanine, and also the basement or cellar of a detached house when its height exceeds 1.80 m. A storey added on top creates floor area, even though it does not take up a single square metre of land.

“Inner face of the external walls”. Measurement starts from the inner face of the external walls: the thickness of these walls does not count. Partitions and internal load-bearing walls, however, are not on the list of deductions: the area they occupy stays in the floor area.

Article R111-22 lists eight deductions. The calculation guidance sheet published with the official forms specifies in a note that the last three, intended for apartment buildings, do not apply to individual housing. For a house, five remain.

1. Walls around doors and windows. The wall thickness in the reveals of openings onto the outside is not added to the room.

2. Stairwell and lift voids and openings. Upstairs, the floor opening where the staircase arrives does not count. The area taken up by the staircase on the lower level, however, is not deducted.

3. Areas with a ceiling height of 1.80 m or less. This is the deduction that weighs most under the roof: in an attic, the whole strip where you cannot stand upright drops out of the calculation.

4. Areas laid out for parking vehicles, motorised or not, including access ramps and manoeuvring areas. A house's garage is therefore not floor area, nor is the bike store.

5. Attics that cannot be converted for living or for a professional, craft, industrial or commercial activity. A loft cluttered with trusses or too low to live in does not count. The criteria that distinguish convertible attics from unusable ones are set out in our guide to convertible attics.

The three deductions reserved for apartment buildings cover a building's plant rooms, cellars and storerooms served by a common area, and 10% of the residential areas when the dwellings open onto internal common areas. One consequence, often overlooked: in a house, a cellar or basement more than 1.80 m high counts in the floor area, unless it is used as a garage.

The official guidance sheet follows a simple method: for each level, the area measured from the inner face of the walls, minus the deductions. It sets out the calculation in four columns: existing area (A), area demolished or removed (B), area created (C), and total area A − B + C. This sheet is not attached to the application, but it is what gives the figures to enter on the form.

Take a fictitious house, as an example: a ground floor of 100 m² measured from the inner face of the walls, including a 20 m² integral garage and a 3 m² stairwell opening on the floor above; an attic floor of 70 m² of floor, of which 18 m² lies under the sloping ceilings at 1.80 m or less; a 30 m² loft above, which cannot be converted.

Out of 200 m² of floors, the house has 129 m² of floor area. The difference comes almost entirely from the garage, the strips under the sloping ceilings and the loft. Three tips for an accurate calculation:

Measure the 1.80 m line on site, with a tape, under each sloping ceiling: it is what shifts the result by several square metres upstairs, and it depends on the actual thickness of the floor and of the insulation under the roof.

Do not reuse the living area from a listing or a survey: it is calculated differently, as shown below.

Calculate the existing building and the project separately. It is the area created that is compared with the prior declaration and building permit thresholds, and the total area after the works that is compared with the 150 m² threshold.

Three areas overlap in a project, and they are often confused.

Footprint (emprise au sol) is defined by article R420-1 of the French planning code: it is the vertical projection of the building's volume, all overhangs and projections included. Think of the house's shadow in the midday sun: walls included, overhanging balcony included, canopy on posts included. Excluded are façade ornaments, such as mouldings and glass canopies, and roof overhangs that are not supported by posts or corbels. According to service-public.gouv.fr, a ground-level terrace without deep foundations is not part of it; a swimming pool is. Note: the town's local plan (PLU) may define footprint in its own way for its own rules, for example by excluding balconies.

Living area (surface habitable) falls under another code, the construction and housing code (article R156-1). You start from the built floor area, then also deduct walls, partitions, steps and stairwells, service ducts and reveals. Unconverted attics, cellars, basements, sheds, garages, terraces, loggias, balconies and conservatories do not count either, nor do parts under 1.80 m high. It is the area used in property listings and leases, not the one used for planning permissions.

Taxable area, finally, is used to calculate the development tax (taxe d'aménagement). It is measured like floor area, from the inner face of the external walls, but only deducts reveals, openings and parts of 1.80 m or less: an enclosed garage is included. The full tax calculation, at 2026 rates, is in our article on the cost of a building permit.

The first two areas are monitored together, because the permission thresholds target an extension that creates floor area or footprint: it is enough for one of the two to exceed the threshold. Adding a storey creates floor area without footprint; a carport or canopy on posts creates footprint without floor area; a single-storey extension creates both, the footprint being slightly larger since it includes the walls.

Converting an attic. Attics that cannot be converted are deducted; once converted for living, all their parts over 1.80 m become floor area. An attic conversion therefore creates area, and is declared as such.

Converting a garage. The garage is not floor area; the living space that replaces it is. Article R*421-17 of the French planning code, as amended by the decree of 20 February 2026, requires a prior declaration for converting more than 5 m² of enclosed and covered area into floor area. Our guide to converting a garage into living space covers this case in detail.

Creating a mezzanine or an intermediate floor in a high space: it is a new level, and its area more than 1.80 m high counts.

Enclosing a conservatory or a canopy. A covered terrace is not floor area; as soon as it is enclosed, it becomes floor area.

Adding a storey. The added storey counts in full, from the inner face of its external walls, minus the parts under 1.80 m if the roof is pitched. Our benchmarks are in our guide to adding a storey to a house.

The town hall filing. According to the thresholds published by service-public.gouv.fr, which our guide to the prior declaration of works sets out: in an urban zone of a town covered by a local plan (PLU), an extension requires a prior declaration between 5 and 40 m² of area created, and a building permit above 40 m². Between 20 and 40 m², the permit becomes mandatory again if the extension brings the total area of the house to more than 150 m². Outside an urban zone, or in a town without a PLU, the prior declaration applies between 5 and 20 m², the permit above that.

The architect. Above 150 m² of floor area, a private individual building for themselves must have the drawings for their permit prepared by an architect, and for an extension it is the area of the house after the works that counts.

VAT. Works that increase the floor area of the existing building by more than 10% are excluded from the reduced rates and invoiced at 20%; the details are in our guide to VAT on renovation works.

Let us go back to the example house, with 129 m² of floor area, in an urban zone of a local plan (PLU). A 25 m² extension would stay under 40 m², but would bring the house to 154 m²: it would require a building permit and drawings prepared by an architect. Reduced to 20 m², it would leave the house at 149 m² and fall under a prior declaration. In both cases, it would increase the floor area by more than 10% and be invoiced with 20% VAT. That is why you need to calculate before drawing, not once the plans are finalised. The timescales and budgets for an extension are brought together in our guide house extension: cost, permit and timescales.

For an extension, adding a storey or an attic conversion, we start by surveying the existing house on site: internal dimensions of each level, heights under the sloping ceilings, stairwell openings, garage, basement. The calculations for the existing building and for the project are done separately, level by level, using the method in the official sheet, and checked against the town's local plan (PLU): permitted footprint, height, setbacks.

When a project comes close to a threshold, we tell you before the final design, along with what it changes: a permit instead of a declaration, two months' processing instead of one, an architect becoming mandatory, 20% VAT. Design, project management and the works then stay with us, carried out by our salaried craftsmen, each in their own trade, on a single quote.

One last point: the declared area is binding. It is the basis for the permission and the development tax; an approximate figure can lead to an incomplete application or to a permission that does not match what is built.