Converting a garage
into living space.

Under the Planning Code, a closed and covered garage is not floor area (surface de plancher): it is expressly excluded from it. The day it becomes a bedroom, an office or a living room, that area enters the calculation. It is this switch, more than the works themselves, that triggers the formalities: a planning permission, a declaration to the tax office, and sometimes a different VAT rate.

Technically, the gap is just as clear. A garage was built to shelter a car: a bare slab laid on the ground, uninsulated concrete-block walls, a large door instead of a window, no ventilation and no heating. Converting it means giving it everything a living space requires, without losing sight of the house it is attached to.

Up to 5 m² converted, no formalities. This is the case for a cupboard, a utility room or a toilet created in a corner of the garage, provided nothing changes on the outside.

Above 5 m², a prior declaration, whatever the area: service-public.fr specifies that there is no upper limit. Article R*421-17 of the Planning Code, in the version resulting from decree no. 2026-117 of 20 February 2026, expressly covers the conversion of more than five square metres of closed and covered area into floor area. The same article states that an ancillary space has the same use as the main building: a garage that belongs to your house is already attached to the dwelling, and converting it is not a change of use in planning terms.

Any change to the external appearance also requires a prior declaration, even under 5 m²: a window cut into a wall, a garage door replaced by a glazed sliding door or by a wall with a window. In practice, almost every full conversion therefore goes through the declaration.

Cases outside the standard pattern. If the conversion comes with an extension or an added storey, it is the extension's area thresholds that decide between a prior declaration and a building permit. A building that is not an outbuilding of the house, or a garage converted into a separate dwelling, requires a case-by-case analysis with the planning department. Thresholds and timescales are detailed in our guide to the prior declaration: one month's processing in the general case, two months in a protected area.

The prior declaration does not exempt you from complying with the local plan (PLU). For a garage, the rule that most often causes problems is parking: many local plans require a number of spaces per dwelling, and the town hall may ask for another one to be created on the plot to replace the one that disappears. Before drawing anything, you need to read that article of the zone regulations and check that a space can be created elsewhere, in front of the house or along the drive.

Openings are then subject to two sets of rules. First those of the local plan, which govern the appearance of façades, proportions and sometimes the materials of the joinery. Then those of the Civil Code (articles 675 to 680): a window looking directly onto the neighbouring plot must be at least 1.90 m from the property boundary, and 0.60 m for an oblique view. A garage built on the boundary can therefore hardly take a window on that side; the light then comes from the former door, a roof window or glass blocks.

The floor. A garage slab is almost always laid without insulation, and often a few centimetres lower than the rest of the house. This difference is used to lay insulation and a screed that bring the floor up to the level of the adjoining rooms, removing the step. The possible techniques and thicknesses are detailed in our guide to ground floor insulation.

The walls. Bare concrete blocks, sometimes rendered on the outside only: insulated lining on a metal frame is the most common solution, with particular attention to the wall separating the former garage from the house, which may or may not already be insulated depending on the age of the building.

The ceiling or the roof. If there is a heated room above, the ceiling is treated mainly for acoustics and finish. If the garage has its own roof, flat or pitched, that is what needs insulating, and it is often the item that requires the most checks: condition of the waterproofing, available ceiling height once the insulation is fitted.

The former door. Two options: replace it with a glazed sliding door that keeps the full width of the opening and brings in plenty of light, or partly brick it up to fit a window above a masonry sill wall. The second insulates better and preserves privacy on the street side; the first suits a former garage that opens onto the garden.

Air, heat and electrics. A living space must be ventilated: vents are connected to the house's system or the system is extended. Heating is extended from the existing installation where it allows. A garage's electrics, often limited to one light and one socket, must be completely redone to the rules for a habitable room.

Damp. A partly buried garage or one built against a bank may have damp walls that nobody noticed while it housed a car. The diagnosis is made before enclosing anything behind a lining: we detail the causes in damp wall: what to do.

There is no serious price per square metre for “converting a garage”: everything depends on what is already there and what becomes of the former door. An honest quote is read item by item. To give you orders of magnitude, here are the prices taken from our own quotes, published in our dedicated guides:

Insulated wall lining: €75 to €90 excl. VAT per m² fitted (see our plasterboard price guide).

Floor screed: €55 to €135 excl. VAT per m² depending on thickness and what it contains, insulation or heating pipes included.

Glazed sliding door 1.50 to 2.10 m wide: €2,400 to €3,100 excl. VAT fitted, without roller shutter. Standard window (0.6 to 1.5 m²): €550 to €1,200 excl. VAT fitted, plus the masonry for the sill wall if the former door is bricked up.

Electrics: around €60 to €85 excl. VAT per socket fitted, plus the circuits and lighting.

That leaves the items specific to each garage: roof insulation, ventilation, heating, interior joinery, painting and floor covering. That is why a price is only worth anything after a visit.

VAT. In a house completed more than two years ago, fit-out works are in principle subject to 10 % VAT, and eligible insulation works to 5.5 %. But the conversion creates floor area: when the works increase the existing floor area by more than 10 %, they are excluded from the reduced rates and invoiced at 20 %. A 20 m² garage converted in a 100 m² house crosses that threshold. The details are in our guide to VAT on renovation works.

Property tax. A habitable room does not have the same rental value as a garage. Service-public.fr states that the conversion may affect the property tax (taxe foncière), and that changes in the nature or use of a property must be declared within 90 days of completion of the works: form H1 for a detached house, IL for a change in nature or use. Failure to declare is liable to a €150 fine.

Planning taxes. They are declared online, in the “Gérer mes biens immobiliers” area of impots.gouv.fr, at the same time as the property declaration and within the same 90-day period. If a development tax (taxe d'aménagement) is due, it is calculated from that declaration.

1. Visit and feasibility. Measuring the garage, ceiling height, condition of the slab and walls, reading the local plan: parking, appearance of façades, distances to boundaries.

2. Plans and choice of opening. Sliding door, window or a combination of both, position of the room in the circulation of the house, location of the new parking space if the local plan requires it.

3. Prior declaration. Cerfa form no. 13703 for a detached house, plans and visual insertion. One month's processing, then the notice displayed on site that starts the two-month period for third-party appeals.

4. Works. Floor, walls, opening, services, finishes, in an order that encloses nothing damp behind a lining.

5. Declarations after the works. Declaration of completion at the town hall, then declaration to the tax office within 90 days.

At Atelier NovAlp, design, the permission application and supervision are handled by our architect, and the works by our own teams: masonry, plastering, joinery and painting. A single point of contact, with a committed price and timescale.