Expanding Your Home Without a Building Permit: What Are the Allowed Areas?

We want to push a wall, cover a terrace, or place a studio at the back of the garden, and the first question always comes up: do we need to apply for a building permit? The answer depends on the area created, but also on the zoning where the land is located.

Expanding your house without a building permit is still possible for most common projects, provided you respect specific thresholds and do not confuse “without a permit” with “without formalities.”

Development tax and extension without a permit: the tax trap that no one checks

It is often thought that an extension made under prior declaration escapes any taxation. This is false. The development tax applies as soon as a taxable area is created, whether the authorization is a permit or a simple prior declaration.

Specifically, a 18 m² veranda placed after a prior declaration generates the same taxable base as a 50 m² extension subject to a permit. The calculation is based on the enclosed and covered area created, multiplied by a fixed annual value, then by the rate voted by the municipality and the department. The amount therefore varies significantly from one municipality to another.

Before starting the work, you can find all the tips on By Caroline and CO to anticipate these often underestimated fiscal and administrative obligations.

A garden shed open on two sides, on the other hand, does not create an enclosed area: it does not count in the taxable base. The distinction between enclosed and covered area and simple coverage changes the final bill.

Threshold of 20 m² or 40 m² without a building permit: what changes depending on the zoning

The Urban Planning Code sets two thresholds for house extensions, and the second only exists under certain conditions. The rule is often summarized as “40 m² without a permit,” but this 40 m² threshold only applies in areas covered by a local urban plan (PLU) or an equivalent urban planning document, in accordance with Articles R.421-14 and R.421-17 of the Urban Planning Code.

Outside PLU zone: the ceiling remains at 20 m²

If the land is in a rural area not covered by a PLU, any extension whose floor area or footprint exceeds 20 m² requires a building permit. Below that, a prior declaration is sufficient.

In PLU zone: up to 40 m² under prior declaration, but with a lock

In a municipality with a PLU, you can go up to 40 m² of extension without a permit, provided that the enlargement does not cause the existing construction to exceed a regulatory total surface threshold. If the current house plus the extension exceeds this threshold, the permit becomes mandatory again, even for a 25 m² project.

This lock is often overlooked. The area of the extension is checked, but the existing area is forgotten. A visit to the town hall or reading the PLU before submitting any application avoids this mistake.

• Extension of less than 5 m² (footprint and floor area): no urban planning formalities, regardless of the zone.
• Extension between 5 and 20 m²: prior declaration mandatory everywhere.
• Extension between 20 and 40 m²: prior declaration only in PLU zone, subject to the total surface threshold. Outside PLU, building permit required.
• Beyond 40 m²: systematic building permit.

Prior declaration of work for an enlargement: what the file really requires

The prior declaration is not a two-page form that you fill out in ten minutes. The file essentially resembles that of a simplified building permit. It includes a site plan of the land, a detailed mass plan, a cross-section plan, a notice describing the project and its integration into the environment, as well as photos of the land and its surroundings.

The instruction period is one month from the submission of a complete file. If the town hall does not respond within this period, the absence of a response equates to tacit approval, but it is advisable to request a certificate of non-opposition to secure the situation.

A common mistake is to under-declare the created area to stay below a threshold. Feedback varies on the rigor of checks depending on the municipalities, but a neighbor can report a non-compliant construction, and the urban planning service has six years to establish an infringement.

Attached extension, garden studio, veranda: rules vary depending on the type of construction

Not all square meters are equal in the eyes of urban planning. The type of construction changes the obligations.

A garden studio detached from the house follows the same thresholds as an attached extension: prior declaration up to 20 m² of floor area or footprint. Beyond that, a building permit is required. The 40 m² threshold in PLU zones only concerns extensions attached to an existing construction, not independent constructions.

A veranda, because it is enclosed and covered, creates floor area. It is therefore included in the calculation of thresholds exactly like a masonry extension. An open garden shed, on the other hand, only generates a footprint.

• Enclosed veranda: floor area + footprint. Prior declaration or permit depending on the threshold crossed.
• Closed garden shed (type chalet): same regime as an extension. Beyond 20 m², permit required outside PLU zone.
• Open pergola without walls: footprint only. No floor area created.

The distinction between footprint and floor area is often blurry. The footprint corresponds to the vertical projection of the volume of the construction, including roof overhangs. The floor area only counts the enclosed and covered spaces with a ceiling height exceeding 1.80 m. For the same project, the two figures can differ by several square meters.

Before estimating a house enlargement, reading the PLU and visiting the urban planning service at the town hall remain the two steps that avoid unpleasant surprises. A well-calibrated project in terms of area and placement passes through prior declaration without difficulty, whereas a few extra square meters can turn the file into a long and costly procedure.