Extension and works without a permit: prior declaration, 2026 thresholds and regularisation

An owner who wants to enlarge a house, convert a garage into a living room or add a conservatory soon runs into a question that looks simple: is a building permit (permis de construire) needed, a simple declaration, or nothing at all? The answer determines the lawfulness of the works and, above all, the value of the property on resale. A structure built without the required authorisation remains fragile for years, exposed to a demolition action and to criminal penalties. This guide deals exclusively with the position of the owner who builds: which thresholds trigger which formality in 2026, and how to regularise works carried out without authorisation. It does not deal with challenges by neighbours or third parties against an authorisation, which follow a different logic.

The logic of the thresholds: floor area and ground footprint

The choice between a building permit and a prior declaration of works (déclaration préalable de travaux) does not depend on the appearance of the project but on two technical measures defined by the French Urban Planning Code (Code de l'urbanisme). Confusing these two concepts is the leading cause of errors of characterisation.

Floor area

Floor area (surface de plancher), defined in Articles L. 111-14 and R. 111-22 of the French Urban Planning Code, is the sum of the enclosed and covered floor areas with a ceiling height of more than 1.80 metres, calculated from the inner face of the facades. Deductions are made for voids (stairwells and lift shafts), parking areas, non-convertible attic space and the thickness of the walls surrounding door and window openings.

In practical terms, a single-storey extension whose walls enclose a habitable space with a height of 2.50 metres creates floor area over almost all of its internal footprint.

Ground footprint

Ground footprint (emprise au sol), defined in Article R. 420-1, is the vertical projection of the volume of the structure, including overhangs and projections. A carport, a canopy or an open shelter creates no floor area (it is not enclosed) but does generate ground footprint. The application of these concepts to open structures attached to the house is examined in our FAQ on the pergola, between permit and prior declaration.

The regulations apply the more restrictive of the two values. A project that exceeds the threshold on either measure triggers the corresponding formality, even if it remains below the threshold on the other.

Permit or prior declaration: the 2026 thresholds

The 2026 building permit thresholds have not undergone any recent upheaval: they result from Articles R. 421-9, R. 421-14 and R. 421-17 of the French Urban Planning Code, in their stabilised wording. The distinction is read in three tiers, to which is added a derogatory regime for urban zones.

Under 5 sq. m: in principle no formality

A new structure or an extension that creates no more than 5 sq. m of floor area or ground footprint is in principle exempt from any authorisation, subject to a limited height. This is the case for a small garden shed or a technical room. Note, however, that this principle does not apply in protected areas (surroundings of a historic monument, remarkable heritage site, classified site), where a prior declaration is again mandatory from the first square metre.

From 5 to 20 sq. m: the prior declaration of works

Above 5 sq. m and up to 20 sq. m of floor area or ground footprint created, a prior declaration of works is sufficient (Article R. 421-9 for new structures, R. 421-17 for works on an existing structure). This lighter regime covers most conservatories, shelters and small extensions. The particular case of an external entrance lobby and the steps it requires is dealt with separately in our article on that type of work.

The prior declaration is examined by the commune within one month in principle, extended to two months in certain cases (in particular in a protected area). Failure to respond within the period amounts to a decision of non-opposition, that is, tacit approval.

Above 20 sq. m: the building permit

An extension that creates more than 20 sq. m of floor area or ground footprint requires a building permit. The examination period is two months for a single-family house and its outbuildings, and three months for other structures.

The derogatory regime for urban zones: the 40 sq. m threshold

Article R. 421-17 f) of the French Urban Planning Code raises the prior declaration threshold from 20 to 40 sq. m for extensions of existing structures located in an urban zone (zone U) covered by a local urban plan (plan local d'urbanisme, PLU) or an equivalent document. In those areas, an extension of up to 40 sq. m can be carried out by simple prior declaration, without a permit.

This favourable regime has a decisive limit. If the extension takes the total floor area of the structure above 150 sq. m, a building permit is again mandatory, even for an extension of between 20 and 40 sq. m. At that same threshold of 150 sq. m of floor area, the use of an architect becomes compulsory (Law No. 77-2 of 3 January 1977 on architecture, threshold lowered from 170 to 150 sq. m by Decree No. 2016-1738 of 16 December 2016). The thresholds, exceptions and the architect's role in the authorisation file are set out in our article on the architect and the building permit.

An example illustrates how this works. An owner of a 120 sq. m house in zone U wishes to add a 35 sq. m master suite. In principle, a prior declaration is sufficient (extension of less than 40 sq. m in zone U). But the extension takes the total to 155 sq. m, above the 150 sq. m threshold: the building permit and the architect become mandatory. If the owner reduced the project to 28 sq. m, the total would remain at 148 sq. m and a prior declaration would be sufficient.

Common extensions: how to characterise a project

Practical characterisation goes beyond simply reading the thresholds. Several frequent situations deserve to be examined.

Converting a garage into a living room, without changing the external volume, creates floor area (the space becomes enclosed, covered and habitable). It requires a prior declaration where the area created exceeds 5 sq. m, because it involves a change of use of part of the structure. If the works alter the external appearance (removal of the garage door replaced by a window opening), a prior declaration is required in any event on account of the alteration of the facade (Article R. 421-17 a).

Raising a roof to fit out an attic creates floor area where the height reaches 1.80 metres. The threshold is assessed on that usable area alone, which sometimes leads to the project being underestimated. A roof raising that changes the roof pitches requires authorisation in every case on account of the alteration of the external appearance.

Installing a swimming pool is subject to its own rules: prior declaration for a pool whose water surface area is between 10 and 100 sq. m, building permit above 100 sq. m or if the cover exceeds 1.80 metres in height (Article R. 421-9). Below 10 sq. m, no formality outside a protected area.

One point deserves the attention of the prudent owner: splitting a project in order to stay below the thresholds is risky. The administration and the administrative court assess the project as a single operation. Carrying out two extensions of 18 sq. m a few months apart to avoid the building permit may be requalified as a single operation of 36 sq. m, revealing an insufficient prior declaration or even the absence of a permit.

Building without authorisation: the real risks

An owner tempted to save the cost of a formality must bear in mind that a planning offence does not easily become time-barred and that it durably encumbers the property. Three sets of consequences accumulate.

Criminal penalties

Building a structure without the required authorisation, or in breach of the authorisation obtained, is an offence punished by Article L. 480-4 of the French Urban Planning Code. The fine is between €1,200 and an amount which may not exceed, in the case of a structure creating floor area, €6,000 per square metre of floor area built. In the event of a repeat offence, a prison sentence of six months may be added.

Public prosecution is time-barred after six years from completion of the works (the ordinary limitation period for offences, Article 8 of the French Code of Criminal Procedure since Law No. 2017-242 of 27 February 2017). Until that period has expired, an infringement report drawn up by a sworn officer or by the mayor may trigger prosecution.

Demolition

This is the heaviest risk for the owner who builds. The criminal court may, under Article L. 480-5, order demolition, bringing the works into compliance or restoration of the site, in addition to the fine.

In parallel, Article L. 480-13 provides for a civil action for demolition. Since Law No. 2015-990 of 6 August 2015, known as the Macron Act, this action has been subject to strict conditions: the civil court may order demolition only if the structure is located in certain sensitive zones that are exhaustively listed (classified sites, surroundings of historic monuments, protected coastal zones, etc.) and on condition that a court decision has first annulled the permit. Outside those zones, a civil demolition action brought by a third party is severely restricted, which does not in any way protect the owner from public prosecution and from demolition ordered by the criminal court.

Civil and asset-value consequences

Beyond the criminal sphere, a structure built without authorisation remains legally irregular and this irregularity weighs on the owner's assets. On resale, the notary records the absence of authorisation, which reduces the value of the property, deters buyers and complicates obtaining a loan. A buyer who discovers after the sale that the extension was not authorised may bring a claim on the basis of defects in consent or of the warranty, depending on the circumstances.

The administration may also refuse final connection to the networks (water, electricity) of a structure built without authorisation, on the basis of Article L. 111-12 of the French Urban Planning Code. The regularity of the structure also determines whether it can be insured in the event of a loss.

Regularising a structure built without authorisation

Regularisation of a structure built without a permit is possible in most cases, but it follows a method. The principle is simple: the authorisation that should have been obtained beforehand is requested after the event.

The regularisation permit or prior declaration

Regularisation takes the form of an application for authorisation covering the structure already built: a regularisation building permit if the work exceeded the permit threshold, a regularisation prior declaration below it. The file describes the existing structure as it was built.

The substantive condition is decisive: the structure must comply with the planning rules in force on the day the administration rules on the regularisation application. In other words, regularisation is possible only if the PLU and the easements allow the structure today as it exists. If the extension breaches a rule on set-back (distance from boundaries), maximum ground footprint or height, it can be regularised only after effective bringing into compliance (for example partial reworking of the structure).

A regularisation made after an infringement report does not necessarily bring an end to criminal proceedings already begun, but it ends the nuisance for the future and secures the property. In practice, it is always preferable to start a regularisation voluntarily before any inspection.

The limitation periods to know

Two distinct periods work in the owner's favour over time, but neither should be overestimated.

The first is the criminal limitation period of six years from completion of the works: after that period, the offence can no longer be prosecuted and demolition ordered by the criminal court is no longer incurred on that ground.

The second is the administrative limitation period of ten years laid down by Article L. 421-9 of the French Urban Planning Code. Once ten years have passed since completion of a structure, the administrative authority may no longer, in principle, challenge the conformity of the structure or refuse to issue a new authorisation on the ground of its original irregularity. This period has notable exceptions: it does not apply where the structure was built without any authorisation in a protected area (classified site, nature reserve, area of the maritime public domain, etc.), nor where works were continued in breach of an order to stop.

The ten-year administrative limitation period does not cure the criminal irregularity any faster than the six-year period and, above all, it does not remove the need to regularise when new works are planned. An owner who wishes to enlarge an older unauthorised structure again will most often have to regularise the whole so that the new project can be examined without difficulty.

Conclusion

The characterisation of an enlargement project cannot be guessed: it is calculated, with floor area and ground footprint in hand and the 2026 thresholds as the basis. The dividing line is clear: up to 20 sq. m (40 sq. m in zone U of a PLU), prior declaration; above that, building permit; and from 150 sq. m of total floor area, permit and architect are mandatory. The owner who builds has every interest in having this characterisation checked beforehand, because an extension without a permit remains a legal thorn for years: a fine that can reach €6,000 per square metre, a risk of demolition ordered by the criminal court, and a lower value and blocked resale.

The good news is that regularisation remains open as long as the structure complies with current planning rules. The wrong reflex is to wait for the limitation periods to run out and hope for the best. The right reflex: before starting an extension project, have a characterisation note prepared (floor area, ground footprint, PLU zone, applicable thresholds) and, if an older structure has never been authorised, start a regularisation application before any new project rather than after an inspection.

Frequently asked questions

What size of extension can I build without a building permit in 2026? A prior declaration is sufficient up to 20 sq. m of floor area or ground footprint created. This threshold is raised to 40 sq. m for the extension of an existing structure located in an urban zone (zone U) of a local urban plan. Above that, or if the extension takes the total area above 150 sq. m, a building permit is mandatory (Articles R. 421-14 and R. 421-17 of the French Urban Planning Code).

Does a 30 sq. m extension need a building permit? It depends on the location. In an urban zone of a PLU, a 30 sq. m extension requires a prior declaration, unless it takes the structure past the threshold of 150 sq. m of total floor area. Outside zone U (for example in a natural zone or where there is no PLU), the 20 sq. m threshold applies and a building permit is required.

Can a structure built without a permit be regularised after several years? Yes, by filing a regularisation permit or prior declaration application covering the existing structure. The condition is that the structure complies with the planning rules in force on the day the administration rules. The criminal offence is time-barred six years after completion of the works and Article L. 421-9 provides for an administrative limitation period of ten years, with exceptions in protected areas.

What is the risk of demolition of a structure built without authorisation? The criminal court may order demolition or restoration under Article L. 480-5 of the French Urban Planning Code, in addition to the fine. The civil action for demolition under Article L. 480-13 has, since 2015, been reserved for certain protected zones that are exhaustively listed and requires the prior annulment of the permit. The risk of demolition therefore does exist, mainly through the criminal route for as long as the offence is not time-barred.

What fine applies for building without a permit? Article L. 480-4 provides for a fine of between €1,200 and an amount which may not exceed €6,000 per square metre of floor area built irregularly. In the event of a repeat offence, a prison sentence of six months may be imposed. These penalties are in addition to measures to bring the works into compliance or to demolish.

Does converting a garage into a habitable room require a declaration? Yes, in almost all cases. The conversion creates habitable floor area and often involves a change of use of part of the structure, which triggers a prior declaration above 5 sq. m. If the works alter the external appearance (replacement of the garage door with a window), a prior declaration is required on account of the alteration of the facade.

Does the 40 sq. m threshold apply everywhere? No. The 40 sq. m threshold of Article R. 421-17 applies only to extensions of existing structures located in an urban zone (zone U) of a PLU or an equivalent planning document. Outside those zones, the prior declaration threshold remains at 20 sq. m. And even in zone U, a permit is again mandatory if the total area exceeds 150 sq. m.