I. The extension you bought is unlawful: who can act against you, and for how long

A. Demolition, fines and daily penalties can follow the building, not the builder

French planning law starts from a simple rule. Under Article L421-1 of the Code de l’urbanisme, almost every new construction, even one without foundations, must be preceded by a permis de construire (planning permission). Extensions, outbuildings, converted garages and swimming pools each have their own threshold between permission and prior declaration, but the principle is constant: a building put up without the required authorisation is unlawful from the day it is finished, whoever built it.

That unlawfulness does not disappear when the property changes hands. The new owner inherits the physical situation and the exposure that goes with it. Three different routes can then converge on a foreign buyer who thought the charming stone extension was simply part of the house.

The first route is criminal. Building without a required permit is a criminal offence punishable, under Article L480-4 of the Code de l’urbanisme, by a fine of between 1,200 euros and 6,000 euros per square metre of floor area built, demolished or rendered unusable. A 40-square-metre extension can therefore represent a theoretical fine of up to 240,000 euros, and the penalties may be imposed on users of the land, beneficiaries of the works and the professionals who carried them out. In practice, prosecutions target the person who ordered or executed the works, and the criminal action for this type of offence is subject to the standard six-year limitation period running from completion. A buyer who did not build the extension is rarely the criminal target, but the existence of the offence matters because it keeps the file alive: a report drawn up by a sworn officer can trigger the administrative route described below, and it colours every negotiation with the seller.

The second route is the civil action of the commune. Under Article L480-14 of the Code de l’urbanisme, the commune or the intercommunal body responsible for the local plan may apply to the civil court for demolition or for the building to be brought into conformity where it was erected without the required authorisation: « L’action civile se prescrit en pareil cas par dix ans à compter de l’achèvement des travaux. » In other words, the commune has ten years from completion of the works to sue for demolition or compliance. Note the contrast drawn by Article L480-13 of the Code de l’urbanisme, which protects buildings erected in accordance with a permit: a court may only order their demolition if the permit was first annulled by the administrative court. Where no permit was ever obtained, that shield does not exist, and the commune’s action runs directly against whoever owns the building when proceedings start, which may be the foreign buyer rather than the seller who built it.

The third route is administrative and increasingly used. Under Article L481-1 of the Code de l’urbanisme, once an infringement report has been drawn up, the competent authority, usually the mayor, may impose a fine of up to 30,000 euros and serve a formal notice ordering the owner either to bring the building into conformity or to file an application to regularise it. That notice can carry a daily penalty of up to 1,000 euros per day of delay, capped at 100,000 euros in total. One hundred days of inaction therefore cost 100,000 euros, quite apart from the works themselves. Where installations present a proven risk to health or safety, or sit outside urban areas, and the notice has gone unheeded, the authority may even carry out the measures itself at the owner’s expense, or have the buildings demolished at the owner’s expense with the authorisation of a judge. For a non-resident owner who learns of the notice months later through forwarded mail, this administrative track is often the most dangerous, because the clock runs whether or not the owner is in France.

Neighbours add a fourth front. A neighbour who suffers direct harm from an unlawful extension, loss of light, water runoff, overlooking, can sue in the civil courts. The limitation rules here were clarified by the Cour de cassation on 4 December 2025 (Third Civil Chamber, no. 23-23.980, official judgment), which recalled the two tracks of Article 2224 of the Civil Code and Article 2227 of the Civil Code: « les actions personnelles ou mobilières se prescrivent par cinq ans » while real property actions prescribe in thirty years. In plain terms, a neighbour’s claim for damages for personal harm generally expires five years after they knew or should have known the facts, while an action asserting a real property right, such as recovery of common parts appropriated without right, can run for thirty years. A buyer should therefore never assume that an old extension is safe simply because the neighbours have stayed quiet; silence may mean the five-year clock has run, but it may also mean a thirty-year real action is still available, and in a subdivision or jointly owned courtyard the distinction is decisive.

One administrative reflex should be abandoned immediately: the land registry is not a planning authority. An extension can appear on the cadastral plan, be included in the property tax base and be photographed by every estate agent in the department, and still be entirely unlawful. Registration for tax purposes proves existence, not conformity. Only the planning file held by the mairie, the permissions granted and the certificates issued, tells the buyer what the administration considers legal.

B. Ten years of silence is not a permit: the Thalamy trap for buyers who want to build

Buyers are often told that old works are protected by time. There is a kernel of truth in that advice, surrounded by conditions that advisers sometimes forget to mention. The mechanism is Article L421-9 of the Code de l’urbanisme, which provides: « Lorsqu’une construction est achevée depuis plus de dix ans, le refus de permis de construire ou la décision d’opposition à déclaration préalable ne peut être fondé sur l’irrégularité de la construction initiale au regard du droit de l’urbanisme. » In other words, once a building has stood for more than ten years, the administration cannot refuse a new planning application merely because the existing building was irregular. That is a real protection for owners who want to extend, convert or sell with a project attached.

But the same article lists exceptions that swallow many foreign-buyer cases whole. The ten-year reset does not apply where the building endangers its users or third parties, where a demolition action has already been brought, where the building stands on public land or in a protected area, and, crucially, where the building was put up without any planning permission when one was required (« Lorsque la construction a été réalisée sans qu’aucun permis de construire n’ait été obtenu alors que celui-ci était requis » — Article L421-9 of the Code de l’urbanisme). An extension that needed a permit and never got one therefore never benefits from the reset, however old it is. A declaration-level irregularity may be absorbed after ten years; a missing permit is not. This is the single most misunderstood point in cross-border purchases, and it should be checked before the preliminary contract is signed, not after.

Even where the ten-year mechanism could theoretically help, the case law adds a second lock known as the Thalamy rule, after the Conseil d’Etat decision of 9 July 1986 (Epoux Thalamy, no. 51172). A 2024 analysis by a Paris planning lawyer traces the rule and its limits with unusual clarity: a new planning application concerning a property must, as a matter of principle, regularise the existing unlawful works at the same time, because the administration cannot authorise new works that rest on an unlawful base. The Conseil d’Etat extended the logic on 13 December 2013 (Commune de Porspoder, no. 349081), holding that even an extension that does not physically rely on the unlawful building must still be accompanied by regularisation. The only softening comes from an earlier decision of 25 April 2001 (Epoux Ahlborn, no. 207095): where several separable buildings stand on the same land, only the building connected with the new project needs to be regularised.

The practical consequence for a foreign buyer is severe. Suppose the 1990s extension lacks a permit, and the buyer plans a pool house in 2026. The mairie will not simply examine the pool house; it will require the file to address the 1990s extension too. If that extension cannot be regularised, because the setback, height or footprint rules have since tightened, the pool house cannot be authorised either. The buyer’s entire project freezes on works the seller carried out thirty years ago. Worse, drawing the administration’s attention to the property through a new application can be the event that triggers the infringement report, the L481-1 formal notice and the daily penalties described above. This is why experienced conveyancers in France treat any new-project ambition as a reason to audit the old works first, and why a buyer who declares no immediate project at the compromis stage should still secure the file, because intentions change and the trap does not expire.

Two evidentiary points complete the picture. First, the ten-year periods, for the commune’s action and for the L421-9 reset alike, run from completion of the works, and completion is a question of fact the buyer will struggle to prove decades later without invoices, photographs or the seller’s cooperation. Second, a demolition action already engaged blocks the L421-9 reset, so a buyer who completes while the commune’s claim is pending buys a lawsuit, not a reset. Both points argue for documentary conditions in the preliminary contract rather than verbal reassurances at the viewing.

II. The buyer’s counter-attack: the seller, the notaire and the Paris playbook

A. Three liability tracks confirmed by the Cour de cassation in 2025

The most important recent decision for buyers was handed down on 11 September 2025 by the Third Civil Chamber (no. 23-17.751, official judgment). After an authenticated sale of co-owned lots in 2013, the buyers discovered that works had been carried out in the building without prior planning permission and sued the sellers on the basis of the delivery obligation. The Paris Court of Appeal had rejected the claim, reasoning that the absence of a conforming permit belonged to hidden-defects warranty rather than to delivery, since the deed said nothing about permits and the buyers had not proved a risk of having to restore the property. The Cour de cassation quashed that reasoning outright. It held: « Il résulte de ce texte que le vendeur doit délivrer une chose conforme à la convention des parties et, sauf stipulation contraire, à la réglementation en vigueur. » In other words, unless the deed says otherwise, the seller must deliver a property that conforms to the regulations in force, and that includes planning permissions. The breach exists even if the seller did not carry out the unlawful works themselves, even if the deed is silent about permits, and even if the buyer cannot yet show a demolition order. For a foreign buyer, this is the primary weapon: the non-conforming extension is not merely a defect to be weighed, it is a failure to deliver what was promised, governed by Articles 1603 and 1604 of the Civil Code. The remedy follows the general law of obligations, damages, price reduction and, in the gravest cases, resolution of the sale, without the two-year guillotine of hidden defects described below, subject instead to the ordinary five-year limitation for contractual liability.

The second track is the warranty against hidden defects, the vices cachés (hidden defects) regime of Article 1641 of the Civil Code: « Le vendeur est tenu de la garantie à raison des défauts cachés de la chose vendue qui la rendent impropre à l’usage auquel on la destine, ou qui diminuent tellement cet usage que l’acheteur ne l’aurait pas acquise, ou n’en aurait donné qu’un moindre prix, s’il les avait connus. » An extension that exposes the buyer to demolition, blocks every future project through the Thalamy rule or renders the house partly unusable plainly meets that test where the buyer could not have detected the irregularity by an ordinary inspection. But the track has a strict timetable. Under Article 1648 of the Civil Code, « L’action résultant des vices rédhibitoires doit être intentée par l’acquéreur dans un délai de deux ans à compter de la découverte du vice. » Two years from discovery, not from the sale. Discovery usually means the day the buyer receives the formal notice, the permit refusal or the expert report establishing the absence of authorisation, and the buyer must be able to prove that date. The remedies are cancellation of the sale with restitution, or retention of the property with a price reduction assessed by experts. One strategic subtlety matters: professional sellers and sellers who knew of the defect cannot contract out of the warranty, and knowledge is readily inferred where the seller built the extension themselves or lived through the works. Conversely, a private seller who genuinely did not know, and says so credibly, still owes the warranty but may escape the additional damages a bad-faith seller would pay. The 23-17.751 decision does not abolish this track; it adds the delivery track alongside it, and a well-drafted claim now pleads both, so that if the court hesitates on one characterisation, the other sustains the action.

The third track is fraud, the dol (fraudulent misrepresentation) of Article 1137 of the Civil Code: « Le dol est le fait pour un contractant d’obtenir le consentement de l’autre par des manœuvres ou des mensonges. » The same article adds that the deliberate concealment of information known to be decisive for the other party is also fraud. A seller who declares in the deed questionnaire that no works were carried out without authorisation, who hides the correspondence with the mairie or who stages the extension as original construction, commits exactly that concealment. The sanction is annulment of the sale, available even where the defect might otherwise be debated, and the limitation clock is buyer-friendly: the nullity action runs for five years, but time only starts when the fraud is discovered. Specialist guidance on French property disputes regularly underlines the evidentiary flip that makes dol attractive: once concealment of a decisive fact is shown, the seller’s protestations of good faith carry little weight, and damages can accompany annulment. The cost is proof. The buyer must establish not only the irregularity but the seller’s knowledge and intent, through the replies to pre-contract questionnaires, the content of the planning file the seller claimed to have checked, or testimony from neighbours and builders. Where that proof exists, dol is the sharpest blade; where it does not, delivery and hidden defects carry the claim.

Two 2025 decisions then frame the notaire (public conveyancing officer), the figure foreign buyers most often misunderstand. On 8 October 2025, the First Civil Chamber (no. 23-18.336, official judgment) dealt with buyers of a villa whose extension had been built without a permit in breach of a four-metre boundary setback; their own 2011 permit application was refused for that very reason, and they sued the notarial firm for failing to warn them. The Court approved the appeal court’s finding that the notaire’s information had been ineffective, then recalled the governing principle: « Les conséquences d’un manquement à un devoir d’information et de conseil ne peuvent s’analyser qu’en une perte de chance dès lors qu’il n’est pas certain que, mieux informé, le créancier de l’obligation d’information se serait trouvé dans une situation différente et plus avantageuse. » In other words, the notaire’s failure is compensated as a lost chance, to have walked away or to have negotiated the price down, quantified as a proportion of the loss, not as automatic full reimbursement of demolition and rebuilding costs. But the same decision contains a buyer-friendly second half: the judge may identify and compensate that lost chance even though the buyers had asked for full repair, because, in the Court’s words, a reparable loss of chance is « la disparition actuelle et certaine d’une éventualité favorable », and a judge cannot refuse to repair damage whose existence in principle has been established. The lesson is balanced: sue the notaire, but frame the loss correctly as a lost chance with a serious percentage, supported by evidence that a warned buyer would have acted differently, such as a withdrawn offer on a comparable compliant property or a documented price negotiation.

The companion decision of 9 July 2025 (First Civil Chamber, no. 23-20.456, official judgment) sets the limits of that liability and corrects a common foreign-buyer illusion that the notaire verifies everything. A buyer who had signed a bilateral promise for a house on agricultural land later refused to complete and blamed the notaire for not obtaining the planning certificate before the promise. The Court rejected the claim: « Le notaire n’est pas tenu d’obtenir la délivrance d’un état de l’immeuble préalablement à la conclusion d’une promesse de vente, dès lors que cet avant-contrat est destiné à arrêter la volonté des parties sans attendre l’expiration des délais utiles à l’obtention des documents administratifs et hypothécaires nécessaires à la perfection de la vente, ni de vérifier la possibilité de modifier la construction existante, qui n’est pas mentionnée à l’acte et dont il n’a pas été avisé, à moins qu’il n’ait pu raisonnablement l’ignorer. » In other words, at the preliminary stage the notaire is not the buyer’s surveyor; the duty to warn attaches to what the file shows and to the project the buyer has actually declared, and the buyer who never mentions an extension project weakens any later claim that the notaire should have investigated buildability. The combined message of the two rulings is coherent and practical: declare the project in writing at the promise stage so the notaire’s duty engages, then hold the notaire to the effectiveness of the warning given, not to an imaginary duty to detect every irregularity unaided. Readers seeking a structured overview of how these property claims fit within French real-estate litigation can consult the firm’s Paris real-estate law practice for the broader framework before drilling into the extension-specific steps below.

B. The foreign buyer’s playbook: evidence, deadlines, money and Paris specifics

Everything above converges into a sequence that should start before the compromis de vente (preliminary sale agreement) and continue for years after the keys change hands. Foreign buyers who follow it spend a few hundred euros early to avoid hundreds of thousands later.

Before signing, commission a planning audit, not just a building survey. The surveyor checks cracks and damp; the planning audit checks legality. Its components are fixed: an extract of the local plan with the applicable zone rules, setbacks, heights and footprint ratios; every permit on the property file at the mairie with its plans, completion declarations and conformity certificates; the two planning certificates, the simple information certificate and the operational certificate stating whether the buyer’s project is feasible; and a comparison between the authorised plans and the existing building, measured on site. In Paris and the inner suburbs, add three local layers. First, many buildings sit within the surroundings of historic monuments or remarkable heritage sites, where the architect of state buildings must be consulted and unlawful works are effectively un-regularisable. Second, co-owned buildings require the general meeting’s authorisation for works affecting common parts or the external appearance, on top of planning permission, so a Paris extension needs two green lights, not one. Third, several communes apply density and green-space rules that make older extensions non-compliant by today’s standards, which is precisely when the Thalamy rule bites hardest. The audit should be a written condition of the promise: a clause suspending completion on confirmation that the existing buildings match the permits, or on obtaining an operational certificate covering both the existing state and the buyer’s project, converts a verbal assurance into an exit right with deposit refund.

At the promise stage, declare the project and load the questionnaire. State in the deed the intended works, even vaguely described, so the notaire’s duty to warn is engaged under the 23-20.456 logic. Require the seller to answer in writing whether works were carried out, with or without authorisation, during their ownership and, to their knowledge, before it. Ask for the invoices, the builders’ names and the planning file references. A seller who answers precisely creates the paper trail for a delivery or hidden-defects claim if the answers prove false; a seller who refuses to answer signals the risk and justifies a price reduction or a specific warranty clause. Never accept a generic clause stating that the buyer takes the property as is as a substitute for these answers: such clauses do not cure a lack of delivery conformity under the 23-17.751 reasoning where the regulations point the other way, and they collapse entirely if concealment is later proved.

After discovery, move on three clocks at once. First, the administrative clock: respond to any formal notice within its deadline, seek a stay or extension in writing, and file the regularisation application immediately if the extension is regularisable, because a pending compliant application changes the negotiation with the mairie and with the judge. Engage a local architect at once; the file needs measured plans, insertion views and a demonstration that the current rules are met or that a variance is available. Second, the judicial clock against the seller: place the two-year hidden-defects deadline under diary control from the proven discovery date, send the formal demand letter that fixes that date in writing, and commission the expert report that establishes both the absence of authorisation and the consequences, demolition risk, loss of use, blocked project, price impact. Plead delivery non-conformity alongside hidden defects so the claim survives a characterisation dispute, and add dol with annulment as an alternative where concealment evidence exists, remembering that its five years run from discovery. Third, the notaire clock: notify the notarial firm and its insurer promptly, quantify the lost chance with comparable transactions rather than bare assertion, and coordinate the actions so findings in one forum serve in the others.

On money, think in four layers and put figures on each before deciding to litigate. Layer one is regularisation cost: architect fees, application taxes, compliance works and the development tax that a new permit can trigger, often 5 to 8 percent of the construction value in Ile-de-France. Layer two is administrative exposure: up to 30,000 euros of mayoral fine plus daily penalties of up to 1,000 euros per day capped at 100,000 euros, against which the buyer weighs the cost of immediate compliance. Layer three is the property loss: the discount a warned buyer would have obtained, typically assessed by a court-appointed valuer comparing compliant and non-compliant values, which forms both the delivery-damages base and the hidden-defects price reduction. Layer four is the notaire’s contribution, a percentage of that loss reflecting the lost chance, commonly argued between 30 and 70 percent depending on how decisive the missing warning was. A Paris example ties the layers together: a 35-square-metre rear extension without permit in a conservation-sensitive street, regularisable only after removing 8 square metres to meet the setback, might cost 25,000 euros in works and fees, carry 15,000 euros of accrued penalties if a notice has run for two months at a moderated daily rate, justify a 60,000-euro price-reduction claim against the seller, and support a 40-percent lost-chance claim against the notaire. The figures are illustrative, but the method, valuing each layer separately before negotiating globally, is what turns an emotional dispute into a recoverable file. An overview of how Paris property disputes of this type are handled in practice is available through the firm’s real-estate team in Paris, which regularly coordinates architects, valuers and bailiffs on such multi-front files.

Finally, insure and finance with eyes open. The buyer’s multi-risk home policy may exclude damage to unlawful parts and may decline to defend planning proceedings; notify the insurer of the situation rather than letting it discover the exclusion after a claim. Lenders, for their part, can treat a proven irregularity affecting value as a breach of the loan’s asset covenants on refinancing, and a future buyer’s lender will demand the regularisation the current buyer skipped, which is why curing the file also protects resale. None of this requires panic. It requires the file, the dates and the three liability tracks to be handled together, quickly, and in writing.

Conclusion

An extension built without permission is not a cosmetic footnote to a French purchase; it is a parallel legal status that travels with the building into the buyer’s hands. The commune has ten years to demand demolition or compliance, the mayor can fine and penalise daily without going to court first, neighbours keep their own actions on separate clocks, and any future project reopens the entire file through the Thalamy rule. The ten-year reset of Article L421-9 helps only where a permit existed and was exceeded, never where a required permit was simply missing. Against those risks, the Cour de cassation’s 2025 case law gives the buyer three structured claims: delivery of a conforming property under Article 1604 even when the deed is silent, the two-year hidden-defects warranty from discovery under Articles 1641 and 1648, and annulment for concealed fraud under Article 1137, completed by a calibrated lost-chance action against the notaire whose warning was ineffective. The buyers who prevail are not those who hoped the old works would stay buried; they are those who audited the planning file before signing, declared their project in the promise, dated their discovery in writing and ran the administrative, seller and notaire tracks together. For a foreign owner, that discipline is the difference between a house with a past and a lawsuit with an address.