You live in France, file a French tax return every spring, and hold a carte de séjour (French residence permit) that Brexit made mandatory. Back in Britain you still own the London flat you kept as a safety net, and now you have accepted an offer on it. Completion day in London will feel like the end of the story, but for tax it is the beginning: two countries will want a share of the gain, on two different calendars, under two different sets of rules. Britain taxes the sale first because the flat stands on British soil, and HMRC gives you only 60 days after completion to report and pay. France taxes the very same gain because you live in France, and the French tax office expects its own filing within a month of the sale, plus the gain on your next annual return. The instrument that stops this from being double taxation is the Franco-British tax treaty signed in London on 19 June 2008, which organises a tax credit in France capped at the French tax on the gain. The same treaty logic extends to French social charges on the gain, the courts have spelled out exactly what proof unlocks the credit, and French procedure gives you a clear path to challenge any bill that ignores the treaty. This guide works through each step in order: why France taxes a London sale, how the treaty divides the bill, which forms and deadlines apply on each side of the Channel, what evidence to keep, and how to contest double taxation before the French courts.

I. Why France Taxes Your London Sale and What the Treaty Lets Britain Take First

A. Living in France Makes Your Worldwide Gains Taxable in France, Including Your London Flat

The starting point is brutal in its simplicity. Article 4 A of the French General Tax Code (code général des impôts, the main French tax statute) provides: “Les personnes qui ont en France leur domicile fiscal sont passibles de l’impôt sur le revenu en raison de l’ensemble de leurs revenus. Celles dont le domicile fiscal est situé hors de France sont passibles de cet impôt en raison de leurs seuls revenus de source française.” In plain English: if your tax home (domicile fiscal, the tax residence concept used by French domestic law) is in France, you pay French income tax on all of your income and gains, from everywhere. Your London salary of years past is history, but the gain on your London flat is current worldwide income in French eyes, even though the property, the buyer, the conveyancer (solicitor) and the Land Registry are all British. Only people whose tax home is outside France are taxed merely on their French-source income.

And the tax home test is easy to meet without noticing. Article 4 B of the same code provides: “Sont considérées comme ayant leur domicile fiscal en France au sens de l’article 4 A : a. Les personnes qui ont en France leur foyer ou le lieu de leur séjour principal ; b. Celles qui exercent en France une activité professionnelle, salariée ou non, à moins qu’elles ne justifient que cette activité y est exercée à titre accessoire ;” Your foyer (family home) or your séjour principal (main place of stay) in France is enough on its own. Any one of the three statutory tests being met makes you a French tax resident: home, main stay, or professional activity. Keeping a British passport, a UK bank account, or even the London flat itself does not keep your tax home in Britain if your life is in France. Readers who split the year between the two countries and face competing residence claims from both HMRC and the French tax office should work through the treaty tie-breaker first, because everything that follows assumes France is your treaty residence.

The mirror image in the statute confirms the trap. Article 164 B of the General Tax Code lists income treated as French-source, starting with: “Sont considérés comme revenus de source française : a. Les revenus d’immeubles sis en France ou de droits relatifs à ces immeubles ;” A flat in Hackney or Manchester is not a revenu de source française. Had you stayed in London, France would have no claim on the sale at all. It is your French residence that pulls a purely British gain into the French net, not the location of the property.

Once inside that net, the gain falls under the standard French regime for private property sales. Article 150 U of the General Tax Code provides: “Sous réserve des dispositions propres aux bénéfices industriels et commerciaux, aux bénéfices agricoles et aux bénéfices non commerciaux, les plus-values réalisées par les personnes physiques ou les sociétés ou groupements qui relèvent des articles 8 à 8 ter , lors de la cession à titre onéreux de biens immobiliers bâtis ou non bâtis ou de droits relatifs à ces biens, sont passibles de l’impôt sur le revenu dans les conditions prévues aux articles 150 V à 150 VH .” A plus-value immobilière (property capital gain) realised by a private individual on the sale for consideration (cession à titre onéreux) of a built or unbuilt property is taxable under that regime, wherever the property stands. Do not expect the main-home exemption to save you: the statute exempts property “Qui constituent la résidence principale du cédant au jour de la cession” (which is the seller’s main home on the day of sale), and the flat you are selling in London is by definition not the French home you live in.

The French tax administration says all of this openly on its official website. Its guidance page for residents selling abroad states: According to the French tax administration’s guidance for residents selling property abroad, where no tax treaty exists with the country where the building stands, a French resident selling a foreign property faces the same taxes as if the property were in France. Without a treaty, your London flat would be taxed exactly as if it stood in Lyon. With a treaty, the picture changes: The same official guidance explains that where a treaty is signed, gains on building sales are in principle taxable where the buildings stand, while the treaty can also preserve a right to tax in France alongside the situs state. Taxation in Britain first, plus taxation in France: that double exposure is lawful, and the treaty then neutralises it with a credit, which is the subject of the next section.

Fortunately, there is a treaty, and it is the modern one. The French official tax commentary (the Bulletin officiel des finances publiques, the administration’s published doctrine) records: a double-taxation convention covering income tax and capital gains was signed in London on 19 June 2008 between France and the United Kingdom (BOI-INT-CVB-GBR-10). The treaty was published in France by Decree No. 2010-20 of 7 January 2010, and its detailed commentary by article sits in the same official bulletin at BOI-INT-CVB-GBR-10-20. Brexit did not terminate it: tax treaties are bilateral instruments independent of European Union membership, and this one continues to allocate taxing rights between Paris and London exactly as before.

B. How the 2008 Treaty Splits the Bill Between Britain and France, and Why the Credit Is Capped at the French Tax

The treaty gives Britain the first right to tax, and France a second right with a credit attached. Because the flat is immovable property situated in the United Kingdom, the gain is taxable there under the treaty’s situs rule, which the French administration summarises exactly as quoted above: gains on the sale of buildings are in principle taxable where the buildings stand. HMRC therefore collects British Capital Gains Tax computed under purely British rules: British conception of the chargeable gain, British acquisition costs, British reliefs such as any period when the flat genuinely was your only or main home, and the British annual exempt amount for the tax year of completion. Those British figures are a matter for HMRC and, if needed, a British adviser. What matters for your French position is that the British tax is real, assessed, and paid, because it is the key that unlocks the French credit.

France then computes its own tax on its own gain, and the two computations will almost never agree. The French gain is the sale price minus the French-law purchase price, with French-law additions and French-law holding-period relief, converted into euros. The British gain follows British law in pounds. Neither administration is bound by the other’s arithmetic. The treaty reconciles them with a credit that runs only one way and only up to a ceiling. The administration’s summary of the mechanism, on the same guidance page, is that a credit against the French tax is available and measured by the treaty: it equals the French tax computed on the gain, or the foreign tax where that is lower, and never more than the French tax. Read that sentence twice, because a surprising amount of expatriate commentary gets it backwards. The credit equals the French tax on the gain, or the foreign tax if that is lower, and it can never exceed the French tax. If the British bill is higher than the French computation, France wipes its own tax out but refunds nothing. If the British bill is lower, France collects the difference. If the French computation produces no tax at all, for example after full holding-period relief, there is no French tax to credit against and nothing to pay in France, but the British tax stands exactly as HMRC assessed it.

The treaty text itself, as interpreted by France’s highest administrative court, says the same thing with full legal force. Article 24 of the 2008 convention provides that the French credit is, “pour les revenus non mentionnés à l’alinéa (ii), au montant de l’impôt français correspondant à ces revenus à condition que le résident de France soit soumis à l’impôt du Royaume-Uni à raison de ces revenus” (for income not listed in subparagraph (ii), equal to the amount of the French tax attributable to that income, on condition that the French resident is subjected to United Kingdom tax on it). The Conseil d’État (the supreme court for administrative and tax disputes, whose decisions bind the French administration) examined this machinery in a formal opinion, Opinion of 12 February 2020, No. 435907 (ECLI:FR:CECHR:2020:435907.20200212), given on questions referred by the administrative tribunal of Cergy-Pontoise about British-source income and the same Article 24. Three of its answers matter directly to your London sale.

First, France cannot use the treaty’s allocation of taxing rights to Britain as an excuse to tax without giving the credit. The court held: “que l’expression ” nonobstant toute autre disposition de la présente Convention ” qui figure à la première phrase de ce paragraphe, signifie qu’alors même que d’autres stipulations de la convention prévoient que certains revenus sont imposables ou ne sont imposables qu’au Royaume Uni, ces revenus peuvent néanmoins être pris en compte pour le calcul de l’impôt français.” That opening phrase, operating notwithstanding any other provision of the Convention, means France may indeed include the British gain in its calculations, but the inclusion triggers the credit mechanism rather than replacing it. A French assessment that adds your London gain to your taxable income while refusing any credit is therefore contrary to the court’s reading of the treaty, and that contradiction is precisely what you would put at the centre of a challenge.

Second, the condition for the credit is lighter than most taxpayers fear. The court explained: “signifie que les revenus en cause doivent être compris dans la base de ” l’impôt du Royaume-Uni ” au sens du a) du paragraphe 1 de l’article 2 de la même convention, sans que le résident de France en soit exonéré à raison de son statut ou de son activité. En revanche, cette condition n’exige pas que les revenus en cause aient été soumis à une imposition effective.” In other words, the gain must fall within the scope of one of the British taxes covered by the treaty (income tax, corporation tax, capital gains tax), without the seller being exempted by personal status, but there is no requirement of actual effective taxation. The court drew the practical conclusion in the very next paragraph: “la condition prévue à l’alinéa (i) du a) du paragraphe 3 de l’article 24 de la convention doit être regardée comme satisfaite s’il est établi par le résident de France qu’il a déclaré les revenus en cause au Royaume-Uni, parce que ces revenus étaient compris dans la base de l’un des impôts énumérés au a) du 1 de l’article 2 de la convention, alors même qu’il n’aurait acquitté dans cet Etat aucun impôt à raison de ces revenus.” Declaring the gain to HMRC, within the scope of a covered British tax, satisfies the condition even if no British tax was ultimately paid on it. For your sale, the lesson is concrete: file the British return and keep the filing receipt whatever the final British figure, because that declaration is the documentary key to the French credit.

Third, the credit machinery covers French social charges (prélèvements sociaux, the CSG-CRDS family of levies charged alongside income tax on investment and property gains) as well as income tax itself. The court noted: “S’agissant des contributions sociales françaises, qui font partie de ” l’impôt français ” défini au b) du paragraphe 1 de l’article 2 de la convention”, and held that no provision of Article 24 makes the credit against those contributions conditional on the income having entered the base of some equivalent British levy. This matters because the social charges on a property gain are often the part of the French bill that surprises British sellers most. They count as French tax for treaty purposes, they enter the capped credit in the same way, and France cannot deny the credit on the ground that Britain has no CSG of its own. The statutory base confirms the charge exists on gains: the Social Security Code provides that “Les personnes physiques fiscalement domiciliées en France au sens de l’article 4 B du code général des impôts sont assujetties à une contribution sur les revenus du patrimoine assise sur le montant net retenu pour l’établissement de l’impôt sur le revenu”, expressly including “Des plus-values, gains en capital et profits soumis à l’impôt sur le revenu” (capital gains and profits subject to income tax).

Put the pieces together with a simple illustration using round figures rather than anyone’s real rates. Suppose HMRC assesses £24,000 of British tax on the sale. Suppose the French-law computation on the same sale produces €20,000 of French income tax plus €10,000 of social charges, meaning €30,000 of total French tax on the gain. The treaty credit equals the French tax up to its full amount, so France credits €30,000, the French bill falls to zero, and the £24,000 paid in Britain is the final cost. Reverse the figures: British tax £10,000 against a French-law total of €30,000. The credit is capped at the foreign tax actually within the French ceiling, so France credits the equivalent of the £10,000 and collects the balance. Either way the same gain is never fully taxed twice, provided you declare on both sides and claim the credit where it belongs, which is the operational subject of Part II.

II. Paying on Time on Both Sides and Building the File That Cancels Double Tax

A. Which Forms to File and When: Sixty Days in Britain, One Month in France, Then the Annual Return

The British deadline runs first and it runs fast. The official GOV.UK guidance for sellers of British property requires any Capital Gains Tax due on UK residential property to be reported and paid within 60 days of completion (GOV.UK, UK property sales on or after 6 April 2020). Completion (completion, the day the buyer’s money is transferred and the keys change hands) starts the clock, not exchange of contracts and not registration at the Land Registry. The same guidance warns that interest and penalties can follow late reporting and payment. The report is made through HMRC’s online UK property account, a system separate from Self Assessment, and it applies to you as a French resident exactly as it would to a British resident seller. If you owned the flat jointly, for example with a spouse or civil partner, each owner reports their own share: the guidance confirms each owner reports their own share of the gain or loss. Remember that the GOV.UK pages describe the general position: gains can be taxable in more than one country, and relief may be claimable where tax is paid twice (GOV.UK, selling overseas property). For you, the other country is France, and the relief runs through the French credit described in Part I rather than through any British form.

The French deadline surprises even careful sellers, because there is no French notary (notaire, the public officer who normally computes, withholds and files property gains on French sales) standing between you and the tax office when the property is in London. On a French sale the notary files everything at the signing appointment; on a London sale you are your own notary. The impots.gouv.fr guidance described in Part I continues with the operative instruction: sellers whose foreign gain is not exempt from French tax under a treaty must file return 2048-IMM within the month after the sale at their local tax office (impots.gouv.fr, property outside France). Form 2048-IMM is the standard French property-gain return, and within the month following the sale at the tax office covering your French home (service des impôts dont vous relevez) is a shorter fuse than most British sellers expect. For readers in Paris and the Île-de-France, that means the service des impôts des particuliers (local personal tax office) for your arrondissement or commune, not a specialised central desk: the filing follows your French address. Attach the computation of the French-law gain, pay any balance the form shows, and keep the stamped copy. If the gain is fully exempt in France under the treaty computation, the same official page relieves fully exempt sellers of that filing: where the foreign gain is exempt from French tax, only form 2047, the return for foreign-source income, is required, with no 2048. Form 2047 travels with your annual income tax return the following spring in any event, so the treaty position on the sale appears twice in your French paperwork: once within a month on form 2048-IMM, once the next year on form 2047.

Two common misunderstandings deserve explicit correction because they each cost money. The first is the belief, repeated in much expatriate commentary, that you need only declare the sale in France the following April on the online return. The official guidance requires the one-month 2048-IMM filing in addition, and missing it exposes you to late-filing interest and penalties on the French side even where the treaty credit would ultimately have reduced the French tax to nothing. Penalties computed on a nil balance are small, but penalties computed on a gain the office reconstructs without your evidence are not, which leads directly to the proof section below. The second misunderstanding is the belief that paying HMRC in full ends the matter. Britain’s 60-day payment settles the British debt; it does not file anything in France, it does not claim the French credit for you, and the exchange of information between the two administrations means the French office will very likely learn of the sale with or without your filing. A British completion statement plus an HMRC payment reference, unaccompanied by any French filing, is how double taxation happens in practice: each country taxes correctly under its own law, and nobody applies the treaty because nobody was asked to.

Order the paperwork chronologically and the system becomes manageable. Before completion, assemble the purchase file: the original acquisition deed with its price, acquisition costs such as stamp duty and conveyancing fees, and every invoice for works. At completion, note the exact completion date for the British 60-day clock and keep the completion statement showing the sale price and deductions. Within 60 days, file the HMRC UK property return online and pay the British tax, keeping the submission receipt and payment reference. Within one month of the sale, file form 2048-IMM at your local French tax office with the French-law computation, claiming the treaty credit. The following spring, report the gain on form 2047 with your annual French return so the two filings reconcile. Readers who let the flat before selling it, and therefore have rental years behind them as well as a sale, face a second treaty computation for the rents: the mechanism is explained in our companion guide on how British landlords living in France declare UK rental income and claim the treaty credit against French tax and social charges. Readers travelling in the opposite direction, leaving France and selling a French home from Britain, face the mirror-image regime with its own exemption windows, which is covered in our guide on selling a former French home after returning to Britain.

B. What Proof to Keep, How the Courts Test It, and How to Challenge a Bill That Ignores the Treaty

The French-law gain is where files are won or lost, because the purchase price can be increased only with evidence that meets strict conditions. The administrative appeal court of Douai set out the rule in a judgment on a disputed property gain, CAA Douai, 25 January 2024, No. 22DA02566: “le cédant d’un immeuble peut majorer, pour la détermination du montant de la plus-value immobilière réalisée lors de la cession de ce bien, le prix d’acquisition du montant des dépenses qu’il a exposées pour y faire réaliser, par une entreprise, une ou plusieurs des prestations de travaux qu’elles mentionnent.” Three conditions emerge from that single sentence, and each has a practical consequence for a London sale. The works must have been carried out by a business (par une entreprise): invoices from a VAT-registered builder, a roofing firm, or a kitchen company count, while do-it-yourself materials receipts and cash payments to an unregistered handyman do not. The expenditure must be proved with documents (sur justificatifs, as the underlying statute puts it): keep the dated invoices, the corresponding bank transfers from your own account, and ideally the firm’s confirmation that the work concerned your flat. And the works must be of a qualifying kind, construction, rebuilding, extension or improvement, not mere repairs or letting expenses already deducted elsewhere. In the Douai case the seller’s six-figure uplift failed precisely because the paper trail ran through his own company’s shareholder loan account with no filed accounts or tax returns to support it, a reminder that the tax office, and then the court, follows the documents rather than the story. For a London refurbishment, that means asking your British contractor now, before memories fade, for itemised VAT invoices matching the transfers on your bank statements, and having the key pages translated if the French inspector cannot read them: our guide on getting British documents accepted in France, from apostille to sworn translation explains which formalities make a British file receivable.

The price of weak evidence is not merely a higher gain. Where the administration reassesses under the taxation d’office procedure (assessment made by the office without your cooperation, typically after ignored notices), the burden of proof reverses onto you. The Douai court recalled the mechanism in the same judgment: after noting that the disputed reassessments had been issued under that procedure, it held that the taxpayer bore the burden and had to demonstrate the excessiveness of the charges. The lesson for your sale is procedural as well as evidentiary: answer every French request for information within its deadline, file the 2048-IMM even when you believe nothing is due, and never let a reassessment go unanswered into taxation d’office territory, because from that moment you argue uphill with the burden on your shoulders.

Social charges deserve a separate paragraph because two different defences circulate and only one of them works reliably. The first defence, the treaty credit, works: as shown in Part I, the Conseil d’État treats French social charges as part of French tax covered by Article 24, with no requirement of a matching British levy, so the credit absorbs them up to the French ceiling. The second defence, sometimes suggested, invokes European social-security coordination to argue that a person insured in Britain should not pay French charges at all. The legal anchor for that idea is real but its post-Brexit scope needs care. The Court of Cassation recently restated the principle in a published judgment, Cass. 2nd Civil Chamber, 25 September 2025, No. 22-24.634 (ECLI:FR:CCASS:2025:C200872, published in the Bulletin): the European coordination regulations “consacrent le principe d’unicité de la législation de sécurité sociale, selon lequel la personne à laquelle les règlements s’appliquent n’est soumise qu’à la législation d’un seul État membre, en sorte que celle-ci, affiliée à un régime de sécurité sociale d’un État membre, ne doit pas contribuer au régime de sécurité sociale d’un autre État membre (CJUE, arrêt du 26 février 2015, De Ruyter, C-623-13, point 35).” That single-legislation principle (unicité de législation) protects people insured in one Member State from contributing in another, and it is the foundation of the famous De Ruyter line of cases. Since Brexit, however, the United Kingdom is no longer a Member State and the coordination regulations no longer cover British insurance in the general case, with only the specific protections of the Withdrawal Agreement for people already in cross-border situations before the end of 2020. A British seller relying on British National Insurance contributions alone to escape French social charges on a 2026 sale is therefore building on uncertain ground, while the treaty credit route rests on a Conseil d’État opinion about this very convention. Claim the credit in every case; treat any affiliation-based exemption as a secondary argument for situations genuinely covered by the Withdrawal Agreement, documented with the applicable certificate, and never as a reason to skip the French filing.

When the bill is wrong, French procedure offers a structured challenge with a hard deadline. The first step is always the prior complaint to the administration (réclamation contentieuse, the written claim filed with the tax office seeking discharge or reduction of the assessment), which in treaty cases sets out the Article 24 computation line by line: the British gain declared to HMRC with the filing receipt, the British payment reference, the French-law gain with its evidence, and the resulting capped credit. The administration must answer, and its express or implied rejection opens the second step, the appeal to the administrative tribunal (tribunal administratif, the first-instance court for tax disputes). Here the deadline is statutory and unforgiving. Article R. 421-1 of the Code of Administrative Justice provides: “La juridiction ne peut être saisie que par voie de recours formé contre une décision, et ce, dans les deux mois à partir de la notification ou de la publication de la décision attaquée.” Two months from notification of the rejection, not a day more, subject only to the narrow statutory extensions. For Paris residents that appeal goes to the tribunal for your department; the Conseil d’État opinion discussed above reached the supreme court precisely as an advisory opinion requested by the administrative tribunal of Cergy-Pontoise, which shows the ordinary path working as designed: local tribunal, reference on the treaty point, supreme court guidance, then judgment applying it. Typical grounds in a London-sale case are a refused or miscalculated Article 24 credit, social charges assessed without credit, a reconstructed gain that ignores evidenced acquisition costs or qualifying works invoices, or penalties imposed despite spontaneous British and French filings. Each ground must be raised with its document attached, because the tribunal decides on the file, and the file is the one Part II has taught you to build from completion day onwards. If instead the error lies on the British side, an overpaid or misreported figure to HMRC, the correction runs through HMRC’s amendment process for the UK property return within its own time limits, after which the corrected British figure feeds a corrective French claim.

Conclusion

Selling a London flat while living in France means satisfying two complete tax systems that do not talk to each other, and the treaty is the only bridge between them. Britain taxes first because the property is British, within 60 days of completion, under British rules. France taxes second because you are French-resident, starting with a 2048-IMM filing within a month of the sale and continuing onto form 2047, under French rules. The 2008 convention then caps the French bill with a credit equal to the French tax on the gain, a mechanism the Conseil d’État has confirmed covers social charges too and requires no effective British taxation beyond a genuine British declaration. The sellers who pay twice are almost never victims of the law: they are victims of a missed deadline, a missing invoice, or an unfiled form. Keep the HMRC filing receipt and payment reference, keep the purchase deed and every qualifying works invoice from a real business with its matching bank transfer, file the French forms on time, and check that the assessment applied the capped credit before paying a euro of French tax. Where the assessment ignored the treaty, the complaint and then the two-month tribunal appeal will enforce it. Handled in that order, a London sale from a French home is an administrative exercise with a predictable outcome, not a double bill.