When you move from London, Manchester or Edinburgh to Paris, Lyon or the Dordogne, one question follows you across the Channel with your savings: does my ISA stay tax-free? The short answer, which surprises many British newcomers, is no. A Cash ISA, a Stocks and Shares ISA, a Lifetime ISA and even Premium Bonds keep their British tax shelter only while the United Kingdom taxes you. Once you are fiscally domiciled in France, France taxes your worldwide investment income, and the Franco-British double tax treaty of 19 June 2008 decides which country gives way, by how much, and by which method. This guide explains, for a British reader with no French legal training, why the wrapper fails, how each product is taxed, which forms to file, which fines to avoid, and how to challenge a wrong bill from Paris or elsewhere in France. Every French term is explained at first use.

I. Why your ISA loses its shelter the day you become French tax resident

A. French residence taxes your worldwide savings, and the treaty tie-breaker confirms it

French domestic law starts with a wide net. Article 4 B of the French Tax Code (Code général des impôts) provides that “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” — in plain English, anyone whose home (foyer) or principal place of stay is in France is treated as fiscally domiciled here: Article 4 B, Code général des impôts (Légifrance). Two further tests can also catch you: carrying on a professional activity in France, or having the centre of your economic interests here. Meeting any one of the three is enough, and the same article adds that treaty tie-breaker rules can still deprive France of residence where a double tax convention allocates it elsewhere.

In practice, a British retiree renting a flat in the 7th arrondissement of Paris, a remote worker whose family home is in Bordeaux, or a seconded executive whose children attend school in France will normally meet the foyer test from the year of arrival. From that point, Article 4 A makes them liable to French income tax on worldwide income, including interest and dividends that the United Kingdom would have left alone inside an ISA. The French avis d’imposition (tax assessment notice) will therefore expect to see those amounts, converted into euros at the annual rate, even where no British tax was charged.

The treaty does not rescue the wrapper. The France–United Kingdom Convention signed in London on 19 June 2008, in force since 18 December 2009 and effective in France from 1 January 2010, allocates taxing rights between the two states but nowhere recognises the ISA as an exempt vehicle: 2008 UK–France Double Taxation Convention (GOV.UK). Its residence article follows the familiar OECD pattern: where both states claim you under domestic law, the treaty tie-breaker looks first to a permanent home, then to the centre of vital interests, then to habitual abode, then to nationality, and finally to mutual agreement. A Briton who keeps a London flat but lives year-round with family in France will generally lose that tie-break to France, because the permanent home available to them and the closer personal and economic relations point here. Our companion guide to your first French tax return after the move (forms 2042, 2047 and 3916) walks through that residence analysis step by step.

The British side of the story is worth stating precisely, because the misunderstanding starts there. The United Kingdom government presents ISAs as a way to save free of United Kingdom tax (Individual Savings Accounts: overview (GOV.UK)). That promise is a creature of British domestic law — exemption from United Kingdom income tax and capital gains tax on returns inside the wrapper — and it binds His Majesty’s Revenue and Customs, not the French Direction générale des finances publiques. France never signed an agreement exempting ISA returns, and Article 120 of the French Tax Code expressly pulls foreign-source investment income into the French base, including “Les dividendes, intérêts, arrérages et tous autres produits des actions de toute nature” from companies whose registered office sits abroad: Article 120, Code général des impôts (Légifrance). The same article reaches government stock and foreign bonds through “Les intérêts, arrérages, et tous autres produits de rentes, obligations et autres effets publics des gouvernements étrangers”. Cash ISA interest, ISA dividends and most bond fund distributions therefore fall squarely inside French taxable income once you live here.

A useful illustration of treaty method comes from the French supreme court for civil and commercial matters, the Cour de cassation. In a trust and treaty-scope dispute decided on 11 February 2026 (appeal no. 23-14.305), the court recalled the standard formula that “La présente Convention s’applique aux impôts sur le revenu et sur la fortune perçus pour le compte de chacun des Etats contractants, quel que soit le système de perception.”: Cour de cassation, chambre commerciale, 11 février 2026, n° 23-14.305. The wording there concerned the Franco-Canadian convention, but the reasoning method is identical for the Franco-British text: the court reads the treaty article by article, checks which tax each state was entitled to levy, and then applies the elimination-of-double-taxation article. For ISA income, that means asking separately for interest, for dividends and for any other gain which article governs, and then applying the French credit method rather than assuming an exemption.

Two practical consequences follow before a single euro of product detail. First, you should not close everything in panic the week you arrive, but you should stop assuming the wrapper protects you and budget for French tax from the year you meet the foyer test, pro rata where the move happens mid-year. Second, keep every British voucher, consolidated tax certificate and NS&I prize statement from the year of arrival: France will ask for gross amounts, dates of payment and any foreign tax actually withheld, and reconstructing twelve months of ISA statements under time pressure is how errors begin.

B. Cash ISA interest, stocks-and-shares dividends and Premium Bond prizes: what France taxes and what the treaty allows

Start with the simplest product, the Cash ISA. Interest paid inside a Cash ISA is exempt in the United Kingdom, but in France it is ordinary taxable interest. The treaty rule for interest is unusually clean. Article 12, paragraph 1 of the 2008 Convention gives the exclusive right to tax interest to the state where the beneficial owner is resident (Article 12, UK–France Convention (GOV.UK)). In everyday terms, where you are resident in France and beneficially own British-source interest, only France may tax it. The United Kingdom should not withhold, and in practice British banks and building societies pay Cash ISA interest gross. France then taxes it under domestic law: Article 125 A of the Tax Code imposes the withholding mechanics for interest paid through a French paying agent, and states the rate rule plainly — “Le taux du prélèvement est fixé à 12,8 %.” (Article 125 A, Code général des impôts (Légifrance)). For interest paid directly by a British institution with no French intermediary, no French prélèvement (advance levy) is taken at source; the 12.8 per cent instead arrives as part of your final French income tax bill under the flat-tax system described below, plus 17.2 per cent of social levies (prélèvements sociaux) where you are affiliated to the French social security system. Cash ISA interest is therefore fully declarable, fully taxable in France, and credit-free because there is no British tax to credit.

Stocks and Shares ISA dividends are slightly more layered. British domestic law currently pays dividends without withholding, so most investors see no British tax deducted. The treaty nevertheless reserves a taxing right to the source state: Article 11 provides that dividends arising in one state and paid to a resident of the other may be taxed in that other state, and may also be taxed in the source state, but caps the source-state tax at 15 per cent of the gross dividend where the recipient is the beneficial owner (Article 11, UK–France Convention (GOV.UK)). For a French-resident holder of a Stocks and Shares ISA invested in United Kingdom equities, France taxes the gross dividend and, if any British withholding had been levied up to the 15 per cent ceiling, would grant a credit under Article 24 within French limits. Where, as is usual, no British withholding was levied, France taxes the full dividend with no credit to give. The position changes for an ISA holding American or European equities through a British platform: the United States, for example, typically withholds 15 per cent on dividends paid to a French resident claiming treaty relief via a W-8BEN, and France then grants a credit equal to the foreign tax within the limit of the French tax attributable to that income, under the Article 24 mechanics explained in Part II. Keep the American 1042-S or the broker withholding statement: without proof of foreign tax effectively and definitively borne, the French inspector will refuse the credit.

Lifetime ISAs add a British bonus that France ignores. The 25 per cent government top-up — up to £1,000 per year on £4,000 of subscriptions — is a British budgetary incentive, not a treaty-exempt capital sum in French eyes. If you subscribed before the move and the bonus was already vested, France does not retroactively tax the past; but interest and dividends subsequently generated by the Lifetime ISA balance are taxed exactly like any other ISA return, and any withdrawal itself follows the nature of the underlying gain rather than enjoying a French exemption. Do not confuse the Lifetime ISA with the French Plan d’épargne en actions (PEA, a French share-savings plan with its own holding conditions): the two wrappers are strangers to each other, and holding one does not satisfy the conditions of the other.

Premium Bonds deserve their own paragraph because clients consistently misread them. National Savings and Investments, the state-owned savings bank, advertises Premium Bonds as paying prizes free of United Kingdom tax, from £25 up to £1 million: Premium Bonds (NS&I). That advertisement speaks of British tax. In France, monthly prizes drawn on bonds you hold as a French resident are not covered by any treaty exemption for gambling or lottery, and the French administration treats them as taxable movable income within Article 120 — the same family as “lots et primes de remboursement” (prize draws and redemption premiums) — to be declared for the year of receipt and taxed at the flat-tax rate or, on global option, at the progressive scale, plus social levies. No British tax is withheld on prizes, so again there is no foreign credit; the prize is simply added to your French base. Small prizes of £25 are taxable in exactly the same way as a £1,000 win: the threshold that matters is not the size of the prize but your French residence on the date of the draw. Keep the NS&I prize notifications and the annual statement, because the Service des impôts des particuliers (SIP, the local personal tax office) cannot verify them automatically and will ask you for the schedule.

A final product warning concerns funds labelled “accumulating” or “roll-up” inside a Stocks and Shares ISA. France generally taxes the distributed amount for distributing share classes, but for accumulating funds it may tax annually the share of interest and income capitalised within the fund under the bond-fund transparency rules, with the balance taxed as a capital gain on disposal. The detailed fund-by-fund treatment exceeds this guide, but the lesson is constant: the ISA label changes nothing, and each underlying stream — interest, dividend, prize, capitalised income, eventual gain — must be characterised under French domestic law and then run through the treaty article that matches its nature. Where the ISA provider’s consolidated certificate lumps everything as a single ISA figure, ask for the analytical breakdown before filing; inspectors in Paris and the large provincial SIPs now routinely request it.

II. Declaring each product without triggering a fine, and fighting back when the bill is wrong

A. Forms 2042, 2047 and 3916, the 12.8 per cent charge and the flat-tax choice: a declaration method that survives control

French filing starts with Article 170 of the Tax Code, which requires every person liable to income tax to file a detailed return of income, gains, household composition and every element needed to compute the tax: Article 170, Code général des impôts (Légifrance). For a British newcomer this means three coordinated forms. Form 2042 (déclaration des revenus) carries the totals. Form 2047 (revenus encaissés à l’étranger, income received abroad) carries the analytical detail: country by country, nature by nature, gross amount, foreign tax withheld, and credit claimed. Form 3916 (déclaration des comptes ouverts à l’étranger) identifies every foreign account — and each ISA counts as an account, even where the provider calls it a wrapper, a plan or a bond holding.

The account-declaration duty is strict and personal. Article 1649 A, second paragraph, states: “Les personnes physiques, les associations, les sociétés n’ayant pas la forme commerciale, domiciliées ou établies en France, sont tenues de déclarer, en même temps que leur déclaration de revenus ou de résultats, les références des comptes ouverts, détenus, utilisés ou clos à l’étranger.” (Article 1649 A, Code général des impôts (Légifrance)). Concretely: one Cash ISA with Halifax, one Stocks and Shares ISA with Vanguard UK and one NS&I Premium Bond holding mean three 3916 entries, each with the institution’s name and address, the account number, and the opening date — and a closing entry in the year you shut one. The official form and notice are published by the tax administration here: Formulaire n° 3916 (impots.gouv.fr), with the general guidance in English here: Must you declare accounts opened abroad? (Service Public, English). Jointly held ISAs are not joint in Britain — ISAs are individual by statute — so each spouse declares their own; Premium Bonds held for minor children are declared by the parents with the child’s identification.

The sanction for forgetting is automatic and per account. Article 1736, paragraph IV provides: “Les infractions aux dispositions du deuxième alinéa de l’article 1649 A et de l’article 1649 A bis sont passibles d’une amende de 1 500 € par compte ou avance non déclaré.” (Article 1736, Code général des impôts (Légifrance)). The same paragraph raises the fine to 10,000 euros per account only where the account sits in a state without administrative assistance with France — not the British case, since the United Kingdom exchanges banking information — but 1,500 euros per ISA per year still accumulates quickly across a Cash ISA, a Stocks and Shares ISA and Premium Bonds. Beyond the fine, the third paragraph of Article 1649 A creates a presumption that sums transferred abroad or from abroad through undeclared accounts constitute taxable income unless you prove otherwise, which reverses the burden of proof in a way few British taxpayers expect. Late regularisation reduces but does not erase the risk, as the case law below shows.

The income declaration itself follows a fixed order. First, convert each gross receipt into euros and enter interest in the 2TR/2BH boxes series, dividends in the 2DC/2FU series, and Premium Bond prizes alongside other movable income, with the analytical detail on form 2047 showing the United Kingdom as source country and the treaty article relied upon. Second, compute the levy. Since 2018, investment income is subject by default to the prélèvement forfaitaire unique (PFU, single flat levy) of 30 per cent, comprising 12.8 per cent income tax and 17.2 per cent social levies: Article 200 A, Code général des impôts (Légifrance). Article 125 A operates the 12.8 per cent as an advance where a French paying agent is involved; for British-paid ISA income with no French intermediary, the 12.8 per cent is simply assessed on the return rather than withheld upstream. Third, decide whether the flat 30 per cent or the progressive scale (barème progressif) suits you: Article 200 A allows a global, express, irrevocable-for-the-year option to tax all eligible investment income and gains at the progressive scale, exercised when filing. The scale brings the 40 per cent dividend allowance (abattement de 40 %) for qualifying dividends and the deductibility of part of the contribution sociale généralisée (CSG, the main social levy), but it also pushes total income into a higher marginal band and forfeits the flat rate. As a rule of thumb, modest ISA returns alongside a French salary often favour the flat levy, while large United Kingdom dividends received by a household with little other income can favour the scale — but the comparison must be computed household by household, year by year, before ticking the 2OP box, because the option covers every eligible receipt of the year, not the ISA alone.

Fourth, claim the treaty credit exactly where the treaty allows it. The French elimination method in Article 24 of the Convention gives a French resident a credit against French tax for the treaty-covered foreign tax, within the conditions and ceilings stated in that article (Article 24, UK–France Convention (GOV.UK)). For dividends within Article 11, the credit equals the British tax paid within the 15 per cent ceiling, capped at the French tax attributable to the same income; for interest within Article 12, there is normally no British tax and therefore no credit. Attach every withholding certificate, claim the credit on form 2047 in the treaty-credit columns rather than as a deduction from gross, and never deduct foreign tax from the declared gross: deducting instead of crediting is the single most frequent cause of reassessment on British files, because it understates both gross and the French tax base. Our analysis of the 25 per cent pension lump sum France still taxes shows the same credit-versus-deduction mechanics on another class of British receipts.

Two administrative points close the method. Where your reference tax income (revenu fiscal de référence) for the year before last falls below 25,000 euros for a single person or 50,000 euros for a jointly taxed household, you may request exemption from the 12.8 per cent advance on French-paid interest by filing the affidavit (attestation sur l’honneur) with the French payer before 30 November of the preceding year — a useful cash-flow step where a French bank tops up a Livret A alongside the ISA, though it changes nothing for British-paid ISA income. And where you arrive mid-year, split the return: British-source income received before the move, while you were still United Kingdom resident, is generally outside the French return, while income received after the foyer moved is declarable even if it accrued earlier. Date of receipt, not date of accrual, governs — so a dividend declared in March but paid in July after your June arrival falls inside the French return.

B. Fines, presumptions and wrong treaty credits: how to challenge a reassessment from Paris and the Ile-de-France

Controls on British files typically start with a mismatch: the international exchange of Banking information sends the French administration the ISA balances, while the 3916 or 2047 lines are missing or inconsistent. The administration then issues a proposition de rectification (proposed reassessment) or, for missing 3916 entries, a flat fine notification. Understanding the order of battle matters, because each paper has its own deadline and its own judge.

Start with the 3916 fine, the most common shock. The Lyon Administrative Court of Appeal (Cour administrative d’appel de Lyon), second chamber, 19 September 2024, no. 23LY02010, heard a taxpayer holding five undeclared Swiss accounts who had regularised late in 2017 and argued, among other points, that the fines restricted the free movement of capital and were disproportionate where the administration could have obtained information from Switzerland: CAA Lyon, 2e ch., 19 sept. 2024, n° 23LY02010 (Légifrance). The court rehearsed the duty in these terms: “Les personnes physiques (…) domiciliées ou établies en France, sont tenues de déclarer, en même temps que leur déclaration de revenus ou de résultats, les références des comptes ouverts, utilisés ou clos à l’étranger.” The lesson for ISA holders is direct: late disclosure does not retroactively erase the breach for the years when the 3916 was missing, and proportionality arguments succeed only with concrete proof that the penalty exceeds what anti-evasion enforcement requires. Practically, where you discover missing ISA or Premium Bond 3916 entries, file a corrective return immediately, pay the fine under protest where demanded, and plead spontaneous regularisation and good faith for a discretionary reduction (remise gracieuse) rather than expecting annulment as of right. Keep the sent receipts from the online filing portal (messagerie sécurisée), because proof of the date of spontaneous disclosure is routinely decisive.

Next, the income reassessment itself. Where the inspector recharacterises ISA interest as a dividend, denies a treaty credit, or applies the Article 1649 A presumption to transfers through an undeclared ISA-linked instant-access account, reply to the proposition de rectification within thirty days with a reasoned letter plus exhibits: ISA statements showing gross and withholding separately, the treaty article cited, the 1042-S or broker certificate for any American dividends inside the wrapper, and the computation of the Article 24 credit capped at French tax. If the inspector maintains the adjustment, the avis de mise en recouvrement (collection notice) opens the formal claim (réclamation contentieuse) before the SIP, to be filed by 31 December of the second year following the assessment year, followed by appeal to the administrative court (tribunal administratif) within two months of an express or implied rejection. For residents of Paris, the competent court is the Tribunal administratif de Paris at 7 rue de Jouy in the 4th arrondissement; for the inner and outer suburbs, it is Montreuil, Cergy-Pontoise or Melun according to the department — check the summons header rather than assuming Paris. Ask the judge for discharge (décharge) of the precise sum concerned: the additional tax, the credit denied, the fine per account, and interest for late payment (intérêts de retard), each with its own reasoning, because globalised pleadings lose.

Where both states claim the same dividend — for instance a United States dividend inside a British ISA held by a French resident, with American withholding, British platform reporting and French assessment — add the treaty’s mutual agreement procedure (MAP) to the domestic claim. Article 26 of the 2008 Convention lets a resident who considers that the actions of one or both states produce taxation contrary to the Convention present the case to the competent authority of their state of residence within three years of the first notification of the offending measure: Article 26, UK–France Convention (GOV.UK). In France that means the Service du contrôle fiscal et de la lutte contre la fraude via the SIP with a copy to the Direction des affaires juridiques; in practice, file the domestic réclamation first to stop the clock, then the MAP request, and tell each authority about the other. The MAP does not suspend French collection by itself, so request a payment stay (sursis de paiement) with guarantees where the amount is significant.

Paris and Ile-de-France practice adds three concrete points. First, control activity is concentrated: the Paris SIPs and the Direction des vérifications nationales et internationales handle a disproportionate share of Anglo-Saxon files, and their inspectors know ISA vouchers, NS&I statements and Vanguard UK certificates by sight — approximate reconstructions are challenged faster here than anywhere else in France. Second, hearings move quickly: the Tribunal administratif de Paris routinely closes instruction within twelve to eighteen months on documentary tax files, so front-load the exhibits rather than holding documents back for appeal. Third, evidence gathering is easier in the capital: most British banks’ Paris branches, the NS&I postal channel and the large brokers issue French-language balance attestations on request, and a same-week trip to the SIP’s reception desk with originals often resolves a mismatch without litigation. None of this changes the substantive law, but procedure wins cases that substance alone would lose.

Finally, calibrate the decision to litigate. A 1,500-euro fine per account may cost more to contest than to pay where the breach is admitted and only the amount is at stake; a denied treaty credit on a large dividend portfolio, a 40 per cent uplift for deliberate failure (manquement délibéré), or an Article 1649 A presumption applied to capital transfers justifies a full challenge with counsel. In every case, diarise three dates the day the reassessment arrives: the thirty-day reply to the proposal, the 31 December N+2 claim deadline, and the two-month court deadline after rejection. Missed deadlines, not weak merits, end most ISA disputes.

Conclusion

Your ISA was an excellent British shelter and remains one for as long as Britain taxes you. Once France becomes your fiscal home, the wrapper becomes transparent: Cash ISA interest is taxable only in France under Article 12, Stocks and Shares ISA dividends are taxable in France with a capped 15 per cent source-state right and an Article 24 credit under Article 11, Premium Bond prizes join the French movable-income base, and every account must appear on form 3916 alongside forms 2042 and 2047 under penalty of 1,500 euros per omission. The treaty softens double taxation through credits, not through exemption, and the courts enforce the declaration duties strictly even after late regularisation. File gross amounts with proof of foreign tax, choose the flat levy or the scale deliberately each year, and challenge errors through the claim, the administrative court and, where two states collide, the mutual agreement procedure. Handled that way, the ISA remains a convenient British container for your savings — simply one whose French tax you have learned to anticipate, declare and, where the inspector overreaches, contest.