You moved to France with your Individual Savings Account intact, and your British bank keeps telling you the good news: the ISA is tax-free. In the United Kingdom that remains true. In France it is not. From the first year you count as resident for French tax purposes, every pound of interest, every dividend and every gain realised inside your cash ISA or your stocks and shares ISA belongs on your French tax return, taxed in France and declarable account by account on the foreign-account statement. The wrapper that sheltered you in Britain shelters nothing here, and the penalties for forgetting it are automatic: a fine for each ISA left off the statement, plus the tax, plus interest for late payment, plus in serious cases an 80% increase that replaces the fine. This guide explains the position exactly as it stands for a British reader living in France after Brexit. Part I describes what France taxes and how the bill is computed. Part II describes the yearly declaration routine and what to do when the tax office comes asking questions or sends a reassessment you want to challenge.

I. Your ISA loses its British shelter on the day you become French tax resident: what France taxes and how

A. Why the British tax-free wrapper means nothing to the French tax office

Start with the British side, because the misunderstanding always starts there. The official British guidance states the domestic position in one sentence: You can save tax-free with Individual Savings Accounts (ISAs). That sentence is correct as far as His Majesty’s Revenue and Customs is concerned. The same guidance then deals with your exact situation, a saver who leaves Britain: If you open an Individual Savings Account (ISA) in the UK then move abroad and become a non-UK resident, you cannot put money into it (unless you’re a Crown employee working overseas or their spouse or civil partner). You may keep the account, since the guidance adds: However, you can keep your ISA open and you’ll still get UK tax relief on money and investments held in it. So London tells you two things: stop paying in once you are non-resident, and enjoy continued British relief on what stays inside. Neither statement binds Paris in any way.

France taxes you the moment you meet its own residence test, and that test is deliberately wide. Article 4 B of the Code général des impôts, the French tax 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 ; In plain terms, your home or your main place of stay (in French, your foyer or place of principal stay), your professional activity, or the centre of your economic interests can each be enough. Most British settlers meet the first test within months: the family home is in France, the children are at a French school, daily life happens here. From that point you are a person fiscalement domiciliée en France, fiscally domiciled in France, and the whole machinery described below applies to you for the full calendar year concerned.

French residents pay French tax on their worldwide income. Article 158 of the Code général des impôts states the rule without ambiguity: Les revenus nets des diverses catégories entrant dans la composition du revenu net global sont évalués d’après les règles fixées aux articles 12 et 13 et dans les conditions prévues aux 2 à 6 ter ci-après, sans qu’il y ait lieu de distinguer suivant que ces revenus ont leur source en France ou hors de France. Net income in each category goes into total net income under the same rules whether its source is French or foreign. And foreign investment income is expressly caught: article 120 of the same code treats as revenue Les dividendes, intérêts, arrérages et tous autres produits des actions de toute nature et des parts de fondateur des sociétés, compagnies ou entreprises financières, industrielles, commerciales, civiles et généralement quelconques dont le siège social est situé à l’étranger quelle que soit l’époque de leur création. Interest credited inside a cash ISA and dividends paid on shares held inside a stocks and shares ISA therefore enter your French taxable income in the year they arise, whether or not you withdraw a penny from the wrapper.

The decisive point is that no French provision recognises the ISA as a shelter. France knows its own exempt wrappers, the Livret A, the plan d’épargne en actions, the assurance-vie after the holding period, each created by a statute that spells out the exemption. The British ISA has no equivalent statute on the French side, and the Franco-British double tax treaty of 19 June 2008, published by decree of 7 January 2010, contains no article preserving foreign savings wrappers either. The treaty allocates taxing rights over dividends and interest, which helps you avoid double taxation, but it never says that income a contracting state exempts domestically must be exempt in the other state. So the interest that London ignores, Paris taxes; the dividend that London ignores, Paris taxes; and the gain realised on a disposal inside your stocks and shares ISA, which London ignores, Paris taxes in the year of disposal even if the proceeds stay inside the account. Each sale your broker executes within the wrapper is a taxable disposal for French purposes in that calendar year, with its own acquisition price, sale price and resulting gain or loss to compute and keep evidence for.

Apply this to each flavour of ISA you may hold. A cash ISA produces interest, taxed as investment income in the year it is credited. A stocks and shares ISA produces dividends when portfolio companies distribute, interest when bond funds pay out, and capital gains whenever holdings are sold at a profit, each stream taxed in France in the year it arises. A Lifetime ISA adds two wrinkles: the 25% British government bonus is a receipt with no exempting provision on the French side, so it joins worldwide taxable income like any other receipt, and the British 25% withdrawal charge you suffer on an unauthorised withdrawal is not a deductible French expense. A Junior ISA held for a child who lives with you in France follows the same logic: the child is generally fiscally domiciled with the parents, the income is taxable, and the declaration duties described in Part II fall on the parents to perform. Finally, respect the British contribution rule quoted above. Paying into the ISA after you have become non-resident breaches British ISA rules and can cost the account its British relief, while changing nothing on the French side, where the income was taxable in any event. The clean routine is to freeze contributions when you leave, keep the account open, and handle everything through the French return.

B. How your ISA interest, dividends and gains are actually taxed: the flat tax and the progressive-scale option

France taxes investment income under a system whose official name you will meet on every notice: the prélèvement forfaitaire unique, universally called the PFU or flat tax. Article 200 A of the Code général des impôts sets its income-tax component in these terms: Le taux forfaitaire mentionné au premier alinéa du présent 1 est fixé à 12,8 % ; The official service-public guidance confirms the architecture: Le prélèvement forfaitaire unique est constitué de l’impôt sur le revenu (12,8 %) et des prélèvements sociaux. In the standard case for investment income this produces the familiar overall charge of 30%, combining the 12.8% income-tax charge with the social levies, and that is the figure to use when you budget. One composition detail is worth knowing exactly as the administration states it for people affiliated to a neighbouring social security system: Si vous résidez en France, travaillez dans un pays de l’EEE ou en Suisse et êtes affilié à la sécurité sociale obligatoire de ce pays, vous n’êtes pas soumis à la CSG et à la CRDS. Seul le prélèvement de solidarité de 7,5 % est dû. Since Brexit the United Kingdom is no longer in that group, so most British settlers in France pay the full social-levy element; if you hold an S1 certificate or a cross-border affiliation, have the levy mix checked rather than assuming either rate.

Two withholding mechanisms sit in front of the final bill, and both name you expressly as the person concerned. For dividends, article 117 quater of the Code général des impôts provides: Les personnes physiques fiscalement domiciliées en France au sens de l’article 4 B qui bénéficient de revenus distribués mentionnés aux articles 108 à 117 bis et 120 à 123 bis sont assujetties à un prélèvement au taux de 12,8 %. For interest and similar fixed-income products, article 125 A of the same code opens in the same way for Les personnes physiques fiscalement domiciliées en France au sens de l’article 4 B qui bénéficient d’intérêts, arrérages et produits de toute nature de fonds d’Etat, obligations, titres participatifs, bons et autres titres de créances, dépôts, cautionnements et comptes courants. In practice your British ISA provider operates no French withholding at all, so nothing is taken during the year and the whole charge is settled when you file the return; the mechanism matters mainly if you also hold French-paying investments, where you can ask the paying institution for exemption from the advance withholding when your reference income is modest, a request the administration says must reach the institution au plus tard le 30 novembre de l’année précédant celle du paiement, no later than 30 November of the year before payment.

You are not locked into the flat tax. Article 200 A continues: Par dérogation au 1, sur option expresse du contribuable, l’ensemble des revenus, gains nets, profits, plus-values et créances mentionnés à ce même 1 est retenu dans l’assiette du revenu net global défini à l’article 158. On your express election, all the income and gains otherwise under the flat tax go into total net income and face the progressive scale instead, and Cette option globale est exercée lors du dépôt de la déclaration prévue à l’article 170, et au plus tard avant l’expiration de la date limite de déclaration. The election is all-or-nothing for the year: every item of investment income and every gain on securities follows the same route. The official guidance explains the trade-off plainly. Under the flat tax you cannot use the 40% allowance on dividends, you cannot deduct part of the CSG, and you cannot deduct costs; under the scale you are taxed at your marginal band but you recover those three advantages, and the election covers all your investment income and securities gains together. Households with little other income, typically taxed at 11% or not at all, often gain from the scale; households already in the 30% band or above usually gain from the flat tax. Run both computations each year before ticking the box, because the right answer moves with your other income.

The treaty then decides what happens to any British tax on the same income. For dividends, article 11 of the Franco-British convention provides: Les dividendes provenant d’un Etat contractant et payés à un résident de l’autre Etat contractant sont imposables dans cet autre Etat. Both states may tax, but si la personne qui les reçoit en est le bénéficiaire effectif, l’impôt ainsi établi ne peut excéder 15 % du montant brut des dividendes, the source-state charge cannot exceed 15% of the gross where the recipient is the beneficial owner. For interest the treaty’s answer is cleaner. Article 12 of the convention provides: Les intérêts provenant d’un Etat contractant et dont le bénéficiaire effectif est un résident de l’autre Etat contractant ne sont imposables que dans cet autre Etat. Interest whose beneficial owner lives in France is taxable only in France. In practice Britain levies no withholding on either stream for you, so there is generally no British tax to credit; where a withholding has been operated, article 200 A answers that L’impôt retenu à la source est imputé sur l’imposition à taux forfaitaire dans la limite du crédit d’impôt auquel il ouvre droit, dans les conditions prévues par les conventions internationales. The foreign withholding is credited against the flat-tax charge within the treaty credit limit. Readers who also receive British dividends outside the ISA will find the same mechanics explained step by step in our guide to British dividends and interest paid from the United Kingdom, which works through declaration lines and the credit in detail.

Two short worked pictures fix the orders of magnitude. Suppose your cash ISA credits £1,000 of interest in the year and you are under the flat tax with no British withholding: France charges the flat-tax treatment on the full £1,000 converted to euros at the year’s rate, roughly £300 of total charge at the standard combined rate, all of it payable in France. Suppose instead your stocks and shares ISA pays £1,000 of dividends and you elect for the scale while your marginal band is 11%: only £600 enters the base after the 40% allowance, the income-tax element is about £66, and the social levies apply on top, with part of the CSG deductible next year. Change the marginal band to 41% and the same election costs about £246 of income tax on the £600, which is why the flat tax usually wins higher up the scale. These are illustrations, not computations for your file: convert each receipt at the proper rate, apply the elections you actually made, and keep the broker statements that prove every figure.

II. Declaring the ISA every year on form 3916 and challenging the bill when it goes wrong

This guide belongs to our ISA cluster whose first comprehensive treatment is Your ISA Is Not Tax-Free in France After Brexit: How a British Resident Declares It, Pays French Tax, and Challenges the Bill; what follows deepens it with the 2008 treaty mechanics and the September 2025 Cour de cassation ruling on undeclared foreign accounts.

A. The yearly 3916 declaration: which statement, which timing, which fines for each undeclared account

The declaration duty is separate from the tax charge and it is the duty that traps most British savers, because nothing in Britain prepares you for it. France requires residents to list their foreign accounts every single year, even accounts that earned nothing, even accounts you never touched. The official service-public guidance states the scope exactly: You must report to the tax authorities the foreign accounts if they were open, held, used or closed in the taxation year. An ISA you simply left invested all year is a held account. An ISA you closed in March is a closed account. Both go on the statement. The same page explains which establishments count: You must declare accounts domiciled outside France with a banking institution or with any financial services provider (notary, stockbroker, etc.). Your cash ISA with a British bank and your stocks and shares ISA with a British broker or platform are both caught, and each ISA counts as its own account, so a saver with one of each files two lines, not one. If you doubt where an online account is domiciled, the administration gives a practical test based on the IBAN code of the account: a French IBAN starts with FR, and anything else is reported each year as a foreign account.

Timing and paperwork follow one simple rule. The guidance says: Accounts opened abroad must be reported at the same time as your annual tax return. You tick the foreign-accounts box on the income return and attach one statement per account, the form 3916 series, currently form 3916 and 3916 bis under Cerfa number 11916, which also covers digital-asset accounts and foreign life assurance. The legal anchor behind the form is article 1649 A of the Code général des impôts, and the sanction for skipping it is in article 1736 of the same code, which 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. That is 1,500 euros per undeclared account, so two forgotten ISAs mean 3,000 euros before any tax is even discussed. The same paragraph continues: Toutefois, pour l’infraction aux dispositions du deuxième alinéa de l’article 1649 A, ce montant est porté à 10 000 € par compte non déclaré lorsque l’obligation déclarative concerne un Etat ou un territoire qui n’a pas conclu avec la France une convention d’assistance administrative en vue de lutter contre la fraude et l’évasion fiscales permettant l’accès aux renseignements bancaires. The 10,000-euro level applies where the account sits in a state with no administrative-assistance treaty with France. The United Kingdom has such assistance arrangements with France, so the British ISA normally attracts the 1,500-euro fine rather than the 10,000-euro one, but do not read that as leniency: the service-public presentation of the sanctions adds that an Increase of 80 % of the duties due on the amounts shown on the undeclared account(s). This increase then replaces the fine of €1,500 or €10,000. Where tax was actually evaded on the sums in the account, an 80% increase on the duties can dwarf the flat fine, and it substitutes for it rather than adding to it.

Build the routine once and repeat it every May. First, list every British account in your name and every account on which you hold a signing authority, including each ISA, and check whether any was opened, held, used or closed during the year. Second, collect the yearly statements from each provider showing interest credited, dividends paid, proceeds of disposals with acquisition prices, bonuses, charges and the sterling-to-euro conversions you will use, keeping the documents for at least the reassessment period. Third, complete one 3916 statement per account with the exact identification details the form asks for: the provider’s name and address, the account number, the dates of opening and closing where relevant. Fourth, report the income itself in the investment-income sections of the return, choosing flat tax or scale for the whole year as computed in Part I, and report securities gains in the gains section with their own detail. Fifth, verify before validating that the number of 3916 statements equals the number of foreign accounts you listed in step one. The most common errors are declaring the income but omitting the 3916, which still draws the fine; declaring one ISA and forgetting the second; treating a stocks and shares ISA as a single shareholding rather than an account to declare plus income to tax; and assuming that an account with no movements needs no statement, when a merely held account is expressly reportable. If you discover an omission from a past year, a spontaneous correction almost always costs less than waiting for the administration to find it through automatic exchange of bank information, which between France and the United Kingdom now runs as a matter of course.

B. When the tax office reassesses you: answering the information request, filing the claim and going to court

Trouble usually announces itself politely, with a letter asking where numbers came from. The administration routinely sends information requests about foreign assets before any reassessment, and a recent decision of the Cour de cassation shows how heavily the rest of the case then weighs on your reply. On 17 September 2025 the commercial, financial and economic chamber of the Cour de cassation, appeal number 23-10.403, decision number 441 FS-B, rejected the appeal of a taxpayer pursued over Swiss bank accounts, and its statement of the facts deserves reading in full by anyone tempted to ignore such letters: le 3 février 2014, à la suite de la transmission par un procureur de la République, sur le fondement de l’article L. 101 du livre des procédures fiscales, d’informations selon lesquelles M. et Mme [H] auraient été titulaires de comptes bancaires ouverts dans les livres d’une banque établie en Suisse, the tax office questioned them about the origin of the funds, and then, the court records, Par une proposition de rectification du 6 novembre 2015 notifiée à M. [H], l’administration fiscale a procédé à la taxation d’office aux droits de mutation à titre gratuit des avoirs figurant sur les comptes bancaires litigieux au cours de la période précitée, en application des articles L. 71 du livre des procédures fiscales et 755 du code général des impôts. Silence led to automatic taxation, taxation d’office, of the account balances as undeclared gifts, and the courts upheld it. The full reference is available from the Cour de cassation, commercial chamber, 17 September 2025, appeal 23-10.403, decision 441 FS-B (rejection). Your ISA case would run under income-tax provisions rather than gift-duty provisions, but the procedural moral is identical: answer information requests completely, within the stated deadline, with documents, because non-response authorises the administration to assess on assumptions that are far harder to undo later.

If a reassessment, a proposition de rectification, arrives, treat it as the decisive round. The notice states the adjustments item by item with their legal basis and gives you a period to respond with observations, and a reasoned response at this stage closes more files than any later appeal. Work through the notice line by line against your own file. Where the office taxed ISA interest you can show was already reported, point to the return line and attach the statement. Where it applied the flat tax but your scale election was ticked, or ignored the election, produce the return and recompute both routes. Where it denied a foreign-tax credit, produce the withholding certificate and the treaty article, dividends under the 15% cap or interest taxable only in France as set out in Part I. Where it fined an account you did declare, produce the filed 3916 and ask for the fine to be cancelled. Where it fined an account you genuinely omitted, consider paying the tax and the fine quickly while arguing good faith and spontaneous cooperation for relief on penalties, rather than contesting the principle and paying late-payment interest on top. Send everything in writing, keep proof of sending, and diary the deadline printed on the notice, because missing it moves the dispute to the next stage on worse terms.

That next stage is the formal claim, the réclamation contentieuse. Article L190 of the Livre des procédures fiscales, the tax procedure code, sends claims for wrongly assessed tax and penalties to the contentious jurisdiction, and in practice you first write to the tax office that issued the assessment, setting out each head of claim with its amount, its legal basis and its evidence. A well-built claim file for an ISA dispute contains the yearly 3916 statements as filed, the provider statements for the years reassessed, your computation of each stream of income with conversions, the treaty articles on dividends and interest, the flat-tax and scale computations side by side, and the withholding certificates where a credit is claimed. If the administration rejects the claim expressly or lets it sit unanswered past the statutory period, you carry the case to the administrative court within the appeal period stated in the rejection, where the judge re-examines both the tax and the penalties. Fines for undeclared accounts are frequently litigated on proportionality and on proof of holding, and the 80% increase on proof of the duties it multiplies, so a file that separates admitted omissions from genuinely disputed points puts you in the strongest position. Throughout, keep paying what is clearly due and contesting only what is genuinely arguable: undisputed sums left unpaid accrue late-payment interest while the argument runs, and no procedural victory refunds interest that timely payment would have avoided.

Conclusion

Your ISA survives the move to France as an account, not as a shelter. Keep it open if you wish, since British relief continues on what stays inside, but stop paying in once you are non-resident, declare each ISA every year on its own 3916 statement, return every pound of interest, dividends, bonuses and realised gains in France in the year it arises, and budget for the flat-tax charge unless your own figures show the progressive scale wins. Keep the broker statements that prove each number, because the first letter from the tax office will ask for them and the courts show little sympathy for savers who cannot produce them. And when a notice arrives, answer it on time, in writing, with evidence, then claim and litigate only the points genuinely worth disputing. Handled that way, the British saver’s favourite account remains perfectly usable in France: fully visible, fully declared, and taxed once, in the right country, at the right rate.