You moved from Manchester to Montpellier, kept your stocks-and-shares ISA with your London broker and your Premium Bonds with NS&I, and you assumed that the British tax shelter travelled with you. It does not. Once France treats you as a French tax resident, it taxes your worldwide income, including every pound of interest, dividend and gain earned inside an ISA that London still calls exempt, and every Premium Bond prize that lands in your inbox. The French tax administration, the administration fiscale, does not recognise the ISA wrapper: it looks straight through it and taxes each item by its nature, under rules written long before anyone in Bercy had heard of a Cash ISA. On top of income tax comes a second duty that catches out almost every British newcomer, namely the declaration of each foreign account on form 3916, punished by a 1,500-euro fine per undeclared account, and a strategic choice between the 12.8% flat-rate levy and the progressive scale, a choice the courts treat as irrevocable for the year once made. The tax treaty signed by France and the United Kingdom on 19 June 2008 softens the blow with a tax credit, but only if you prove your position with the right documents, and the social levies follow their own logic, with only narrow exemptions. This guide explains, with the exact statutes and court decisions behind each statement, what France taxes, how to declare it on the French returns, and how to challenge a reassessment, a social-levy bill or a fine when the administration gets it wrong.

I. Is my UK ISA still tax-free and are my Premium Bonds prizes taxable now I live in France?

A. Is my ISA still exempt from French tax now that I am a French tax resident?

The starting point is residence, because everything else flows from it. Article 4 A of the French General Tax Code, the code général des impôts, at https://www.legifrance.gouv.fr/codes/article_lc/LEGIARTI000006302200, 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, a French tax resident pays French income tax, the impôt sur le revenu, on worldwide income, while a non-resident pays only on French-source income. If your life is now in France, your ISA income is worldwide income, and it falls inside the French net.

Whether your tax home, your domicile fiscal, is in France depends on article 4 B of the same code, at https://www.legifrance.gouv.fr/codes/article_lc/LEGIARTI000051202565: “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 means your family home and the centre of your personal life, your séjour principal means the place where you spend most of your time, and any one of the three tests is enough on its own. A British retiree who owns a house in the Dordogne, spends eight months a year there and keeps only a small flat near the children in Leeds will normally satisfy the first test without any serious discussion.

The courts apply these tests with a wide lens, and the leading modern statement comes from the Conseil d’État, the supreme administrative court, in a decision of 11 May 2022, case number 450692, which is published on Légifrance at https://www.legifrance.gouv.fr/ceta/id/CETATEXT000045790796. The court held that “le foyer d’un contribuable célibataire s’entend du lieu où il habite normalement et a le centre de sa vie personnelle, sans qu’il soit tenu compte des séjours effectués temporairement ailleurs en raison des nécessités de la profession ou de circonstances exceptionnelles. Le lieu du séjour principal de ce contribuable ne peut déterminer son domicile fiscal que dans l’hypothèse où il ne dispose pas de foyer.” For a British reader the message is direct: the judges look for the place where you normally live and keep the centre of your personal life, they disregard temporary stays elsewhere for work or exceptional events, and they turn to the place of your main stay only if you have no such home at all. Short consultancy trips back to London, a few weeks with family at Christmas, or a temporary posting do not move your tax home when your settled life is in France.

Once you are a French resident, the ISA wrapper becomes invisible. No French statute extends a domestic exemption to the British Individual Savings Account, while article 120 of the General Tax Code expressly catches foreign-source investment income, providing that “Sont considérés comme revenus au sens du présent article : 1° 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”. Read the full article on Légifrance at https://www.legifrance.gouv.fr/codes/article_lc/LEGIARTI000038613483. Interest credited inside a Cash ISA is interest within this definition, dividends received inside a stocks-and-shares ISA are dividends within it, and gains realised when you sell holdings inside the ISA are caught separately by article 150-0 A, which states that “les gains nets retirés des cessions à titre onéreux, effectuées directement, par personne interposée ou par l’intermédiaire d’une fiducie, de valeurs mobilières, de droits sociaux, de titres mentionnés au 1° de l’article 118 et aux 6° et 7° de l’article 120 , de droits portant sur ces valeurs, droits ou titres ou de titres représentatifs des mêmes valeurs, droits ou titres, sont soumis à l’impôt sur le revenu”, readable at https://www.legifrance.gouv.fr/codes/article_lc/LEGIARTI000053543761. France does not tax the ISA as a container; it taxes each fruit the container produces, exactly as if the wrapper did not exist.

The British side of the story confirms the trap rather than removing it. The official GOV.UK guidance on ISAs for people who leave the country states: if you open an ISA in the United Kingdom and then move abroad and become non-resident, you cannot pay further money into it, unless you are a Crown employee working overseas or their spouse or civil partner, and you must tell your ISA provider as soon as you stop being a United Kingdom resident, but you may keep the ISA open with continuing United Kingdom tax relief on the money and investments it holds. See https://www.gov.uk/individual-savings-accounts/if-you-move-abroad. Every word of that is true for British tax purposes and irrelevant for French tax purposes: keeping the account open with UK relief does not shelter one penny from the French administration, and the duty to warn your provider means your file already records the exact date your residence changed, a date the French administration can ask about later.

The same applies to savings held with NS&I, National Savings and Investments, the state-owned bank that runs Premium Bonds. NS&I warns its own customers that some of its accounts may attract local tax abroad and tells savers to check the laws of their new country, and it adds a practical sting, namely that its customer agreement requires holders to keep a United Kingdom bank or building society account open in order to operate an NS&I account. See https://www.nsandi.com/help/join-nsandi/using-nsandi-outside-uk. Many British residents in France discover that their UK high-street bank wants to close their account after Brexit, which can in turn jeopardise their NS&I holdings; our separate guide on bank closures explains the French right to an account and how to challenge a closure from France at https://kohenavocats.fr/en/2026/09/27/british-uk-bank-closed-french-account-droit-au-compte-challenge-brexit/. For tax purposes the lesson is simple: an ISA or NS&I product that remains perfectly lawful in London is fully visible to the tax office in Paris.

B. Are my Premium Bonds prizes and my UK savings interest taxable in France?

Premium Bond prizes need separate treatment because they are neither dividends nor interest. A prize is a windfall attached to a holding, not a dividend voted by a company, not interest paid on a deposit, and not a gain realised on the sale of a security, so it sits outside the categories of article 120 and outside the disposals taxed by article 150-0 A. That does not make it exempt. The worldwide principle of article 4 A quoted above catches everything you receive once you are a French resident, and prizes must therefore be declared as other taxable income and bear the ordinary progressive scale, the barème progressif, rather than the 12.8% flat-rate mechanism, which the statute reserves to the listed investment income and securities gains. prizes are modest for most holders, a 25-pound win here and there, but regular winners should keep the NS&I prize history for the year, because the administration can ask for the total, and an undeclared stream of winnings discovered during a wider audit undermines your credibility on every other point.

Interest on ordinary UK savings, a Cash ISA, easy-access accounts, fixed-term deposits, follows a clearer path. Article 125 A of the General Tax Code sets up a levy collected at source: “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, ainsi que d’intérêts versés au titre des sommes mises à la disposition de la société dont elles sont associées ou actionnaires et portées sur un compte bloqué individuel, sont assujetties à un prélèvement lorsque la personne qui assure le paiement de ces revenus est établie en France, qu’il s’agisse ou non du débiteur, ce dernier étant établi en France ou hors de France.” The full article is at https://www.legifrance.gouv.fr/codes/article_lc/LEGIARTI000037526745. The decisive words for a British saver are “lorsque la personne qui assure le paiement de ces revenus est établie en France”: when the payer is your London bank or broker, no French levy is collected at source, and the machinery shifts entirely onto your shoulders. You declare the gross interest yourself on the foreign-income return, form 2047, which feeds into the main return, form 2042, and the tax is computed in Paris, not in London.

Where the United Kingdom has already taken a slice, the treaty of 19 June 2008 prevents you from paying twice, through a credit mechanism interpreted authoritatively by the Conseil d’État in an opinion of 12 February 2020, case number 435907, published at https://www.legifrance.gouv.fr/ceta/id/CETATEXT000041569463. The court quotes the treaty directly: “a) nonobstant toute autre disposition de la présente Convention, les revenus qui sont imposables ou ne sont imposables qu’au Royaume-Uni conformément aux dispositions de la présente Convention sont pris en compte pour le calcul de l’impôt français lorsqu’ils ne sont pas exemptés de l’impôt sur les sociétés en application de la législation interne française. Dans ce cas, l’impôt du Royaume-Uni n’est pas déductible de ces revenus, mais le résident de France a droit, sous réserve des conditions et limites prévues aux alinéas (i) et (ii) et au paragraphe 4, à un crédit d’impôt imputable sur l’impôt français”, and the treaty fixes the measure as follows: “Ce crédit d’impôt est égal : / (i) 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 ;”. Two holdings in that opinion matter enormously in practice. First, the condition of being subject to United Kingdom tax “n’exige pas que les revenus en cause aient été soumis à une imposition effective”, so it does not require tax to have been effectively paid across the Channel. Second, the condition turns on whether the income falls within the British taxable base and on the absence of any exemption by reason of status or activity, rather than on tax effectively paid. In practice, a British resident of France who keeps filing a United Kingdom return, for example a self-assessment return showing the savings income even where allowances wipe out the bill, holds the exact proof the Conseil d’État demands, since inclusion in the British taxable base suffices without any tax effectively paid. Keep those United Kingdom returns and assessments forever; they are the key that unlocks the French credit. Note the ceiling built into the treaty for the listed categories: the credit equals the United Kingdom tax paid but “toutefois, ce crédit d’impôt ne peut excéder le montant de l’impôt français correspondant à ces revenus”, so France never writes you a cheque for the difference when London taxed more heavily than Paris.

On top of income tax come the social levies, the prélèvements sociaux: the generalised social contribution, the contribution for the repayment of the social debt, and their companions on capital income. French residents owe them on savings income as a matter of principle, and the only reliable escape is to show that you are subject to the social-security legislation of another state, under the single-legislation principle of European coordination. The administrative court of appeal of Marseille restated the rule on 17 March 2020, case number 19MA00148, published at https://www.legifrance.gouv.fr/ceta/id/CETATEXT000041814324, quoting the European regulation: “Les personnes auxquelles le présent règlement est applicable ne sont soumises qu’à la législation d’un seul Etat membre. Cette législation est déterminée conformément au présent titre”. The court recalled that “un ressortissant de l’Union européenne relevant d’un régime de sécurité sociale d’un Etat membre est exonéré des prélèvements sur les revenus du capital au titre d’une cotisation au régime de sécurité sociale instauré par un autre Etat membre en raison du principe de l’unicité de la législation applicable en matière de sécurité sociale”, citing expressly “son arrêt du 26 février 2015, ministre de l’économie et des finances contre Gérard de Ruyter (C-623/13)”. But the same judgment shows how narrow the door is: the claimant produced only certificates of private health cover, and the court answered that “M. D… se borne à produire des attestations relatives au bénéfice de prestations de maladie de la part d’assurances privées. Il ne peut ainsi être regardé comme relevant d’une législation d’un régime de sécurité sociale d’un autre Etat membre de l’Union européenne.” Private insurance is not a social-security scheme, and since Brexit the United Kingdom is no longer a member state, so British nationals need properly documented affiliation, typically through an S1 healthcare certificate or a genuine posting, rather than assumptions. Readers holding an S1 should consult our dedicated guide on the S1, the CSG and the CRDS at https://kohenavocats.fr/en/2026/09/27/british-s1-holder-france-csg-crds-exemption-claim-back-challenge-brexit/ before conceding a social-levy bill.

II. How do I declare my UK savings in France and how do I challenge the bill or the fine?

A. Which forms, which choices and which rate: form 3916, returns 2047 and 2042, and the 12.8% flat rate or the progressive scale?

Declaration comes in two layers, and missing either one creates its own penalty. The first layer is the account itself. Article 1649 A of the General Tax Code provides: “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.” The article is at https://www.legifrance.gouv.fr/codes/article_lc/LEGIARTI000045764822. The vehicle is form 3916, whose official notice states: private individuals, associations and non-commercial companies must declare, using the current form 3916 – 3916 bis filed together with the income return, bank accounts and digital-asset accounts held abroad as well as capitalisation contracts and similar placements, notably life-assurance contracts, taken out outside France. See https://www.impots.gouv.fr/formulaire/3916/declaration-par-un-resident-dun-compte-letranger-ou-dun-contrat-de-capitalisation-o. Every current account, savings account and share-dealing account you hold in the United Kingdom needs its own entry each year, including accounts you barely use and accounts closed during the year, because the statute says “ouverts, détenus, utilisés ou clos”, opened, held, used or closed. A stocks-and-shares ISA held on a broker platform counts as an account to declare even in a year when you realised no gain inside it. Premium Bonds sit in a greyer zone, since they are neither a bank account nor a classic capitalisation contract, so the cautious course is to list the NS&I holding or take advice on a 3916 bis entry as a placement of the same nature, rather than to leave the administration to discover it. The price of silence is fixed by article 1736 of the code: “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.” The article is at https://www.legifrance.gouv.fr/codes/article_lc/LEGIARTI000054373979. Three undeclared accounts over two years can therefore mean nine thousand euros of fines before a single euro of tax is even discussed.

The second layer is the income itself, declared on form 2047 for foreign-source items and carried onto the main form 2042. Here you face the central strategic choice of French savings taxation: the single flat-rate levy, the prélèvement forfaitaire unique, or the progressive scale. Article 200 A of the code builds the default rule: “L’impôt sur le revenu dû par les personnes physiques fiscalement domiciliées en France au sens de l’article 4 B à raison des revenus, gains nets, profits, distributions, plus-values et créances énumérés aux 1° et 2° du A du présent 1 est établi par application du taux forfaitaire prévu au B du présent 1 à l’assiette imposable desdits revenus, gains nets, profits, distributions, plus-values et créances.” The article is at https://www.legifrance.gouv.fr/codes/article_lc/LEGIARTI000053546896. The Conseil d’État described the mechanism in its decision of 5 April 2024, case number 490411, published at https://www.legifrance.gouv.fr/ceta/id/CETATEXT000049375386: “si les revenus de capitaux mobiliers et plus-values de cession de valeurs mobilières qu’elles énumèrent sont en principe imposés au taux forfaitaire de 12,8 %, ces revenus peuvent, sur option du contribuable portant sur la totalité des revenus entrant dans leur champ pour l’année en cause, être imposés par application du barème progressif de l’impôt sur le revenu.” The same decision then locks the door: “Il ressort de la lettre même de ces dispositions qu’une telle option revêt un caractère irrévocable.” The option covers all the income within its scope for the year, it is exercised when filing the return, and once exercised it cannot be withdrawn during an audit or within the claim period. For a British saver with modest French income and large ISA gains, the progressive scale can beat 12.8%, while for a higher-rate household the flat rate usually wins; either way the calculation must be done before signing the return, because the signature is final. Remember that social levies apply on top of whichever income-tax route you take, unless you hold a genuine exemption as discussed above.

B. How do I challenge a reassessment, a social-levy bill or a 1,500-euro fine?

When the notice arrives, an avis d’imposition swollen by reassessed ISA income, a demand for social levies you believe you do not owe, or a 1,500-euro fine per undeclared account, the procedure is always the same at the start: file a formal claim, a réclamation, with the tax office that issued the assessment, setting out facts and law, and then, if the administration maintains its position expressly or by silence, appeal to the administrative court, the tribunal administratif. Observe strictly the time limits printed on your notice, because a late claim fails whatever its merits, and pay what is asked or arrange a guarantee where the rules require it so that the dispute concerns the principle rather than collection. Four lines of argument, each anchored in the decisions quoted above, cover almost every British savings dispute.

First, residence. If the administration taxes you as a French resident for a year when your centre of life was arguably still in the United Kingdom, deploy the test from the Conseil d’État decision 450692 quoted in part I: show where you normally lived and kept the centre of your personal life, produce leases, utility bills, school registrations, medical records and travel calendars, and neutralise the administration’s reliance on isolated stays by invoking the rule that temporary stays for work or exceptional circumstances do not count. The burden is practical rather than theoretical: whoever holds the fuller file usually wins the residence fight, so British newcomers should keep a residence folder from the year of the move, with dated proof of daily life on each side of the Channel.

Second, the treaty credit. When France taxes United Kingdom savings income on which London also took tax, or even reported income on which London took nothing because allowances covered it, claim the credit under article 24 of the 2008 treaty exactly as the Conseil d’État construed it in opinion 435907. The winning exhibit is your United Kingdom tax return for the year showing the income included in the British taxable base, because the court held the condition satisfied where the resident proves declaration in the United Kingdom, even where no tax was effectively paid there. Attach the United Kingdom return, the computation and any assessment, translate the essentials, and compute the cap yourself so the inspector sees a finished, checkable figure rather than a bare demand. The most common administrative error here is to refuse the credit on the ground that no British tax was effectively paid; that refusal contradicts the Conseil d’État, and saying so politely in the claim, with the reference, resolves a surprising number of files.

Third, the 3916 fine. Be realistic about which arguments fail. The administrative court of appeal of Lyon upheld the 1,500-euro fine on 19 September 2024, case number 23LY02010, published at https://www.legifrance.gouv.fr/ceta/id/CETATEXT000050268447, against taxpayers holding five undeclared Swiss accounts who pleaded European law and the fact that the administration could have learned of the accounts through exchange of information. The court answered: “L’amende forfaitaire de 1 500 euros susceptible d’être prononcée en cas de défaut de déclaration annuelle d’un compte bancaire ouvert, utilisé ou clos à l’étranger est propre à garantir la réalisation des objectifs poursuivis par le législateur et, eu égard à son montant, apparaît en adéquation avec la gravité du manquement qu’elle vise à réprimer, de sorte qu’elle doit être regardée comme respectant le principe de proportionnalité.” It added that the administration’s theoretical ability to obtain the information from abroad is “sans incidence”, irrelevant, to that assessment. Since Brexit removed even the European-law dimension for British accounts, a proportionality challenge to the amount itself has almost no prospect. Aim instead at the findings: an account that was in fact declared, a duplicate fine for the same account and year, an account that was genuinely closed before the relevant year with proof from the bank, or a fine addressed to the wrong person. And for the future, file late rather than never, because spontaneous regularisation puts you in a far stronger negotiating position than a discovery during an audit.

Fourth, the social levies. If you are billed for the CSG, the CRDS and related contributions on savings income while you claim cover elsewhere, produce a real certificate of affiliation to a foreign social-security scheme for the exact years in dispute, not private health insurance, which the Marseille court held insufficient, and not a letter postdating the years at issue, which the same court discarded. S1 holders have the strongest files and should present the certificate with the claim from the outset. Where no exemption exists, verify the base rather than the principle: prizes taxed as other income should not silently attract computations built for investment income, and income already relieved by a treaty credit should not generate a second, inconsistent base for the levies.

Conclusion

Keeping British savings after settling in France is perfectly lawful, but keeping the British tax treatment is impossible. From the year your tax home moves to France, your ISA interest, dividends and gains become French taxable income item by item, your Premium Bond prizes join your declared worldwide income, each United Kingdom account earns its own 3916 entry on pain of 1,500 euros, and the choice between the 12.8% flat rate and the progressive scale, once made on the return, cannot be taken back. The treaty credit of the 2008 convention and the narrow social-security exemptions then do the work of softening the bill, provided you arrive with United Kingdom returns, affiliation certificates and bank records in hand. Build that file before the administration asks, declare completely and on time, and challenge precisely, with the statute and the decision that fits your facts, when a notice goes too far.