Your London employer is sending you to Paris for six months, or eighteen, to open an office, run a project or sit with a client. Your salary keeps coming from Britain, your contract stays British, and everyone assumes the paperwork will follow the money. It will not. France runs three separate controls over your assignment, and each uses its own test: the labour inspectorate checks the posting declaration, the social-security bodies check your cover, and the tax office checks your residence and your salary independently of both. Days spent in France do not decide any of the three the way most people expect. This guide explains what your employer must declare before you start, how the A1 document protects your British social-security cover, when France can still tax you as its resident, which country taxes your salary under the treaty, and exactly how to challenge a double charge.

I. Posted to France: what must be declared and covered before you start?

A. Has your employer filed the posting declaration and named a local contact?

French labour law treats any temporary assignment by a foreign employer as a posting, a détachement, with formalities that start before the Eurostar leaves. Article L1262-1 of the Labour Code states: “Un employeur établi hors de France peut détacher temporairement des salariés sur le territoire national, à condition qu’il existe un contrat de travail entre cet employeur et le salarié et que leur relation de travail subsiste pendant la période de détachement. Le détachement est réalisé : 1° Soit pour le compte de l’employeur et sous sa direction, dans le cadre d’un contrat conclu entre celui-ci et le destinataire de la prestation établi ou exerçant en France ; 2° Soit entre établissements d’une même entreprise ou entre entreprises d’un même groupe ; 3° Soit pour le compte de l’employeur sans qu’il existe un contrat entre celui-ci et un destinataire.” Three routes cover nearly every British assignment: performing a client contract in France, moving between two sites of the same group, or working in France on the employer’s own account. In each case the British employment contract must survive the move. If it is terminated and replaced by a French contract, the exercise is no longer a posting at all, and a different regime, with French payroll from day one, applies.

The declaration itself is compulsory and advance. Article L1262-2-1 of the same code provides: “I.-L’employeur qui détache un ou plusieurs salariés, dans les conditions prévues aux 1° et 2° de l’article L. 1262-1 et à l’article L. 1262-2 , adresse une déclaration, préalablement au détachement, à l’inspection du travail du lieu où débute la prestation. II.-L’employeur mentionné au I du présent article désigne un représentant de l’entreprise sur le territoire national, chargé d’assurer la liaison avec les agents mentionnés à l’article L. 8271-1-2 pendant la durée de la prestation. III.-L’accomplissement des obligations mentionnées aux I et II du présent article ne présume pas du caractère régulier du détachement.” Two duties, one warning. Before the assignment starts, the employer files a declaration with the labour inspectorate (inspection du travail) where the work begins, through the SIPSI online system, the Information System for International Service Provision. The official Business France guidance confirms the practice in English: any employer established outside France posting employees to French territory files the prior posting declaration with the competent labour inspectorate through the SIPSI online service before the assignment starts (Business France, Posted employees). The employer must also name a representative on French soil to answer the control authorities throughout the assignment. And the closing sentence matters most: filing the papers does not prove the posting is genuine. Inspectors look past the forms to the reality of the work, the subordination link and the temporary character, so a declaration describing a permanent Paris role as a six-month posting protects no one.

The posting also upgrades your workplace rights to the French hard core from the first day. Article L1262-4 of the Labour Code guarantees: “I.-L’employeur détachant temporairement un salarié sur le territoire national lui garantit l’égalité de traitement avec les salariés employés par les entreprises de la même branche d’activité établies sur le territoire national, en assurant le respect des dispositions légales et des stipulations conventionnelles applicables aux salariés employés par les entreprises de la même branche d’activité établies sur le territoire national, en matière de législation du travail, pour ce qui concerne les matières suivantes”. What follows is the protected list: individual and collective freedoms at work, non-discrimination and equality between women and men, maternity and family leave, temporary-work guarantees, the right to strike, working time and rest, paid holidays, minimum pay and payment rules, health and safety, and accommodation terms. A British engineer posted to a Lyon building site therefore works Lyon hours for at least the Lyon minimum rates, keeps French-equivalent leave, and answers to French safety rules, whatever the London contract says. Your employer cannot contract out of this core, and a posting letter promising otherwise is the first exhibit in the dispute that follows.

B. Are you still covered at home, or have you joined the French system?

Social security follows its own logic, and the document that controls it is the A1. As a posted worker, you stay insured in your home country while working temporarily abroad, but only if your employer secures the proof in advance. The European guidance states the mechanism plainly: the employer obtains a Portable Document A1 from the social-security institution of the home country, which then informs the host country’s competent authorities (Your Europe, Posted workers abroad). Carry the document and keep a copy with the posting file. French control bodies, from labour inspectors to URSSAF, the agency collecting social contributions, treat a worker without an A1 as an ordinary French-insured employee, with contributions due in France. The same guidance sets the normal ceiling at twenty-four months of A1 cover. Beyond two years, either you join the French system or the two countries’ authorities agree an exceptional extension in your interest. British assignments drafted as rolling six-month renewals should therefore be read honestly: a posting that never ends is employment in France by another name, and the A1 will not cover it indefinitely.

The A1 protects your social cover. It does not protect your tax residence, and this is where posted workers lose twice. France can treat you as its tax resident under Article 4 B of the General Tax Code even during a genuine posting, because any one of its four doors is enough: the family home (foyer) where you normally live with the centre of your family life, the main place of stay, a professional activity in France that is more than accessory, or the centre of your economic interests. A posting of eighteen months where your spouse and children remain in the Surrey house keeps your foyer British, but a posting where the family moves to Boulogne-Billancourt, the children enter a French school and your daily management happens from La Défense opens the first door wide. Residence is assessed on facts, year by year, and the posting label on your contract does not bind the tax office.

The Conseil d’État illustrated both halves of that lesson for a French employee seconded abroad, in reasoning that mirrors yours. The court kept him French-resident because “c’est avec la France que ses liens personnels et économiques étaient les plus étroits” and he “devait par conséquent, en application des stipulations du a) du paragraphe 2 du même article 4, être considéré comme un résident de France au sens de cette convention”, yet it discharged three years of additional assessments because the treaty granted him a credit equal to the French tax on the foreign-source salary, adding: “Il résulte de ces stipulations que l’octroi de ce crédit d’impôt n’est pas soumis à la condition que les revenus concernés aient été effectivement imposés dans les Emirats arabes unis” (CE, 20 March 2023, No. 452718, paras. 12 and 13). Losing on residence did not mean paying twice. British secondees should memorise the sequence: establish where you live for treaty purposes first, then claim the treaty credit expressly, because the administration does not always grant of its own motion what the treaty commands. The credit can even survive where the other country taxed nothing at all.

II. Who taxes your salary, and how do you fight a double charge?

A. Do the three Article 15 conditions keep your salary British?

Salary follows a different treaty article from residence, and confusing the two is the costliest mistake in this field. Employment income is governed by Article 15 of the France–United Kingdom convention of 19 June 2008. The Paris court quoted its opening rule: “Sous réserve des dispositions des articles 16, 18, 19 et 20, les salaires, traitements et autres rémunérations similaires qu’un résident d’un Etat contractant reçoit au titre d’un emploi salarié ne sont imposables que dans cet Etat, à moins que l’emploi ne soit exercé dans l’autre Etat contractant. Si l’emploi y est exercé, les rémunérations reçues à ce titre sont imposables dans cet autre Etat” (CAA Paris, 11 April 2025, No. 23PA02576, para. 3). Work performed in France is taxable in France as the starting point. The exception that sends the salary back to Britain has three cumulative conditions: “Nonobstant les dispositions du paragraphe 1, les rémunérations qu’un résident d’un Etat contractant reçoit au titre d’un emploi salarié exercé dans l’autre Etat contractant ne sont imposables que dans le premier Etat si : / a) le bénéficiaire séjourne dans l’autre Etat pendant une période ou des périodes n’excédant pas au total 183 jours au cours de toute période de douze mois consécutifs ; et / b) les rémunérations sont payées par un employeur, ou pour le compte d’un employeur, qui n’est pas un résident de l’autre Etat ; et / c) la charge des rémunérations n’est pas supportée par un établissement stable que l’employeur a dans l’autre Etat.” Short stay, foreign payer, no local cost centre bearing the wage. All three must hold together, and the 183 days are counted over any rolling twelve-month period, not over the calendar year. Company directors should note the treaty expressly pulls management functions in a French-taxed company into salaried employment, so a London-based director running a Paris company cannot sidestep the article by calling the pay something else.

The Paris court applied these conditions to a British-side claim in April 2025 and the salary stayed French. A chief executive, French tax resident, declared multi-million-euro pay in France, then claimed it was really taxable in Britain because he was physically present there more than 183 days a year and had paid British tax on it, seeking a treaty credit of about 3 million euros for 2017 and 8.6 million for 2018. The court reconstructed where the job was actually done: the company’s effective management sat in Paris with its staff, corporate reports gave the Paris headquarters as his business address, his payslips pointed to Paris activity, he belonged to the French social-security system for that work, and even from London he directed the Paris teams daily. The court recorded that “il y a séjourné 193 jours en 2017 et 207 jours en 2018, y compris pour des raisons professionnelles”, with professional premises available to him in London, yet held: “M. B… ne peut être regardé comme ayant exercé son emploi de mandataire social de cette société ailleurs qu’en France”, rejecting the claim in full (CAA Paris, 11 April 2025, No. 23PA02576, paras. 6 to 8). More than 183 days in Britain did not make it British employment, because the place of effective direction was Paris. For posted workers the moral is direct: the treaty protects the salary only while the real work stays abroad in the Article 15 sense, and payslips, social-security registration and board geography testify louder than travel diaries.

Where France does keep the tax, the treaty still caps the damage through the credit article. The court quoted it: “En ce qui concerne la France, les doubles impositions sont éliminées de la manière suivante : / 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. 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” (CAA Paris, 11 April 2025, No. 23PA02576, para. 2). France computes its tax on the worldwide amount, then imputes a credit equal to the French tax on the British-taxed income. Two practical points follow. First, the credit must be claimed, with the British assessment notices attached; the Paris claimant failed partly because he had not declared the British-taxed income as credit-opening in France, and the court reminded him that “il appartient au contribuable de démontrer le caractère exagéré d’une imposition dont il demande la décharge ou la réduction lorsqu’elle a été établie d’après les bases indiquées dans la déclaration qu’il a lui-même souscrite” (CAA Paris, 11 April 2025, No. 23PA02576, para. 4), the burden rule of Article 170 of the General Tax Code flow: whoever files the figures proves their excess. Second, a British non-resident paid from France without treaty protection meets withholding instead: Article 182 A of the General Tax Code imposes source withholding on “les traitements, salaires, pensions et rentes viagères, de source française, servis à des personnes qui ne sont pas fiscalement domiciliées en France”, at 12 percent above 17,275 euros and 20 percent above 50,112 euros on an annual scale. Residence, posting and payroll therefore form one equation: get any of the three wrong and the wrong country collects first.

B. How do you challenge a French posting assessment, step by step?

Start before the dispute hardens, and start in writing. If a French payslip shows withholding that the treaty forbids, if URSSAF bills contributions despite a valid A1, or if the income-tax notice ignores a claimed treaty credit, write first through the secure messaging of the impots.gouv.fr account or to the service des impôts des particuliers, the local personal tax office, and to URSSAF through its own channels for contribution disputes. Many posting misunderstandings, a British salary coded as French wages, an A1 not yet recorded, a missing credit line, are corrected at this informal stage, and the exchange creates the written record the later stages require. Do not telephone and consider the matter handled. Written questions produce written answers, and only written answers bind the file. Keep every posting document in one bundle from the first day: the SIPSI declaration receipt, the designation of the French representative, the A1, the posting certificate or assignment letter, travel logs, payslips from both countries, and the British tax assessments. The Paris cases reward the taxpayer whose bundle reconstructs an ordinary working week without gaps, and punish the one who rebuilds years from memory.

If the informal route fails, the formal claim, the réclamation contentieuse, is compulsory before any court. Article R*190-1 of the Tax Procedure Book directs: “Le contribuable qui désire contester tout ou partie d’un impôt qui le concerne doit d’abord adresser une réclamation au service territorial, selon le cas, de la direction générale des finances publiques ou de la direction générale des douanes et droits indirects dont dépend le lieu de l’imposition.” No court will hear a tax case without this prior claim, and it must go to the office of the place of taxation, not to the most convenient counter. The deadline is strict and it forgives nothing. Article R*196-1 of the same book requires: “Pour être recevables, les réclamations relatives aux impôts autres que les impôts directs locaux et les taxes annexes à ces impôts, doivent être présentées à l’administration au plus tard le 31 décembre de la deuxième année suivant celle, selon le cas : a) De la mise en recouvrement du rôle ou de la notification d’un avis de mise en recouvrement”. For income tax assessed in 2026, the claim must therefore reach the administration by 31 December 2028 at the latest. Draft the claim as if the judge will read it, because the judge will: invoke Article 15 or the credit article expressly, state which of the three salary conditions holds with dated evidence, attach the A1 and the SIPSI receipt in numbered order, and keep proof of sending. A claim that merely says the double charge feels unfair, without a treaty ground and without documents, fails at the next stage for lack of material, not for lack of sympathy.

The court stage follows the administration’s reply, express or implied after six months of silence. Article R421-1 of the Administrative Justice Code sets the rhythm: “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 the rejection letter, not a day more, and the competent court is the one covering the place of taxation, as the official English-language guidance confirms: claims go to the administrative court of the place of taxation within two months of the administration’s decision (service-public.gouv.fr, Tax claims and court appeals). For readers posted in Paris and the inner suburbs this has a concrete consequence worth stating plainly. If the place of taxation is in Paris, the file belongs to the Paris tax offices and any appeal belongs to the Paris administrative court, the very court that decided the two Franco-British employment and residence cases discussed above. Readers posted in the wider Île-de-France follow the same rule with their own geography: the tax office of the place of taxation designates the forum, so check the issuing office printed on the assessment before assuming the venue. Once resident and filing annually, the mechanics of the French return itself, the 2042, the 2047 and the treaty credit boxes, are walked through in our companion guide: British Newcomer, First French Tax Return After Brexit: the 2042, the 2047, the 3916 and the Treaty Credit.

Conclusion

A posting is not a shield. It is a timetable with three clocks running at once. The labour clock starts before arrival with the SIPSI declaration and the local representative, and filing the forms never proves the posting is genuine. The social-security clock runs on the A1 for up to twenty-four months, after which France expects its own contributions unless the authorities agree otherwise. The tax clock runs on facts, not labels: residence through the family home, the workplace and the money, salary through the place where the job is really done, with the 183-day test deciding only the narrow Article 15 exception over a rolling twelve months. The two Paris rulings point the same way. The executive who spent more than half the year in London stayed taxable in France because Paris directed the company, while the secondee who lost on residence still won discharge through the treaty credit. Declare early, carry the A1, count duties before days, claim the treaty expressly and on time, and challenge through the claim and the court within their strict deadlines. Paper that is organised before the inspector writes beats arguments that are invented after the assessment arrives.