Is sudden high bill from new French tax office correct?

If you believe the extra demand is incorrect, a réclamation can be made

Tax offices can carry out checks and make rectification demands
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Reader Question: Having lived in France for over 20 years, we were asked to submit our tax returns to a different tax office last time and have been asked to pay thousands of euros in back tax. They said we had been getting away lightly and we were lucky they could not go back further than three years. Is this justified? 

You presumably submit by paper (which is now unusual) as normally when submitting online there is no specific choice of tax office but the declaration goes automatically to the service des impôts des particuliers (SIP) assigned to your address.

Usually, you would only change SIP if you move, however reorganisations of SIPs can happen.

Tax offices from time to time carry out checks and make rectification demands, up to the end of the third year after the one in which tax should have been paid. 

The main exception to this is where deliberate fraud is suspected (in which case they may go back further).

Valid reasons for a demand for back payments could include if you have been filling in boxes incorrectly leading to incorrect tax levies.

Points to check include whether extra amounts are income tax or social charges (prélèvements sociaux) and if the levies are correct bearing in mind both French and international rules, eg. tax treaties if there is income from abroad.

If you believe the extra demand is incorrect, you can make a réclamation (official claim) via online messaging in your account at the Impots website (click je signale une erreur sur le calcul de mon impôt) or with a registered post letter with reception slip.

The extra tax is still payable, unless you also request and are granted un sursis de paiement (suspension of payment) while a decision is taken. You will be refunded if your complaint is upheld.

If the tax offices reject a réclamation (explicity, or implicitly after six months), there are two months to appeal to the administrative court.

If large sums are involved, it may be worth consulting an avocat fiscaliste (tax lawyer).