France offers several perfectly legal ways to reduce your tax bill — but none of them are applied automatically. Miss them and you overpay. Here are the five that UK expats most commonly leave on the table.
France won't tell you about these.
There's no letter from the tax office saying "by the way, you might be overpaying." No automatic adjustment. No prompt. The savings exist in the system — but only if you know where to look and actively claim them.
Most of the UK expats overpaying their French taxes aren't doing anything wrong. They're just not doing everything right. That's a different problem — and a fixable one.
Here are the five levers worth checking every year.
1. The S1 tick box — the most expensive missed saving in French expat tax
If you hold a valid S1 certificate and you're not ticking boxes 8SH or 8SI on your tax return, you are paying social charges you don't owe. Every year. And the tax office will not tell you.
Social charges are the second layer of tax France levies on top of income tax — up to 18.6% on investment income and 17.2% on rental income in 2026. With an S1, the rate drops to 7.5% on both. On pension income, it drops to zero.
Here's what that looks like in practice. A UK couple, both S1 holders, with €2,500 in savings interest and €12,000 in UK rental income — €14,500 of income in total. Without the tick boxes:
- Social charges at 18.6%: €2,697
With the tick boxes:
- Social charges at 7.5%: €1,088
That's €1,609 a year. Over three years — the window the tax office can go back and correct — that's nearly €4,830 overpaid.
The fix is two ticks on one form. Box 8SH for Declarant 1, box 8SI for Declarant 2, on Form 2042-C. No amount needed — just the tick. But it must be done every single year. It doesn't carry over.
For the full picture on the S1 and how to get one, see our S1 guide.
2. The RFR-based pension rate reduction — lower income means lower social charges
If you don't hold an S1, social charges on your pension income aren't fixed at one rate. France uses a sliding scale based on your household income — and if your income is modest, you could be paying significantly less than the standard rate. Or nothing at all.
The rate is based on your RFR — your household reference income — from two years ago. It's the number printed on the front of your Avis d'Impôt (tax assessment notice), usually top left or top right, clearly labelled Revenu fiscal de référence.
For 2026, a married couple (2 household shares) pays zero social charges on their pension if their RFR is below €20,016. They pay the maximum rate only if it exceeds €40,604. Between those two figures, reduced rates of 3.8% and 6.6% apply.
Four possible rates:
| Rate | What it means |
|---|---|
| 0% | Fully exempt — income below the lower threshold |
| 3.8% | Reduced rate — lower income band |
| 6.6% | Median rate — middle income band |
| 8.3% | Standard rate — above the upper threshold |
The catch: this isn't applied automatically. You have to declare your pension income for social charges in Form 2047, Section 9, using the band that matches your RFR. If you skip that section, the tax office doesn't calculate social charges on your pension at all — and may come back with a backdated bill for up to three years.
Full rate table and current thresholds are on the RFR thresholds section of the Taxpert data page →
3. The CSG deductible — money you already paid, reducing next year's bill
If you paid social charges on pension or investment income last year, part of what you paid is deductible against this year's income. Not the whole amount — but a meaningful slice. And it appears on your Avis d'Impôt, calculated for you, ready to use.
Look for a line on your Avis labelled:
CSG déductible sur les revenus du patrimoine/valeurs mobilières
That figure — in euros — should automatically appear in Box 6DE on the following year's return. If your Avis shows a number but Box 6DE is blank when you start your next return, check it. You may be paying tax on income you've already handed over to the state.
The deductible amounts by income type:
- Investment and rental income: 6.8% of gross income
- Pension income at the standard rate: 5.9%
- Pension income at the reduced 3.8% rate: the full 3.8% is deductible
This only applies if you chose the sliding scale (box 2OP) rather than the flat tax. If you used the flat tax, this saving isn't available — which is one of the reasons the flat tax vs sliding scale decision is worth revisiting every year.
4. The flat tax vs sliding scale — a decision worth making fresh every year
When you have investment income — dividends, interest, capital gains — France gives you a choice about how it's taxed. The default is a flat rate of 31.4% on everything (12.8% income tax plus 18.6% social charges). Simple, no calculation needed.
The alternative is to switch to the sliding scale by ticking box 2OP. This taxes your investment income at the same rates as your regular income — which could be 0%, 11%, or 30% depending on your total household income.
If you're in the 0% or 11% income tax band, the flat 12.8% income tax element of PFU is costing you money. On the sliding scale, that same income would be taxed at 0% or 11%.
If you hold an S1, your flat rate drops to 20.3% (12.8% + 7.5%). On the sliding scale at 11%, you'd pay 11% + 7.5% = 18.5% — still cheaper.
Three things that change the calculation and are worth reviewing each year:
- Your income dropped — pension ended, rental income fell, one-off income last year inflated the figure
- You gained an S1 — your flat rate has just fallen; the comparison changes
- You had a large capital gain — the all-or-nothing rule means switching to the sliding scale affects all your investment income, not just one part of it. A big gain on one asset dragged into a higher band by the switch could wipe out the saving elsewhere
The decision must be modelled every year. What was right in 2024 may not be right for 2025. Our PFU vs sliding scale guide walks through how to assess it.
5. The mixed household split — when only one partner has an S1
This is the trap that catches couples where one partner holds an S1 and the other doesn't — one is a UK pensioner, the other is still working in France or hasn't yet reached state pension age.
Here's what goes wrong. John has an S1 — his social charges rate is 7.5%. His wife Sarah doesn't — her rate is 18.6%. They hold a joint investment account that generates €10,000 in gains.
By default, the tax software can't split that automatically. It sees a joint account, applies the higher rate to the whole lot, and charges 18.6% on €10,000. John's half gets taxed at Sarah's rate.
Box 8RC is how you fix it. You manually declare John's share of the joint investment income — in a 50/50 account, that's €5,000 — so it's isolated and taxed at his 7.5% rate rather than Sarah's 18.6%.
On that one joint account, the saving is €555 a year. Miss it across multiple joint assets and the overpayment adds up quickly.
Common mistakes
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Registering the S1 with CPAM and assuming the tax office knows. They don't. CPAM and the tax office don't share information. You must tick 8SH/8SI every year — it doesn't carry over from the previous return.
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Assuming the flat tax is always simpler. It is simpler. It isn't always cheaper. For anyone in the 0% or 11% income tax band, the sliding scale almost always produces a lower bill.
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Missing the CSG deductible. It's on your Avis d'Impôt. It should auto-populate in Box 6DE. Check it's there before you submit.
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Not splitting joint income in a mixed S1 household. If one partner has an S1 and the other doesn't, joint investment income needs to be split manually via Box 8RC. The software won't do it for you.
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Not rechecking the RFR band year on year. Your RFR changes. The thresholds change. A band that didn't apply two years ago might apply now.
Frequently Asked Questions
Are there legal ways to reduce my French tax bill?
Yes — several. The most common are: declaring your S1 certificate correctly (boxes 8SH/8SI), checking your RFR band for pension social charges, using the CSG deductible from your previous year's Avis d'Impôt, reviewing the flat tax vs sliding scale decision annually, and splitting joint income correctly in a mixed S1 household. None of these are applied automatically — you have to claim them.
What is the biggest tax saving for UK pensioners in France?
For most UK state pensioners with an S1, the biggest saving is declaring the S1 correctly on the tax return. Two tick boxes — 8SH and 8SI — can save over €1,600 a year for a couple with modest investment and rental income. Missing them is the single most common and most expensive mistake in UK expat tax filing.
How do I know if the flat tax or sliding scale is better for me?
It depends on your marginal income tax rate. If your total household income puts you in the 0% or 11% band, the sliding scale is almost always cheaper. If you're in the 30% band or above, the flat tax is usually better. The decision needs to be made fresh each year — income changes, S1 status changes, and one large capital gain can change the calculation completely.
Where do I find the CSG deductible on my Avis d'Impôt?
Look for a line labelled CSG déductible sur les revenus du patrimoine/valeurs mobilières in the social charges section of your Avis. The figure shown should auto-populate in Box 6DE on next year's return. If Box 6DE is blank when you start filing, check your previous Avis and enter it manually.
What is Box 8RC and when do I need it?
Box 8RC is used when only one partner in a household holds an S1. It lets you declare that partner's individual share of joint investment income separately, so it's taxed at the reduced 7.5% solidarity levy rather than the full rate. Without it, the full rate is applied to the entire joint income by default.