How to Reclaim Overpaid Tax: P800 Letters, Tax Codes and the 4-Year Window

If you have been on the wrong tax code, the money is not gone. HMRC lets you reclaim overpaid income tax for four tax years back, and the claim is free to make yourself. A single wrong code can be worth £2,514 in a year, and codes usually stay wrong for months before anyone notices, so the four-year total is often larger than people expect.

This guide walks through the whole recovery: how to read a P800 calculation, how to check and correct your code inside your HMRC Personal Tax Account, which years are still open, why codes go wrong in the first place, and what a refund company actually costs if you use one.

Every figure here is an estimate based on published 2026/27 rates and thresholds. Confirm your own position with HMRC before acting on it.

What you can claim, and by when

The general time limit for a tax claim is four years from the end of the tax year concerned. HMRC's Self Assessment Claims Manual points to TMA 1970 s43 as the section that sets those time limits. You can see the same four-year reach in HMRC's live guidance: the Marriage Allowance page currently says "you can backdate your claim to 6 April 2022 (the 2022 to 2023 tax year) for any years you were eligible".

Here is what that means in practice, as at August 2026.

The ordering matters. 2022/23 is the year with the shortest runway, and it is also the year people are least likely to have paperwork for. If you have ever suspected a code was wrong that far back, check it before you check anything else.

For the current year the mechanism is different. You do not usually claim 2026/27 back as a lump sum. You correct the code, and your employer refunds the overpayment through your next payslip once HMRC issues the new code.

How to read your P800, section by section

A P800 is HMRC's own reconciliation of a completed tax year. HMRC's guidance on tax overpayments and underpayments states that the letters "are sent out between June and March of the following tax year", and that you only get one if you are employed or receive a pension. If you pay tax only through Self Assessment, no P800 is coming and your refund route is your tax return instead.

A P800 is short, but every line is checkable. Work down it in this order.

1. The tax year at the top. Confirm which year you are looking at before anything else. If you have several letters, they will not be in date order in the envelope.

2. Your income, listed per employment and pension. This is the section that catches most errors. Each employer and pension provider should appear once, with the pay figure that matches the P60 or final payslip for that job. If a job you left is still listed with income after your leaving date, or a job is missing entirely, the whole calculation below it is built on the wrong number.

3. Allowances and deductions. Your Personal Allowance appears here, normally £12,570 for each of the years currently claimable, since the allowance has been frozen. Below it you may see deductions such as a company car, medical insurance, an untaxed interest estimate, or an amount described as tax underpaid from an earlier year being collected through your code. Every deduction here reduces your tax free income, so every one of them is worth verifying. If you handed back the company car in 2024 and it still appears in 2025/26, that is a live error.

4. Taxable income and the tax due. HMRC applies the rates to whatever survived step 3. For a rest of UK taxpayer in 2026/27 that is 20% up to £50,270, 40% up to £125,140, and 45% above. Scottish taxpayers have a different band structure, so if your code starts with S the arithmetic on your letter will not match the figures above. Our Scottish tax codes guide sets out those bands.

5. Tax you actually paid. Cross check this against the "tax deducted" total on your P60 for that year. A mismatch here is worth querying directly rather than accepting.

6. The result. Either "you paid too much tax" with a refund figure, or "you paid too little" with an amount to settle. If the figure is a refund, move to the next section. If it says you owe, do not assume it is correct simply because it came from HMRC. The same input errors that produce refunds produce underpayments.

When the refund arrives on its own, and when you have to act

This is the part most guides get vague about, so here are the published timings from HMRC's own guidance.

You will need the reference number printed on the P800 and your National Insurance number to claim online.

The trap is the fourth row of the "why did nothing happen" list: no P800 at all. HMRC only reconciles automatically where its records flag a discrepancy. If the wrong figure was in its records all along, for example a benefit in kind that was never removed, the year can reconcile cleanly to a wrong answer and no letter is generated. Nobody will tell you. That is the most common reason a four-year claim sits unclaimed.

Check and correct your tax code in your Personal Tax Account

Your Personal Tax Account is the free HMRC service that does everything a refund company would do on the code side. Here is the walkthrough.

Step 1. Go to the right service. For the year in progress, use Check your Income Tax for the current year at tax.service.gov.uk. HMRC states this service covers 6 April 2026 to 5 April 2027, and that it is not available if Self Assessment is the only way you pay tax.

Step 2. Sign in or set up a Government Gateway ID. If you are creating one, HMRC may ask you to confirm your identity, which it says "usually involves using photo ID like a passport or driving licence". Have one to hand before you start rather than getting halfway and stopping.

Step 3. Read your current code and Personal Allowance. The service shows the code in force right now and the allowance behind it. Compare it with the code on your most recent payslip. They should match. If the payslip is behind, your employer has not yet applied a newer code.

Step 4. Open the breakdown. This is the screen that matters. HMRC shows how it built your allowance: the standard Personal Allowance, then every addition and deduction. Read each deduction and ask whether it is still true. Common stale entries are a car you no longer have, medical cover that ended, an estimate of untaxed interest that is far above what you actually earn, and an old underpayment still being recovered after it was already settled.

Step 5. Check your jobs and pensions. The service lists your estimated income from each employment and pension. Employers you have left should not be showing ongoing income. A second job you never started should not be there at all.

Step 6. Update what is wrong. HMRC's guidance is explicit that you can "update details of your income from jobs and pensions", and warns "you may pay too much or too little tax if they're not up to date". You can also tell HMRC about a change in circumstances that affects the code, such as a benefit ending.

Step 7. Watch for the new coding notice, then check the payslip. HMRC issues a P2 coding notice confirming the change and sends the new code to your employer. Check that your next payslip actually uses it. If the year is still running, the correction usually arrives as a lower deduction, or a refund, on that payslip.

For completed years, use Check your Income Tax for previous years. HMRC describes it as a service you can use "to get a tax refund or pay tax you owe" for a year that has already been calculated.

If you would rather see the arithmetic before you log in, our free tax code checker estimates what your code should be for your circumstances and what the gap is worth. It is an estimate to arm you with a number, not a substitute for confirming the position with HMRC.

The five reasons codes go wrong, and what each one costs

Every figure below assumes a rest of UK taxpayer and 2026/27 rates. Scottish and Welsh positions differ.

1. An emergency code that never got switched off

Codes ending W1, M1 or X are non cumulative. They tax each pay period in isolation instead of spreading your allowance across the year. Our guides to W1 codes and M1 codes cover the mechanics.

On a steady salary from April, an emergency code costs little. The damage comes when you start a job part way through the year. Say you begin work in October on £32,000, after months with no taxable income. On a cumulative code your first payslip would carry seven months of accumulated allowance, £7,332.50, against gross pay of £2,666.67, so the tax due would be nil. On an M1 code, only one month of allowance applies and roughly £324 is deducted. That gap repeats every month until the code is corrected, and none of it comes back until the year is reconciled.

2. A basic rate or flat rate code on your main job

BR taxes every pound at 20% with no Personal Allowance. 0T removes the allowance and applies the normal bands. D0 taxes everything at 40%. Each is perfectly correct on a second job or a second pension, and each is expensive on your only income.

The BR figure is not a coincidence. Losing the whole £12,570 allowance at 20% is £2,514 every year, at any basic rate salary.

3. A second job that was set up against the wrong employment

When you take a second job, HMRC has to decide which employment carries your allowance. It should be the higher paying one. When it lands on the smaller job, part of your allowance goes unused while your main income is taxed flat. This is common after a job change where the old employer was slow to submit a leaver record.

4. A benefit in kind that changed and was never removed

Company cars, fuel, and medical insurance are collected through your code as a deduction from your allowance. When the benefit ends, the deduction should end too. It frequently does not. A £6,000 car benefit left in your code costs a basic rate taxpayer £1,200 a year and a higher rate taxpayer £2,400. If it survived two years untouched, that is the whole claim on its own.

The reverse also happens. A K code means your deductions exceed your allowance, so there is no tax free pay at all. K codes are sometimes right and sometimes the fossil of a benefit that ended years ago.

5. Marriage Allowance never claimed, or claimed the wrong way round

Marriage Allowance lets the lower earner transfer £1,260 of Personal Allowance to a spouse or civil partner. HMRC states it "reduces their tax by up to £252 in the tax year", where the lower earner's income is normally below £12,570 and the higher earner's is between £12,571 and £50,270.

Because backdating currently reaches 6 April 2022, an eligible couple who never claimed can pick up as much as £252 for each of 2022/23, 2023/24, 2024/25, 2025/26 and 2026/27. That is up to £1,260 in total, and it is the single most overlooked item on this list.

For the full picture on how codes break and how to get them corrected, see our guides on what to do about a wrong tax code and how to change your tax code, or browse every UK tax code explained.

What four years actually adds up to

Here is a realistic composite. Sam is employed, earns £29,000, is married to a partner earning £41,000, and has never looked at a coding notice.

Nothing in that table is dramatic. It is four ordinary administrative gaps, none of which generated a letter, adding up to more than a month's take home pay. This is why the four-year sweep is worth doing even when no single year looks obviously wrong.

Run your own numbers year by year with the tax code checker, then confirm the final position with HMRC before you claim.

Refund companies: what 36% actually costs

Refund companies are legitimate businesses and some people genuinely prefer to hand the paperwork over. The point of this section is that you should know the price before you sign, because the fee is taken from money HMRC would have paid you in full for free.

RIFT, one of the larger UK operators, publishes its fee openly: "36% of your refund (including VAT), with a minimum fee of £60" for a PAYE rebate, and a fixed £354 including VAT for a CIS claim that includes filing a tax return. RIFT notes that VAT "is applied to our fee only, not to your whole refund". Other firms publish different rates. The published RIFT rate is used here because it is stated plainly on their own site and is straightforward to verify.

Applied to real refund sizes, that fee looks like this.

The sector has drawn sustained criticism. The Low Incomes Tax Reform Group has raised concerns including high fees, overinflated promises, unclear or hidden terms, difficulty contacting firms afterwards, and deeds of assignment that customers did not realise they were signing. An assignment routed the refund to the company rather than to the taxpayer, and LITRG documented cases where unconnected refunds were diverted as a result.

HMRC acted on this. In its 11 January 2023 announcement on repayment agents, HMRC set out a package that replaced legally binding assignments with cancellable nominations, introduced a 14 day cooling off period, added a registration process for repayment agents, and required agent communications to be "fair, clear, accurate and do not mislead or conceal material facts". Assignments of income tax repayments made on or after 15 March 2023 are void, meaning HMRC treats them as having no effect.

Two practical consequences. First, if you did sign something years ago, a nomination can now be cancelled, so check where a pending refund is actually going. Second, if a firm tells you a claim can only be made through them, that is not correct. Every route described in this guide is open to you directly, at no cost.

Making the claim yourself, step by step

If you have a P800 showing a refund. Claim online using the reference on the letter and your National Insurance number. Bank transfer is the fast route at around 5 working days. If the letter says a cheque is coming automatically, you do not need to do anything.

If the current year's code is wrong. Correct it in your Personal Tax Account using the seven steps above. The refund comes through payroll once your employer receives the new code, so there is no separate claim to make.

If a previous year looks wrong and no P800 arrived. Use the previous years service to see HMRC's calculation for that year. If it disagrees with your P60 and payslips, contact HMRC with the year, the employer, the figures you hold and what you believe the correct position to be.

If the overpayment came from unclaimed job expenses. Employment expenses have their own claim route through form P87 rather than a coding correction. HMRC's guidance on claiming tax relief for employment expenses sets out what qualifies and how to submit.

If it is Marriage Allowance. Apply through the Marriage Allowance service and ask for backdating in the same application. The lower earner is the one who applies.

What to have ready before you start. Your National Insurance number, a P60 for each year in question, final payslips from any job you left, the P800 reference if you have one, and the dates any benefit in kind started or ended. Missing paperwork is the main reason claims stall rather than fail.

Keeping it from happening again

Getting one refund is useful. Not needing the next one is better.

Check the code on your first payslip after 6 April each year, and again after any change: a new job, a second job, a benefit starting or ending, a pension beginning, a partner's income crossing the Marriage Allowance thresholds. Read the P2 coding notice when it arrives instead of filing it unread, because it is HMRC telling you in advance what it is about to do. Our guide to checking your tax code covers the five places you can find your current code in about two minutes.

One further thing worth knowing if any of that income is self employed. Making Tax Digital for Income Tax began in April 2026 for sole traders and landlords with qualifying income over £50,000, with lower thresholds following in 2027 and 2028. If your side income is heading that way, digital records and quarterly updates stop being optional. Our guide to MTD for sole traders explains who is in scope and when.

People also ask

How many years back can I claim overpaid tax?

How long does an HMRC tax refund take?

What if I never received a P800 but think I overpaid?

Is it worth using a tax refund company?

Will fixing my tax code refund me automatically?