Your situation
Paying Self Assessment through your tax code
Independent site, not HMRC. Only HMRC can change your code: use your HMRC online account or app.
A balance from your tax return, collected by your employer or pension provider over the next tax year: who qualifies and what it does to each payslip.
Pay a Self Assessment bill through your code
Can be collected through the code
£100 a month
| Under the limit | yes |
| Usual tax on the PAYE pay | £3,486 |
| Total no more than twice the usual tax | yes |
File online by 30 December (paper by 31 October) for this to apply.
A Self Assessment balance can be collected through your PAYE tax code instead of being paid to HMRC in one go, provided three conditions all hold, according to GOV.UK: the bill is under £3,000, you already pay tax through PAYE as an employee or with a workplace or private pension, and your return reached HMRC by 31 October on paper or by 30 December online. When they do, HMRC sets it up on its own unless you asked on your return for it not to. The amount is then spread evenly over 12 months and deducted from your salary or pension, on top of your normal deductions. A £1,200 bill means about £100 more each month. HMRC refuses when your PAYE income is too small, when the total would exceed half of that income, or when you would end up paying more than twice your usual tax: on £30,000 of salary the usual tax is £3,486.00, so the ceiling is generous; on a small part-time wage it is not.
Owed tax collected through your code
Code next tax year
657L
| Allowance taken off | £6,000 |
| Extra each month over 12 | £100 |
| Limit for coding out | under £3,000 |
HMRC reduces the code by the amount owed divided by your rate, so the extra tax adds up to the debt.
Rules read on HMRC’s texts by Radif Partners · Editorial policy · Method
The three conditions and the filing dates
Coding out a Self Assessment balance is not something you apply for separately. HMRC does it when each of these is true (GOV.UK):
- you owe less than £3,000 on the tax bill;
- PAYE already takes your tax, for instance as an employee or through a company pension;
- your paper return arrived by 31 October, or your online return by 30 December.
The two dates are the practical hurdle. Filing on paper gives you a shorter window than filing online, and a return sent after the relevant date cannot use your code, even if the amount and the income would otherwise qualify. If you want the balance spread through your pay, file online early in the autumn and do not ask, on the return, for the balance to be kept out of your code.
The cases where HMRC will not use your code
Meeting the three conditions is not the end of it. GOV.UK lists four situations in which the bill stays outside PAYE: your PAYE income is too low for HMRC to collect it; the collection would mean paying more than half of your PAYE income in tax; you would end up paying more than twice your normal tax; or you owed £3,000 or more and paid part of it to get under the limit. The table applies the two income tests to a £1,500 bill at several salaries, with the 2026 to 2027 rates for England and Northern Ireland.
| PAYE pay | Usual tax | Usual tax plus bill | Twice the usual tax | Through the code? |
|---|---|---|---|---|
| £12,000 | £0 | £1,500 | £0 | no, pay directly |
| £16,000 | £686 | £2,186 | £1,372 | no, pay directly |
| £20,000 | £1,486 | £2,986 | £2,972 | no, pay directly |
| £25,000 | £2,486 | £3,986 | £4,972 | yes |
| £40,000 | £5,486 | £6,986 | £10,972 | yes |
The pattern is clear. On a modest wage the usual tax is small, so even a mid-sized bill more than doubles it and the twice-the-usual test fails. Once the usual tax reaches the size of the bill, the twice-the-usual test is met, and on these salaries the half-of-pay test is met too. The calculator at the top runs the same checks on your own figures.
What the coded balance does to your payslips
Once accepted, the balance is deducted in equal amounts from your salary or pension over 12 months, together with your usual tax. In practice HMRC lowers the number in your code so that the allowance lost, taxed at your rate, matches the debt; the same arithmetic as for an ordinary underpayment. A basic rate payer with a £1,500 balance loses £7,500 of allowance, so 1257L becomes 507L.
What a new code number changes
Change in tax each month
+£125.00
| Allowance change | −£7,500 |
| Tax over the year with 1257L | £3,484.00 |
| Tax over the year with 507L | £4,984.00 |
If the balance is larger than the allowance you have left, the code can turn into a K code. The K code rule still caps the deduction at half of your pay before tax in each period, so the tax taken never swallows a whole payslip.
Checking that the balance has been coded
Your HMRC online account shows your current code and tells you whether it has changed. Compare the new number with your previous code: a drop that matches the balance divided by your rate is the coded Self Assessment debt. If it does not appear and the payment date is approaching, check your Self Assessment statement in the same account and pay the balance directly; the coding route only covers what HMRC has actually put into the code.
A Self Assessment taxpayer does not receive a P800 letter. Under- and overpayments are settled through the return itself, which is why the filing date decides whether your pay can absorb the balance.
Class 2 National Insurance and other exceptions
If you are self-employed as well as employed, watch the National Insurance part of the bill. Class 2 National Insurance cannot go through the code unless it has been due since before 6 April 2015. Any part of the bill that cannot be coded has to be paid by the deadline using another method, so check your statement line by line rather than assuming the whole balance is covered.