If you're a CIS subcontractor working as a sole trader or partner, your 2025–26 Self Assessment return is due online by 11:59pm on 31 January 2027. Any tax you owe is due the same day. This checklist puts each step in date order, with GOV.UK's own lead times, so nothing is left to the last weekend. One date to note now: 31 January 2027 is a Sunday.
The short answer
- Return: online by 11:59pm on 31 January 2027 (GOV.UK deadlines). The paper deadline, 31 October 2026, has passed.
- Payment: any 2025–26 tax still owed, plus any first payment on account for 2026–27, by 11:59pm on 31 January 2027.
- Second payment on account: 31 July 2027, if you have to make them.
- The tax year covered: 6 April 2025 to 5 April 2026. Your CIS deductions for that year count towards the bill. GOV.UK says HMRC "will work out how much tax you owe and take off any deductions taken by contractors".
Now (November 2026): check you can get in
1. Are you registered for Self Assessment? GOV.UK says you had to tell HMRC by 5 October 2026 if you've not sent a return before. If you register after that, HMRC "will send you a letter or email with a different deadline to send your tax return by", 3 months from the date on it. But "You must still pay the tax you owe by 11:59pm on 31 January 2027". GOV.UK's penalties page says that if you register after 5 October and don't pay all your bill by 31 January, "you may get a 'failure to notify' penalty". Our guide to your first tax return as a CIS subcontractor covers registering.
2. Do you have your UTR? GOV.UK's register page says that if you register online, your UTR "will be in your personal tax account within 72 hours". By post, "You'll usually get your UTR by post in 15 working days (21 days if you're abroad)", and it "may take longer during busy periods".
3. Can you sign in? You need sign-in details and your UTR to file online. Lead times from GOV.UK:
| Step | GOV.UK's timing |
|---|---|
| Already registered, but didn't use the online service last year | Activation code "in the post within 7 working days (21 if you're abroad)" |
| Identity check | You'll be told when you sign in; it "usually involves using photo ID like a passport or driving licence" |
| Locked out after wrong details | "Your account will unlock after 2 hours" |
Access codes for signing in can come by text, voice call or authenticator app (sign-in help). If you're waiting on HMRC, its reply-time tool covers Self Assessment and UTRs.
November to December: statements and totals
4. Collect every statement for the year. GOV.UK's get paid page says contractors must give you one "within 14 days of the end of each tax month". For 2025–26 that's the tax months ending 5 May 2025 to 5 April 2026, from each contractor you worked for. The last was due by 19 April 2026. If any are missing, our guide Is there a CIS end-of-year statement? covers who to ask and HMRC's address.
5. Total them. You need two figures: pay before deductions and total CIS deductions. See How to add up your CIS deductions for your tax return. For which boxes they go in, see Where CIS deductions go on your 2025–26 tax return.
30 December 2026: only if you also have a job or pension
6. Paying through your tax code. GOV.UK says HMRC can collect your bill through your PAYE tax code if you owe less than £3,000, you already pay tax through PAYE, and you file online by 30 December. "If you miss this deadline, you'll have to pay another way."
January 2027: send the return, then pay
7. Send the return. GOV.UK says sending it earlier means you "find out what you owe" and "have time to pay your bill by 31 January". The online return doesn't have to be done in one go. You "can save your entry and go back to it later".
8. Work out what's due. By 31 January GOV.UK says you pay any tax owed for 2025–26 (the balancing payment) and your first payment on account for 2026–27, if you have to make one. Your online account shows the amounts. Our guide CIS payments on account explains when they apply and how CIS deductions affect them.
9. Pay in time for a Sunday deadline. GOV.UK's pay your bill page says that if the deadline falls on a weekend, your payment must reach HMRC "on the last working day before", unless you pay by Faster Payments or by debit or credit card. For 31 January 2027 that's Friday 29 January.
| How you pay | GOV.UK's timing | Latest day to pay, by our count |
|---|---|---|
| Debit card online | Accepted "on the date you make it", including weekends | Sunday 31 January (the weekend rule doesn't apply) |
| Faster Payments | "Usually" same or next day, including weekends | The weekend rule doesn't apply, but GOV.UK only says "usually" |
| CHAPS | Usually same working day | Friday 29 January |
| Bacs | Usually 3 working days | Tuesday 26 January |
| Direct Debit, already set up with HMRC | 3 working days | Tuesday 26 January |
| Direct Debit, first time | 5 working days | Friday 22 January |
Pay with your 11-character reference: your 10-digit UTR followed by "K". GOV.UK warns a wrong reference can mean the payment is "delayed or used to pay a different tax bill".
10. If you can't pay in full. GOV.UK says you "might be able to set up a payment plan (also known as a 'Time to Pay' arrangement)". You can check online whether you're eligible.
31 July 2027
11. Second payment on account, if you have to make them. Contractors keep taking deductions through 2026–27, and those count towards that year's bill, which is settled on the 2026–27 return.
What happens if you're late
From GOV.UK's penalties page.
Late return:
- an initial £100 penalty
- after 3 months, £10 a day, up to £900
- after 6 months, a further 5% of the tax due or £300, whichever is greater
- after 12 months, another 5% or £300, whichever is greater
Late payment: 5% of the tax unpaid at 30 days, 6 months and 12 months, plus interest on the amount owed.
"All partners will be charged a penalty if a partnership tax return is late." If you have a reasonable excuse, you can appeal. GOV.UK has a tool to estimate a penalty.
FAQ
Is there a separate CIS deadline? Not for subcontractors' deductions. GOV.UK says sole traders and partners claim them through "your Self Assessment tax return as usual".
Can I file before I've got every statement? GOV.UK lets you send the return any time after 5 April. The CIS box asks for all deductions from 6 April 2025 to 5 April 2026, so a missing statement means a missing amount. Changing a return later is covered in Missed CIS deductions on an earlier tax return?.
How long until a refund? See CIS refund: when you can claim it and how long it takes.
I work through a limited company. Does 31 January apply? Not for CIS deductions. GOV.UK says a company uses its payroll scheme instead.
Getting step 5 done in one go
cissplit reads your CIS statements (PDFs or photos) and checks the deduction rate on each one. It flags gaps between a contractor's months and two statements for the same month, then totals the tax year, with every statement listed by contractor and tax month. Your first 3 statements are free, with no sign-up. It doesn't fill in or send your return, and you still check the figures against your statements.
This isn't tax advice. Check GOV.UK's CIS guidance for subcontractors or an accountant for your situation, and check the figures before you submit anything.