If you started CIS work in the 2025–26 tax year (6 April 2025 to 5 April 2026), your first Self Assessment return is probably due online by 31 January 2027. Most of the work is setting things up: being registered, having a Unique Taxpayer Reference (UTR) and being able to sign in. HMRC's lead times for those are measured in days, so it's worth starting now. This guide covers sole traders. There's a short note on partners.
The short answer
- Do you need a return? GOV.UK says you must send one if, in the last tax year, you were "self-employed as a 'sole trader' and earned more than £1,000", or "a partner in a business partnership".
- Registering. The deadline to tell HMRC was 5 October 2026. You can still register. The deadline for your return may then be later, but the tax is still due by 31 January 2027.
- Lead times. Register online and GOV.UK says your UTR "will be in your personal tax account within 72 hours". By post, it's usually 15 working days.
- On the return. GOV.UK says you record "your total pay before deductions were taken, as 'income'" and "the total deductions contractors have taken, as 'CIS deductions'".
Step 1: check whether you're already registered
Being registered for CIS and being registered for Self Assessment can happen together. GOV.UK's CIS how to register page says the online CIS service needs a UTR. If you don't have one, you "register as a new business for Self Assessment and choose 'working as a subcontractor' when prompted. You'll be registered for Self Assessment and CIS at the same time."
So if you registered that way, you already have a UTR. GOV.UK's find your UTR page says it's a 10-digit number, "It might just be called 'tax reference'". You'll find it in your personal tax account, the HMRC app, or on letters from HMRC.
If you registered for CIS by post, or only gave contractors your details, check you have a UTR before going further.
Step 2: register, if you haven't (and the 5 October rule)
GOV.UK's register for Self Assessment page says you had to tell HMRC "by 5 October 2026" if you hadn't sent a return before. It adds: "If you tell HMRC after 5 October 2026, you could get a penalty."
What registering late means, from GOV.UK's deadlines page:
- HMRC "will send you a letter or email with a different deadline to send your tax return by - this will be 3 months from the date on the letter or email".
- "You must still pay the tax you owe by 11:59pm on 31 January 2027 or you'll get a penalty."
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", based on the amount still left to pay.
To register as self-employed online, you sign in (or create sign-in details), and "You can save your progress and come back to finish registering later." By post, the form is CWF1. Partners use form SA401.
If you've "registered for Self Assessment before but did not send a tax return last year", GOV.UK says you may need to reactivate your account instead.
Step 3: GOV.UK's lead times
| Step | What GOV.UK says |
|---|---|
| UTR after registering online | "within 72 hours", in your personal tax account |
| UTR after registering by post | "usually ... in 15 working days (21 days if you're abroad), but it may take longer during busy periods" |
| Activation code, if you're already registered but didn't use the online service last year | "in the post within 7 working days (21 if you're abroad)" |
| Locked out after wrong sign-in details | "Your account will unlock after 2 hours" |
If you're waiting longer than that, HMRC's reply-time tool covers Self Assessment, including UTRs. GOV.UK says it's updated weekly.
Step 4: sign in to HMRC online services
GOV.UK's sign-in page says there are two ways in:
- Government Gateway: a user ID (up to 12 characters) and password
- GOV.UK One Login: an email address and password
If you've never used HMRC online services, GOV.UK says to select "Create new sign in details", "even if you have GOV.UK One Login sign in details from another government service". When you sign in, you may get an access code by text, voice call or authenticator app.
To file online you also need your UTR. GOV.UK says "You'll be told if you need to prove your identity when you sign in", which "usually involves using photo ID like a passport or driving licence". You don't have to finish in one go: "You can save your entry and go back to it later."
Step 5: what you need from your statements
The return asks for two CIS figures for the tax year, and HMRC's notes are specific about them:
- Turnover includes "the full amount ... before any Construction Industry Scheme (CIS) deductions made by contractors".
- CIS deductions are "all the deductions made by contractors from 6 April 2025 to 5 April 2026. Use the statements that contractors gave you."
That means a statement from each contractor for every tax month you were paid in, from the month ending 5 May 2025 to the month ending 5 April 2026. You don't send them with the return, but GOV.UK says to keep them, "as HMRC might ask for evidence of your deductions".
Our other guides cover the detail:
- what each line means: CIS payment and deduction statements explained
- missing statements: Is there a CIS end-of-year statement?
- totalling them: How to add up your CIS deductions for your tax return
- which boxes: Where CIS deductions go on your 2025–26 tax return
One point for small first years. The SA103S notes say income of £1,000 or less doesn't always need reporting. But if "you're a subcontractor and want to claim back tax deducted under the Construction Industry Scheme (CIS)", you complete boxes 1 to 8 and box 38.
Step 6: what a first-year bill can include
GOV.UK's payments on account page says that if you didn't make any payments on account last year, "for example, because it's your first time filing Self Assessment", you may need to pay both "the full amount of your tax calculation" and "your first payment on account towards your next bill" by 31 January. Whether that applies depends on how much of the bill your CIS deductions covered. Our guide CIS payments on account explains the tests.
Worked example (made-up dates)
Every date and detail here is invented. Sam started CIS work in mid-June 2025 and gave contractors only a name and National Insurance number. Sam never registered for Self Assessment.
| Date | What happens |
|---|---|
| 9 November 2026 | Sam registers online as self-employed |
| By 12 November 2026 | UTR in Sam's personal tax account (GOV.UK: "within 72 hours") |
| Later in November | HMRC writes with Sam's filing deadline: 3 months from the date on the letter |
| 31 January 2027 | Tax owed for 2025–26 is still due by 11:59pm, whatever the filing date |
Sam's statements run from the tax month ending 5 July 2025 to 5 April 2026, so Sam expects up to 10 from each contractor.
FAQ
I'm registered for CIS. Isn't that enough? GOV.UK says the online CIS registration needs a UTR, which comes from Self Assessment. Sole traders claim deductions through "your Self Assessment tax return as usual".
Can I file before 31 January? Yes. GOV.UK says you can file "anytime on or after 6 April following the end of the tax year".
I'm a partner. Is it different? You register with form SA401. GOV.UK's online filing page says you can't use HMRC's service to file "for a partnership". The partnership's return uses commercial software or paper forms.
Your first year's statements, totalled
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 register you, sign in to HMRC or fill in 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.