Self Assessment for CIS subcontractors in the UK
Most CIS subcontractors in the UK are self-employed rather than employees, which means no one deducts your tax for you. You register with HMRC, file a Self Assessment return once a year, and pay what you owe by 31 January. This guide covers what to declare, what you can claim, and the dates that matter.
Do you need to file?
You need to register for Self Assessment if your self-employed income was more than £1,000 in the tax year, before expenses.
The tax year runs 6 April to 5 April. If you started during the 2025-26 tax year, your deadline to register with HMRC is 5 October 2026, and your first return is due by 31 January 2027.
If you earned under £1,000 you may be able to use the Trading Allowance and not file at all. You cannot use the Trading Allowance and claim expenses at the same time.
Which forms you need
| Form | When it applies |
|---|---|
| SA100 | The main Self Assessment return. Everyone files this. |
| SA103S | Self-employment (short) pages. Your self-employed income and expenses go here. |
What you can claim
These are the costs CIS subcontractors can usually claim, where each is incurred wholly for the work:
- Tools and equipment: Tools and equipment used for the work, with larger items claimed as capital allowances.
- Protective clothing: Protective clothing needed for the work, such as boots, gloves and hi-vis.
- Public liability insurance: Public liability cover for injury or damage to others.
- Travel between sites: Travel between different work sites during the working day.
- Materials: Materials used up on jobs and charged into the work.
- Accountancy fees: Fees paid to an accountant for the business.
- CSCS card: The cost of the test and the card itself, required for access to most construction sites, together with any renewal.
- Van running costs: Fuel, servicing and repairs for a work van, apportioned for any private use.
- Phone and data: The business share of your mobile phone and data.
- Training: Training that maintains or updates skills for your current work.
- Home office: A fair share of home working costs, by the flat rate or by apportioning bills.
- Parking: Parking paid legitimately while working. Parking and other fines are never allowable.
The rule most CIS subcontractors get wrong
The 20 percent CIS deduction taken by your contractor is tax already paid. It goes in the CIS deductions box and is set against your bill, which is why most CIS subcontractors are owed a refund rather than a payment. You need every CIS payment and deduction statement.
Keep a simple record you can produce if HMRC asks. If you are not yet within Making Tax Digital, a spreadsheet is enough; HMRC does not require a specific format, only a record.
Deadlines for the 2025-26 tax year
| Date | What happens |
|---|---|
| 5 April 2026 | Tax year ends |
| 5 October 2026 | Deadline to register with HMRC if this is your first return |
| 31 October 2026 | Paper return deadline |
| 31 January 2027 | Online filing deadline and payment deadline |
| 31 January 2027 | First payment on account, if your bill is over £1,000 |
| 31 July 2027 | Second payment on account |
Payments on account catch a lot of first time filers out. If your bill is over £1,000, you pay it in full plus half again towards next year, on the same day.
National Insurance
As a self-employed person you pay Class 4 National Insurance at 6% on profits between £12,570 and £50,270, and 2% above that, for the 2025-26 tax year. Class 2 is no longer compulsory, but you may choose to pay it voluntarily to protect your State Pension record if your profits are low.
Making Tax Digital
From 6 April 2026, if your qualifying income from self-employment and property, before expenses, is over £50,000, you must keep digital records and send quarterly updates to HMRC through MTD-compatible software, then a final declaration after the year ends. The threshold falls to £30,000 from April 2027 and to £20,000 from April 2028. HMRC uses the income on an earlier tax return to decide when you join, and writes to tell you. Below the threshold, you file a normal Self Assessment return.
Direct submission to HMRC via the Making Tax Digital API is in development, pending HMRC production access.
How TaxEase UK helps
TaxEase UK walks you through your Self Assessment step by step, in your own language, with every box explained in plain words.
Scan your statements and invoices with the built-in OCR and the figures fill in themselves. The deductibility checker tells you yes, no or partial for any expense you are unsure about.
Your tax data stays on your device unless you choose to send it to your accountant or to HMRC. No bank account link, no data sharing.
Frequently asked questions
Do CIS subcontractors need to file a Self Assessment?
If your self-employed income was more than £1,000 in the 2025-26 tax year, before expenses, yes. Below that you may be able to use the Trading Allowance instead.
Does HMRC already know what I earned?
Often, yes. Digital platforms and some payers report income to HMRC, so the figures you declare need to match. Declare your gross income and claim your costs separately.
What if I also have a normal PAYE job?
You declare both. Employment income goes on SA102 from your P60 or P45, and your self-employed income on SA103S. Tax already paid through PAYE is credited against your total bill.
I only worked part of the year, do I still file?
If your income for the work was over £1,000 in the tax year, yes, however few months you worked.
Can TaxEase UK do this in my language?
Yes. The whole return is guided in 10 languages, with every box explained in plain words, and the free version covers the main steps with no time limit.
Related guides
TaxEase UK is an independent software product. We are not affiliated with or endorsed by HMRC or the UK Government. Figures shown apply to the 2025-26 tax year.