Your contractor takes 20% from every labour payment before you see it. Your Self Assessment return is how you get the overpaid part back. Here's exactly what to enter, where it goes, what you can claim — and how HMRC works out your refund.
The 60-second version
- No return, no refund. HMRC doesn't send overpaid CIS back automatically — you have to file.
- Most subcontractors need SA100 + SA103S (self-employment short pages, turnover under £90,000). CIS income never goes on the employment pages.
- Three numbers matter: your gross income (before deductions), your expenses, and your total CIS deductions — Box 38 on the SA103S.
- 2025/26 deadlines: paper 31 October 2026, online 31 January 2027. If you're owed money, file now — there's no reason to wait.
Here's a question worth asking before you open the form: did your contractor's 20% take your personal allowance and your expenses into account? It didn't. The CIS deduction is a flat percentage of your labour — it ignores the first £12,570 you can earn tax-free, every tool you bought, every mile you drove, and every week you didn't work. That's why most CIS subcontractors overpay, and why most get a refund when they file.
Your Self Assessment return corrects it. You tell HMRC the full picture — what you earned, what it cost you to earn it, and how much was already deducted — and HMRC works out whether it owes you money or you owe more. This guide covers the 2025/26 tax year (6 April 2025 to 5 April 2026).
"Self Assessment isn't complicated for most CIS subcontractors. It's your income, your expenses and your CIS deductions — three numbers. The form just spreads them across several boxes."
Why CIS Subcontractors Must File a Return
HMRC does not refund overpaid CIS automatically. The deductions taken from your payments sit with HMRC until you file a return and claim them back. Don't file, and you never see that money.
You need to be registered for Self Assessment as self-employed. If you started working for yourself during 2025/26, the deadline to register is 5 October 2026. Registering gets you a UTR (Unique Taxpayer Reference), which you'll need to file.
Want a rough idea of your refund before you start?
Try the free CIS refund calculator →What You Need Before You Start
Gather everything first. It saves time, and HMRC's online service can time out if you stop to hunt for paperwork.
- Your UTR — the 10-digit number on your HMRC letters
- Your Government Gateway login — test it before you sit down to file
- Your National Insurance number
- Every CIS payment and deduction statement for the year, from every contractor
- Records of all business expenses — receipts, mileage log, insurance, phone bills
- Bank statements showing the payments you received, for cross-checking
- P60 or P45 — only if you also had a PAYE job
- Your bank details so HMRC can pay your refund
Which Forms Do You Need?
Self Assessment Forms for CIS Subcontractors
Most CIS subcontractors need SA100 + SA103S. File online and HMRC's system adds the right pages based on your answers — you don't need to download anything.
Filling In Your Return, Step by Step
Box numbers below refer to the paper SA103S. Online, you'll see the same questions with plain-English labels.
- Check your personal details (SA100). Online, most of this is pre-filled — confirm your name, address, UTR and National Insurance number. When asked about income types, tick self-employment (and employment too if you had a PAYE job).
- Enter your gross turnover — Box 9. Add up the gross amounts from every payment and deduction statement: what you were paid before CIS was taken. Not the amount that reached your bank.
- Enter your expenses — Boxes 11–19. Each category has its own box (materials, vehicle and travel, insurance, repairs, accountancy, phone, other costs such as PPE and training). The total goes in Box 20, and your net profit (Box 21) is turnover minus expenses.
- Enter your CIS deductions — Box 38. The total of all deductions from all your statements. This is the credit that creates your refund — don't leave it blank.
- Add your bank details. There's a section on the SA100 for your account name, sort code and account number. Without them, your refund takes longer to reach you.
- Review the calculation and submit. HMRC shows your tax position before you submit — check it against your own records. Save the submission reference.
Where Your CIS Deductions Go
Your CIS deductions are not an expense. They're tax already paid on your behalf, so they have their own field on the self-employment pages:
Where to Enter CIS Deductions
Which Expenses Can a CIS Subcontractor Claim?
This is where most subcontractors leave money behind. You can claim any cost incurred wholly and exclusively for your trade. Anything with a personal element — a phone you also use privately, a van you also drive at weekends — must be split between business and personal use.
| Expense | Claimable? | Notes |
|---|---|---|
| Tools and equipment | Yes | Hand and power tools, blades, bits. Larger plant and machinery may go through capital allowances instead. |
| PPE and protective workwear | Yes | Safety boots, hard hats, hi-vis, overalls. Ordinary clothing isn't claimable, even if you only wear it on site. |
| Mileage (simplified method) | Yes | 45p per mile for the first 10,000 business miles on your 2025/26 return. For journeys from 6 April 2026 (your 2026/27 return), the rate is 55p. Use either mileage or actual costs — not both. |
| Vehicle costs (actual method) | Yes | Fuel, insurance, repairs, road tax — business share only, backed by a mileage log. |
| Materials you supplied | Yes | Materials you bought and used on jobs, where not separately reimbursed. Keep receipts. |
| Phone | Partly | The business-use share. A phone used only for work is fully claimable. |
| Public liability insurance | Yes | A direct cost of running your trade. |
| Accountancy fees | Yes | Including the cost of preparing your return. |
| Training and certifications | Yes | If it keeps your skills up to date for your current trade — CSCS renewals, refreshers. Training for a new, unrelated trade isn't claimable. |
| Travel to a permanent workplace | No | Ordinary commuting isn't claimable. Travel to temporary sites and between sites generally is. |
| Everyday meals | No | Day-to-day food on site doesn't qualify. |
| Fines and penalties | No | Parking and speeding fines are never allowable. |
If your costs are very low, you can use the £1,000 trading allowance instead of itemising. For an active subcontractor, real expenses are almost always higher — so claiming them gives a bigger refund.
How Your Refund Is Calculated
Worked Example — 2025/26 (Illustrative)
Now the "aha" part. Run the same example without the £6,000 of expenses, and the refund drops to about £870. For a basic-rate taxpayer, every £1,000 of genuine expenses is worth roughly £260 more back — 20% income tax plus 6% National Insurance. Your receipts are, quite literally, money.
Plug in your own figures and see what your refund could be.
Try the free CIS refund calculator →National Insurance and Payments on Account
National Insurance for the Self-Employed
Class 4 is worked out on your profit, not your turnover — which is another reason every legitimate expense counts twice.
Deadlines and Penalties for 2025/26
| Date | What happens |
|---|---|
| 6 April 2026 | You can file your 2025/26 return |
| 5 October 2026 | Deadline to register for Self Assessment if you're new |
| 31 October 2026 | Paper return deadline |
| 31 January 2027 | Online return deadline — and deadline to pay any tax owed |
Late Filing Penalties
Never filed before? You can still claim for up to four previous tax years — and the deadline for 2022/23 is 5 April 2027. See how long a CIS refund takes, and which years you can still claim.
Mistakes That Shrink or Delay Your Refund
- Entering net instead of gross income — use the top-line figure from your statements, not your bank balance
- Putting CIS income on the employment pages (SA102) — it belongs on the self-employment pages
- Leaving the CIS deductions box blank — no entry, no credit
- Estimating your CIS figure — mismatches with contractor records pause refunds
- Not claiming expenses — every £1,000 missed costs a basic-rate taxpayer around £260
- Leaving out bank details — your refund takes longer to arrive
- Waiting until January when you're owed money
Your Pre-Filing Checklist
- All payment and deduction statements collected, from every contractor
- Gross income totalled (before CIS deductions)
- CIS deductions totalled
- Expenses totalled, receipts kept
- Mileage log ready — 45p a mile for 2025/26 journeys
- UTR and Government Gateway login ready
- Bank details ready for your refund
- Any other income noted (PAYE job, rent, savings interest)
- Checked whether Payments on Account might apply
Frequently Asked Questions
Do CIS subcontractors have to file a Self Assessment tax return?
Yes. CIS subcontractors working as sole traders are self-employed and need to register for Self Assessment and file a return each year. It's also the only way to reclaim overpaid CIS deductions — HMRC doesn't refund them automatically.
Which Self Assessment form do CIS subcontractors use?
The SA100 main return plus the self-employment pages: SA103S (short) if your turnover is under £90,000, or SA103F (full) if it's £90,000 or more. CIS income never goes on the employment pages (SA102). If you file online, HMRC adds the right pages for you.
Where do I enter CIS deductions on my tax return?
On the SA103S, CIS deductions go in Box 38. On the SA103F, it's Box 82 on recent versions — check the current form. Online, it's the CIS deductions question in the self-employment section. Enter the total from all your payment and deduction statements. It's a tax credit, not an expense.
Do I enter gross or net CIS income?
Always gross — the amount before CIS was deducted. If you were owed £1,000 and paid £800, your income is £1,000 and your CIS deduction is £200, entered in its own box.
What mileage rate can I claim?
On your 2025/26 return, 45p per mile for the first 10,000 business miles in a car or van, and 25p after that. For journeys from 6 April 2026 — your 2026/27 return — the first-10,000-mile rate is 55p. You can use the mileage method or actual vehicle costs, but not both.
When is the Self Assessment deadline for 2025/26?
31 October 2026 for paper returns and 31 January 2027 online. You can file from 6 April 2026 — and if you're owed a refund, filing early means you get it sooner.
Can I file if I'm missing some deduction statements?
Ask the contractor first — they're legally required to provide them. If that fails, contact HMRC's CIS helpline on 0300 200 3210. Don't estimate: figures that don't match your contractors' returns can delay or block your refund.
I also have a PAYE job. How does that affect my return?
Your PAYE income goes on the employment pages (SA102) and your CIS work on the self-employment pages. HMRC combines them to work out your tax. Your CIS deductions still count in full, but your Personal Allowance may already be used by your job, which can reduce your refund.
This article is for general information only and does not constitute tax advice. Box numbers, rates and deadlines can change between tax years — always check the current year's notes on gov.uk or speak to a qualified accountant. QuoteDone is not affiliated with or endorsed by HMRC.