Self Assessment for CIS subcontractors: a step-by-step 2026/27 guide
If you're a subcontractor with CIS taken off your pay, a Self Assessment tax return is how you get that money reconciled — and most subbies are owed a refund.
By Bobby Gardiner·13 July 2026·6 min read
- Under CIS, contractors deduct 20% (if you're registered) or 30% (if you're not) from your labour and pay it to HMRC before you see a penny.
- Because those deductions ignore your personal allowance and expenses, most subcontractors are due a refund — not a bill.
- You need a UTR and to be registered for Self Assessment; the online filing deadline is 31 January.
- Keep your monthly CIS statements and expense records — but if statements go missing, they can be rebuilt from your bank.
- Refunds usually land around two weeks after filing, paid straight to you — never through us, and never as a cut of your money.
- From April 2026, Making Tax Digital applies to subbies whose gross self-employment income (turnover, before expenses) is over £50,000.
If you work on the tools and have tax deducted before you're paid, you're inside the Construction Industry Scheme (CIS) — and that means a Self Assessment tax return every year. It sounds like a chore. In reality, for most subbies it's the moment you get money back.
Here's the thing hardly anyone explains on site: CIS deductions ignore your personal allowance and your expenses. So the tax taken off your labour is almost always more than you actually owe. Filing your return is how you claim the difference.
This guide walks you through the whole thing in plain English — what CIS is, why you're probably due a refund, what you need to get sorted, the deadlines that matter for the 2026/27 tax year, and how the refund lands in your account. No jargon, no scare tactics.
What CIS actually is (and why you're in it)
The Construction Industry Scheme is HMRC's way of collecting tax from construction work at source. If you're a subcontractor, the contractor who pays you is required to take a slice off your labour and send it to HMRC on your behalf.
If you've registered for CIS, they deduct 20%. If you haven't registered, it jumps to 30% — which is a good reason to get registered. Either way, you're still self-employed: a sole trader running your own show, responsible for your own tax return.
That's the part that trips people up. The deduction isn't your final tax bill. It's an advance payment against it. The Self Assessment return is where it all gets squared up.
- Registered subbie: 20% deducted from labour
- Unregistered subbie: 30% deducted from labour
- Deductions apply to labour only — not materials
- You remain self-employed and must file a Self Assessment return each year
Why most subbies get a refund
This is the good news. When a contractor deducts CIS, they don't factor in your personal allowance (the chunk of income you can earn tax-free) or any of your expenses — tools, van, fuel, materials, protective gear, insurance, phone.
So you've effectively overpaid all year. When you file your return, your real taxable profit is worked out properly — allowance applied, expenses knocked off — and the CIS already deducted is credited against that final figure. Nine times out of ten, that leaves HMRC owing you money.
Want a rough idea before you file? Our free CIS refund estimator gives you a ballpark in a couple of minutes. And if you'd rather someone just handle the lot, that's exactly what our CIS accounting service is for.
What you need before you file
Getting organised upfront makes the whole thing painless. Here's your checklist.
Missing a few CIS statements? Don't panic. If they've been lost, we can reconstruct your deductions from your bank records — so a shoebox that's seen better days won't hold you up.
- A UTR (Unique Taxpayer Reference) — your 10-digit tax ID, and registration for Self Assessment
- Your monthly CIS statements — the contractor should give you one each month showing pay and deductions
- Records of your income — invoices and bank deposits
- Records of your expenses — tools, van, fuel, materials, PPE, phone, insurance, use of home
The key 2026/27 deadlines
The tax year runs 6 April to 5 April. Miss the big dates and HMRC charges penalties, so it's worth marking these on the calendar.
One extra thing to know: if your tax bill is over £1,000 and less than 80% was collected at source, you may have to make payments on account — advance instalments towards next year's bill. For most refund-due subbies this won't bite, but it's worth being aware of.
- 5 October after the tax year you first started working for yourself — a one-off deadline to register for Self Assessment (it only applies in your first year; after that you're already registered)
- 31 October — deadline for paper returns
- 31 January — deadline for online returns and any balancing payment due
- Register early — leaving it late is the most common reason refunds get delayed
How to file — and how the refund works
You file your Self Assessment return online through HMRC, or you get an accountant to do it for you. The return totals up your income, deducts your expenses, applies your personal allowance, and credits the CIS already taken. If you've overpaid, the balance is your refund.
Once filed, refunds usually arrive around two weeks later, paid directly to you — into your own bank account. HMRC does run checks, and the January rush can add a little time, so filing earlier in the year gets your money back sooner.
We charge a fixed fee agreed upfront — never a percentage of your refund. Your refund is yours; it comes to you, not through us. If you'd like a real person to take the whole thing off your plate, get in touch and we'll sort it.
A quick word on Making Tax Digital
Making Tax Digital for Income Tax (MTD ITSA) is changing how self-employed people report to HMRC — moving from one annual return to quarterly digital updates.
From April 2026 it applies if your gross self-employment income (your turnover, before expenses) is over £50,000, then over £30,000 from April 2027 and over £20,000 from April 2028. If that's you, it's nothing to fear — it just means keeping digital records and sending updates through the year. Our Making Tax Digital guide explains exactly what it means for subcontractors and how we make it seamless.
Let's get your CIS refund sorted
Deal directly with Bobby Gardiner (CAT FMAAT) — not a call centre. Fixed fee agreed upfront, your refund paid straight to you, and no nasty surprises. Try the free CIS refund estimator for a ballpark, or get in touch and we'll handle the whole return for you.
Common questions
Do CIS subcontractors have to do a Self Assessment tax return?
Yes. As a CIS subcontractor you're self-employed (a sole trader), so you must file a Self Assessment tax return each year. It's how the CIS deducted from your pay gets reconciled against your actual tax bill — and because deductions ignore your personal allowance and expenses, most subbies end up due a refund rather than owing anything.
How much CIS refund will I get and when?
It depends on your income, expenses and how much CIS was deducted, but because the 20% (or 30%) taken off ignores your tax-free personal allowance and your expenses, most subcontractors are owed money back. Refunds usually arrive around two weeks after filing, paid directly into your own bank account. You can get a quick ballpark with our free CIS refund estimator.
What happens if I've lost my CIS statements?
Don't worry — you don't need every piece of paper. If your monthly CIS payment and deduction statements have gone missing, we can reconstruct your deductions from your bank records, so lost statements won't stop you filing or claiming your refund.