Why Self Assessment is the heart of being a CIS subcontractor
If you are a sole-trader subcontractor in the Construction Industry Scheme (CIS), the money taken from your payments during the year is not your final tax bill. It is an advance on account. A contractor deducts 20% from your labour (or 30% if you are not registered) and pays it to HMRC before you ever see it, and that happens regardless of what your expenses are or whether you have used your personal allowance. The place where all of that gets reconciled, where HMRC works out what you genuinely owe and compares it to what has already been taken, is your annual Self Assessment return.
For most registered subcontractors the reconciliation goes one way: too much has been deducted, and the return produces a refund. That is not a quirk or a loophole, it is the predictable result of the scheme taking 20% off your labour before any of your costs or your tax-free allowance are counted. Understanding the return is therefore the difference between getting back what you are owed and leaving a four-figure sum with HMRC by default.
This guide covers who must file, exactly what goes on the return, how the refund is calculated with real numbers, the limited-company alternative that bypasses Self Assessment entirely, the records you need, the deadlines and penalties, the four-year back-claim rule and the Making Tax Digital timeline that now tests subcontractors on gross income. If you want the shorter, practical walkthrough of lodging a claim, our guide to how to claim a CIS tax refund sits alongside this one.
Who must file a Self Assessment return
Every sole-trader CIS subcontractor must file. Being paid under CIS makes you self-employed for tax, and self-employment brings a Self Assessment obligation in its own right, whatever the CIS position. The same applies if you are a partner in a partnership that does CIS work, where the partnership files its own return and each partner files individually.
You must file even when you expect a refund. The deductions taken at source do not discharge the filing duty, and HMRC does not automatically calculate and repay your over-deduction. Nothing comes back until the return is in. Equally, you must file even in a year you made little or no profit, because the return is how the position is confirmed and the refund released.
The one group that does not use Self Assessment for the CIS offset is limited companies. A company recovers CIS through its payroll filings, not through a personal return, and we cover that route in full below. A company director may still file a personal Self Assessment return (for example to report dividends), but the company's CIS is not reclaimed there.
What goes on the return: the three figures that matter
Strip the return down to its CIS essentials and three numbers drive everything.
1. Gross CIS income. This is your total construction earnings for the tax year before any CIS was deducted, not the net amount that landed in your bank. If a contractor paid you £4,000 after taking £1,000 of CIS, your gross income for that job is £5,000. Using the banked figure understates your turnover and corrupts every downstream calculation, including the Making Tax Digital threshold test.
2. Materials and allowable expenses. You deduct two kinds of cost from gross income to reach your taxable profit. First, the materials you bought for jobs (these were already outside the CIS deduction base, and they are an expense here too). Second, your allowable business expenses: tools and equipment, van running costs, fuel or mileage at 55p per mile for the first 10,000 business miles from April 2026, insurance, protective clothing, accountancy fees, use of home and so on. Getting these complete is what turns a modest refund into the full one. Our guide to allowable expenses for CIS subcontractors sets out what counts and what does not.
3. CIS deducted. This is the total amount taken from your payments across the year, and it is the figure that gets set against your actual liability to produce the refund or balance. It comes straight from your CIS payment and deduction statements, the monthly statements every contractor must give you. HMRC matches the figure you enter against what contractors reported, so it has to reconcile to your statements.
How the refund is calculated
The logic is the same every year. Work out the tax and National Insurance actually due on your profit, then compare it to the CIS already taken. Whichever is larger tells you whether you get a refund or owe a balance.
For 2026/27 the building blocks are: a personal allowance of £12,570, basic-rate income tax at 20% up to £50,270 and higher rate at 40% above it; Class 4 National Insurance at 6% on profits between £12,570 and £50,270 and 2% above; and Class 2 National Insurance treated as paid at nil once profits exceed the £7,105 small profits threshold.
Worked example 1: a sole trader's refund
Take a registered subcontractor with a typical year. Gross CIS income of £45,000, made up of £40,000 labour and £5,000 materials, so CIS at 20% was taken on the £40,000 of labour, a deduction of £8,000. On top of the £5,000 materials, they have £5,500 of other allowable expenses (van, fuel, tools, insurance, use of home).
Step by step:
- Taxable profit: £45,000 gross less £5,000 materials less £5,500 expenses = £34,500.
- Income tax: the first £12,570 is covered by the personal allowance. The remaining £21,930 falls in the basic-rate band, taxed at 20% = £4,386. None of the profit reaches the £50,270 higher-rate threshold, so there is no 40% tax.
- Class 4 National Insurance: 6% on the £21,930 between £12,570 and the profit = £1,315.80. Nothing is above £50,270, so no 2% band applies.
- Class 2 National Insurance: profit is above £7,105, so it is treated as paid at nil.
- Actual liability: £4,386 income tax plus £1,315.80 Class 4 = £5,701.80.
- CIS already deducted: £8,000.
- Refund: £8,000 less £5,701.80 = £2,298.20.
The whole refund exists because the £8,000 was taken at 20% on labour before the £12,570 allowance, the £5,000 materials and the £5,500 of expenses were ever counted. Account for all of those on the return and the genuine liability is only £5,701.80, so £2,298.20 comes back. The table below lays the same calculation out as a sequence.
| Step | Figure |
|---|---|
| Gross CIS income | £45,000 |
| Less materials | (£5,000) |
| Less other allowable expenses | (£5,500) |
| Taxable profit | £34,500 |
| Income tax: 20% on £21,930 above the £12,570 allowance | £4,386.00 |
| Class 4 NI: 6% on £21,930 | £1,315.80 |
| Class 2 NI: treated as paid (profit above £7,105) | £0.00 |
| Actual liability | £5,701.80 |
| Less CIS already deducted (20% on £40,000 labour) | (£8,000.00) |
| Refund due | £2,298.20 |
Two things change the result if your numbers are different. If you earn enough that part of your profit clears £50,270, that slice is taxed at 40% income tax and 2% Class 4, so the liability rises and the refund shrinks. And if your expenses are higher (heavier mileage, more equipment), the liability falls and the refund grows. The mechanics are identical, the size of the refund just tracks your profit and your costs.
The limited-company alternative: real-time offset through the EPS
A subcontractor who trades through a limited company recovers CIS in a completely different way, and it is worth understanding even if you are a sole trader, because it is the single biggest reason some subcontractors incorporate.
When a contractor deducts CIS from a payment to a limited company, that deduction is treated as an advance payment of the company's own PAYE and CIS liabilities. So instead of waiting for a year-end return, the company offsets the CIS it has suffered against the PAYE and CIS it owes HMRC each month, by reporting the figure on its monthly Employer Payment Summary (EPS). The EPS is part of the company's payroll filing, not anyone's Self Assessment.
Worked example 2: a company's monthly EPS offset
Take a company subcontractor with £50,000 of gross CIS income for the year, of which £40,000 is labour and £10,000 is materials. CIS at 20% is taken on the £40,000 labour, so the company suffers £8,000 of CIS deductions across the year, roughly £667 a month if income is even.
Suppose the company runs a small payroll and owes around £500 a month in PAYE and employee National Insurance on its directors' and staff wages. Each month it reports the CIS suffered on the EPS:
- PAYE/NIC the company owes that month: about £500.
- CIS suffered that month, reported on the EPS: about £667.
- Net position: the £667 of CIS more than covers the £500 due, so the company pays HMRC nothing that month and carries the £167 surplus forward to set against later months.
Over the year the company suffers £8,000 of CIS and owes roughly £6,000 of PAYE and NIC, so the offset wipes out the PAYE bill entirely and leaves around £2,000 of surplus CIS. That surplus carries forward through the tax year, and any amount still unrelieved at the year end is repaid by HMRC after the final payroll submission, with the company repayment target around 25 working days. The key contrast with the sole-trader route is timing: the company recovers value month by month rather than waiting for a Self Assessment refund after 5 April. The full mechanics, including what to do when CIS suffered repeatedly exceeds PAYE due, are in our guide to how CIS limited companies reclaim deductions.
Note what is not happening here: none of this £8,000 goes on a personal Self Assessment return. Putting company CIS on a director's personal return is a common and expensive error that leaves the company's deductions unreclaimed.
The records you need
A clean refund depends on clean records, and HMRC can ask to see them. Keep:
- CIS payment and deduction statements. One for each month a contractor paid you, showing the gross amount, the materials, the labour and the CIS deducted. These are the source of the CIS-deducted figure on your return, and HMRC matches your claim against contractor reporting. Chase any missing statements before you file.
- Expense receipts and records. Invoices and receipts for materials, tools, equipment, insurance, protective clothing, accountancy and anything else you claim.
- A mileage log. If you claim the 55p-per-mile rate, keep a contemporaneous record of business journeys (date, route, miles, purpose). The flat rate covers the first 10,000 business miles, then 25p thereafter.
- Details of any other income. Employment (with the P60 or P45), savings interest, dividends, rental income and so on, because Self Assessment reports your whole tax position, not just the CIS trade.
HMRC expects business records to be kept for at least five years after the 31 January filing deadline, so do not discard statements or receipts once the return is filed.
Key deadlines and the penalty ladder
The headline date is 31 January following the end of the tax year on the previous 5 April. That is the deadline for filing online and for paying any tax due. You cannot file until after 5 April, and filing promptly after that date is the simplest way to get a refund back sooner, because nothing is repaid until the return is in.
Miss the deadline and the penalties apply to the return itself, even if you owe nothing or are due a refund. This is a frequent and painful surprise: a subcontractor sitting on a refund still gets a £100 penalty for filing late. The ladder, which is separate from the monthly CIS300 penalty ladder that applies to contractors, runs as follows.
| How late | Penalty type | Amount |
|---|---|---|
| 1 day late (missing 31 January) | Fixed | £100 (even if no tax is due or a refund is owed) |
| 3 months late | Daily | £10 per day, up to 90 days (maximum £900) |
| 6 months late | Tax-geared | £300 or 5% of the tax due, whichever is higher |
| 12 months late | Tax-geared | A further £300 or 5% of the tax due (up to 100% in serious cases) |
Late-payment interest and separate late-payment penalties also run on any tax actually owed, on top of the filing penalties above. For a subcontractor who is normally in a refund position the late-payment side rarely bites, but the £100 fixed penalty and the daily charges apply regardless, so there is never a reason to file late.
Claiming back earlier years: the four-year rule
If you have suffered CIS deductions in earlier years and never filed, or filed without claiming all your expenses, the over-deduction is not necessarily lost. Self Assessment lets you reclaim overpaid tax for up to four prior tax years. Within that window you can file the missing returns or amend filed ones and recover the over-deduction, provided you have the deduction statements (or can reconstruct the figures) for each year.
The four-year limit is firm. Once a year falls outside it, the unclaimed refund for that year is gone for good. That is why a back-year review is so often the first thing worth doing for a subcontractor who has not been filing carefully: there can be several years of over-deduction sitting recoverable, but the oldest of them is always about to expire.
Making Tax Digital for Income Tax: the gross-income trap
The way you file is changing. Making Tax Digital for Income Tax (MTD ITSA) requires digital record-keeping and quarterly updates through compatible software, phased in by income:
- From April 2026: sole traders and partnerships with gross income over £50,000 must comply.
- From April 2027: the threshold drops to £30,000.
The point that catches subcontractors out is how the threshold is measured. It is tested on gross income before any deductions, not the net amount you bank. A subcontractor on £60,000 gross who receives £48,000 after 20% CIS is tested on the £60,000 figure, so they are over the £50,000 threshold and in scope from April 2026, even though their bank statements only ever show £48,000 coming in. Many subcontractors will assume their net receipts keep them under the line when their gross figure does not.
In practice, MTD ITSA means four quarterly updates (due by the 7th of the month after each quarter, that is 7 August, 7 November, 7 February and 7 May) plus a final declaration that replaces the old single annual return. The 31 January reconciliation and payment date still applies. HMRC is not charging late-update penalty points in the first year (2026/27), but the late annual return and late-payment penalties still apply, so the grace is narrow. We work through the detail and the timeline in our guide to Making Tax Digital and CIS.
Getting it right, and getting it back
For a sole-trader subcontractor, Self Assessment is not a chore bolted onto the side of the job, it is the mechanism that recovers the money the scheme took from you in advance. The deductions came off your labour at 20% before your £12,570 allowance, your materials and your expenses were counted, so the return almost always tips into refund territory once everything is properly accounted for, as the £2,298 example above shows. The risks are mechanical and avoidable: claiming on net rather than gross income, missing the gross-income MTD threshold, losing deduction statements, leaving expenses unclaimed, filing late and collecting a £100 penalty on a year you were actually owed money, or letting a back year expire under the four-year rule.
We treat the CIS refund as the front door to keeping you compliant for the long term rather than a one-off rebate, which means filing accurately, claiming every legitimate expense and making sure the timing works in your favour. If you want to see your own numbers before you file, run them through our CIS Self Assessment calculator or get a quick figure from the CIS refund estimator, and if you would rather we handled the return and the reclaim end to end, you can get in touch.