Self Assessment for CIS subcontractors: your step-by-step 2026/27 guide

If you work on the tools and have tax stopped before you are paid, you are inside the Construction Industry Scheme (CIS), and that means a Self Assessment tax return every year. It sounds like a chore, but for most subcontractors it is the point at which the money comes back.

What hardly anyone explains on site is that CIS deductions take no account of your personal allowance or your expenses, so the tax stopped from your labour is almost always more than you owe. Your return is how you claim the difference back.

This guide runs through all of it in plain English: what CIS is, why you are probably owed a refund, what to gather together, the deadlines that matter for the 2026/27 tax year, and how the money reaches your account, without jargon or scare tactics.

What CIS is and why you're in it

The Construction Industry Scheme is the way HMRC collects tax from construction work at source. If you are a subcontractor, the contractor paying you has to deduct a percentage from your labour and send it to HMRC on your behalf.

If you are registered for CIS the deduction is 20%, and if you are not registered it rises to 30%, which is reason enough to get your registration sorted out. Either way you remain self-employed: a sole trader running your own business and responsible for your own tax return.

The point that trips people up is that the deduction is an advance payment towards your tax, and your final bill is only worked out when your Self Assessment return is filed, which is where everything is squared up.

• A registered subcontractor has 20% deducted from labour
• An unregistered subcontractor has 30% deducted from labour
• Deductions apply to your labour only, and materials are left out
• You remain self-employed and must file a Self Assessment return each year

Why most subbies get money back

The good news is that when a contractor stops CIS, they take no account of your personal allowance (the amount of income you can earn tax-free) or of any of your expenses, such as tools, van, fuel, materials, protective gear, insurance and phone.

The result is that you have overpaid all year without realising it. When you file, your actual taxable profit is worked out properly, with the allowance applied and the expenses deducted, and the CIS already taken is credited against that final figure. Nine times out of ten, HMRC ends up owing you money.

If you would like a rough idea before you file, our free CIS refund estimator gives you an approximate figure in a couple of minutes. If you would sooner someone handled all of it for you, our CIS accounting service does that.

What you need before you file

Getting organised in advance makes the whole process much easier, and the list below covers what you need.

If a few CIS statements are missing, it is not a disaster. We can rebuild your deductions from your bank records, so a shoebox that has seen better days will not hold you up.

• A UTR (Unique Taxpayer Reference), which is your 10-digit tax reference, together with registration for Self Assessment
• Your monthly CIS statements. The contractor should give you one each month showing your pay and the deductions made
• Records of your income, meaning your invoices and bank deposits
• Records of your expenses, such as tools, van, fuel, materials, PPE, phone, insurance and use of home

The 2026/27 deadlines that matter

The tax year runs from 6 April to 5 April. HMRC charges penalties if the main dates are missed, so it is worth putting them in your calendar.

There is one further point to be aware of. If your tax bill comes to over £1,000 and less than 80% of it was collected at source, you may have to make payments on account, which are advance instalments towards next year's bill. For most subcontractors who are due a refund this will not apply, but it is worth knowing about.

• 5 October after the end of the tax year in which you first started working for yourself is the deadline for registering for Self Assessment. It applies only in your first year, because after that you are already registered
• 31 October is the deadline for paper returns
• 31 January is the deadline for online returns and for paying any balance due
• It pays to register early, because leaving it late is the most common reason refunds are delayed

Filing your return and getting the refund

You file your Self Assessment return online through HMRC, or you ask an accountant to do it for you. The return adds up your income, takes off your expenses, applies your personal allowance and credits the CIS already stopped. If you have overpaid, the balance comes back to you as a refund.

Once the return is filed, a refund usually arrives around two weeks later and is paid directly into your own bank account. HMRC runs 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 in advance, and we never take a percentage of your refund. The refund is paid to you directly and does not pass through us. If you would like a real person to take the whole thing off your hands, get in touch and we will sort it out.

Making Tax Digital, briefly

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, with the threshold falling to £30,000 from April 2027 and £20,000 from April 2028. If that includes you, there is nothing to worry about: it means keeping digital records and sending updates to HMRC through the year, and little more than that. Our Making Tax Digital guide explains what it means for subcontractors and how we take the work off your hands.