CIS for sole traders is one of those areas where the mechanics are fairly straightforward once someone explains them clearly — but until then, it can feel like money is disappearing from your invoices without much explanation.

The Construction Industry Scheme (CIS) is HMRC’s way of collecting tax at source from subcontractors in the building and construction sector. If you work for a contractor as a sole trader — whether you’re a plumber, electrician, groundworker, joiner, or any other trade — the contractor is legally required to withhold a percentage of your labour payment and send it directly to HMRC on your behalf.

That sounds reasonable in theory. In practice, we speak to plenty of sole traders who are being deducted at the wrong rate, aren’t registered when they should be, or have no idea they’re sitting on a refund. This post sets out how CIS actually works for sole traders, and what you should be doing about it.

When you work as a subcontractor under CIS, the contractor paying you doesn’t hand over the full invoice amount. They keep back a percentage of the labour element — not the materials, just the labour — and pay it to HMRC on your behalf.

The rate depends on whether you’re registered with HMRC for CIS:

• Registered subcontractor: 20% deducted from the labour portion of your payment
• Unregistered subcontractor: 30% deducted

That gap — 10 percentage points — is the single biggest reason to register. If a contractor can’t verify you with HMRC, they’re required to deduct at the higher 30% rate. There’s no flexibility on their part; it’s a compliance requirement for them.

Registration is straightforward and free. You do it through your HMRC online account, and once you’re on the system, contractors can verify you before each first payment. From that point, they’ll deduct at 20% rather than 30%.

One thing worth understanding: CIS deductions are not a final tax bill. They’re more like advance payments towards your annual tax liability. Whether you end up owing more, breaking even, or getting money back depends on your total income, allowable expenses, and personal allowance — which is exactly what your self assessment tax return is for.

This is where many sole traders leave money on the table. Because CIS deductions happen automatically, it’s easy to assume the tax side is just handled — but it isn’t. The deductions are calculated on your labour income, not your profit, so they take no account of your legitimate business costs.

When you file your self assessment return, you declare your income and then deduct allowable expenses to arrive at your taxable profit. Those expenses reduce the tax you owe, and because your CIS deductions have already been paid in advance, a lower tax bill often means a refund.

Common allowable expenses for CIS sole traders include:

• Tools, equipment, and small plant you’ve bought yourself
• Vehicle costs — either actual expenses or HMRC’s approved mileage rate of 45p per mile for the first 10,000 miles in the tax year
• Protective clothing and safety gear
• Business phone and relevant broadband costs
• Public liability insurance and professional memberships
• Materials you’ve personally supplied and paid for

What you can’t claim are the materials the contractor supplies, or any personal expenses dressed up as business costs. HMRC is well aware of how CIS works and what expenses are typical for the trades — so claim what you’re entitled to, but keep the records to support it.

In our experience, sole traders who keep even basic records of their mileage and tool purchases through the year are consistently better off at self assessment time than those who try to reconstruct everything in January.

If your CIS deductions during the year are more than the tax you actually owe, HMRC owes you the difference back. The mechanism for reclaiming it is your annual self assessment tax return.

The process looks like this: your contractors issue you with deduction statements showing how much was withheld on each payment. You (or your accountant) use those statements to calculate the total CIS deducted across the year, which then gets entered on your return as tax already paid. If that figure is higher than your actual liability, HMRC issues a refund — usually within a few weeks of the return being processed.

Sole traders on the standard 20% CIS rate commonly receive refunds somewhere in the range of a few hundred to a few thousand pounds, depending on their income, expenses, and personal allowance position. The personal allowance alone (£12,570 for 2026/27) means that lower earners may find their entire CIS deduction refunded, because their taxable income isn’t high enough to generate a matching liability.

The deadline for filing your self assessment return online is 31 January following the end of the tax year. Miss it, and you’ll face a £100 automatic penalty — and the refund still won’t arrive until you actually file. You can also reclaim overpaid CIS from previous years, going back up to four tax years, though late filing penalties for those earlier years may apply.

If you’re not sure whether you should be filing self assessment, or you’ve missed a year, that’s worth sorting sooner rather than later.

Some sole traders find themselves on both sides of the CIS fence — they work as a subcontractor for larger contractors, and they also bring in their own subbies to help on jobs. This is where the compliance picture gets more involved.

If you make payments to subcontractors for construction work, you’re acting as a contractor for CIS purposes. That means you need to register as a contractor with HMRC, verify each subcontractor before you pay them for the first time, deduct at the correct rate, and file a monthly return by the 19th of the following month showing what you paid and deducted.

Late or missing monthly returns attract penalties, and the rules around misclassification — deducting at the wrong rate, or failing to deduct at all — can result in HMRC charging penalties of up to £3,000 for errors on a single return. Many small contractors get caught out here, particularly when they only use subbies occasionally and assume the obligation doesn’t apply to them. It does.

If this is your situation, cloud accounting software with built-in CIS functionality can take a lot of the admin off your plate and reduce the risk of a missed deadline. We help clients set this up through Xero and QuickBooks Online, both of which handle CIS calculations and returns well.