CIS subcontractor expenses you're probably not claiming (2026/27)

If you work under the Construction Industry Scheme, tax comes off your pay before you ever see it. The contractor deducts 20% if you're registered, or 30% if you're not, and pays it straight to HMRC on your behalf.

The bit a lot of subbies miss is this: that deduction comes off your gross pay, before a single penny of your expenses is counted. HMRC has no idea you spent £200 on a new drill this month, or drove 8,000 miles to get to site. So there's a good chance you've overpaid.

Once you file your Self Assessment and put your real costs in, most CIS subcontractors are owed money back. Below is the plain-English list of what you can claim for 2026/27, along with the few traps that catch people out.

Why most CIS subcontractors are owed a refund

Under CIS, the contractor treats your deduction as a payment towards your tax and National Insurance. If you're registered, that's 20% of your labour; if you never registered, it jumps to 30%.

The problem is timing. That 20% or 30% comes off the top, based on what you were paid rather than on your profit. Your profit is what's left after you've covered tools, travel, insurance and everything else the job takes. Those costs are real, and they reduce the tax you owe.

So when you file your return and enter your expenses, the tax bill on your genuine profit is usually a good bit lower than the amount already deducted, and the difference comes back to you as a refund. One point worth knowing: the deducted tax is set against your whole bill, both Income Tax and Class 4 National Insurance, so what comes back is the balance after that, not always the full 20%.

A refund is the usual outcome, though not a certainty. With very few expenses, or other untaxed income alongside your CIS work, the deducted tax might only cover what you owe. For a rough idea before you do anything, try our free CIS refund estimator. It gives you a ballpark in a couple of minutes.

The expenses most subbies forget to claim

These are the costs HMRC accepts as allowable for a self-employed CIS subcontractor. Go through the list and be honest about what applies to you, because small amounts add up fast over a year.

• Tools and equipment: drills, saws, hand tools, hire costs. Bigger kit may be claimed differently, but it still counts.
• Protective clothing and PPE: hi-vis, hard hats, gloves, goggles, ear defenders, knee pads.
• Work boots: safety boots are allowable (ordinary shoes are not).
• Vehicle costs: either 45p a mile for the first 10,000 business miles then 25p, or your actual running costs (fuel, insurance, repairs, tax). Pick one method and stick to it for the vehicle.
• Public transport to temporary sites: train, bus or fares to a site that isn't your permanent base.
• Materials you paid for: anything you bought yourself to complete a job.
• Mobile phone: the business-use share of your bill.
• Accountancy fees: yes, our fee for sorting your return is itself an allowable expense.
• Public liability insurance: this and other genuine business cover count.
• Use of home for admin: a reasonable amount for doing quotes, invoices and paperwork at home.
• Small tools and consumables: blades, screws, drill bits, tape, the bits that vanish on every job.
• Laundry and cleaning of specialist workwear (not your everyday clothes).
• Training to update existing skills: refreshers and tickets that keep you current in what you already do.
• Subsistence when working away: reasonable food and drink for times you're away from your normal base.

Mileage: usually the biggest one people undercount

Travel is where most subbies leave money behind. If you use your own van or car to reach different sites, you can claim 45p per mile for the first 10,000 business miles in the tax year, then 25p a mile after that. You could instead claim the actual running costs and a share of the vehicle's value, but for most subbies the flat mileage rate works out easier and is well worth logging.

Remember that mileage is an allowable expense, which cuts the profit you're taxed on. It isn't handed back pound for pound. A modest 8,000 miles a year at 45p comes to £3,600 of allowable expense; at basic rate that's roughly £700-plus less tax, plus a Class 4 NI saving on top, straight onto your refund. All it takes is a note of your business journeys. A phone log or a cheap notebook in the van is plenty.

If you're not sure how the numbers stack up against your CIS deductions, our tax calculators let you play with the figures.

The traps: what you cannot claim

A few things feel like they should count but don't. Claiming them is the fastest way to get an HMRC query, so it's worth being clear.

• Ordinary everyday clothing: jeans, t-shirts, a normal jacket. Only proper PPE and specialist safety gear count.
• Commuting to a permanent workplace: travel to a site you treat as your regular, fixed base isn't allowable. Travel to a temporary site is.
• Fines: parking tickets, speeding fines and penalties are never allowable.
• Meals on a normal working day at your usual site: subsistence only applies when you're working away from base.

Missing receipts aren't the end of the world

Nobody keeps every scrap of paper, and a missing receipt doesn't mean the cost never happened. For something like mileage you're claiming a set rate rather than proving individual fuel receipts, so a journey log does the job.

For other costs, bank and card statements are strong evidence, and a reasonable, honest estimate for small consumables beats claiming nothing at all. The goal is a fair, defensible return, not a perfect one.

Going forward, snap a photo of receipts on your phone as you go. It takes five seconds and makes next year's refund quicker and bigger.

How we sort it for you

We handle CIS subcontractors day in, day out: pulling your deductions together, capturing every allowable expense, filing your Self Assessment and chasing any refund. Fixed fee, agreed upfront, no nasty surprises, and you deal directly with Bobby rather than a call centre.

You can read more about how we work on our CIS accounting for contractors page, or just get in touch and we'll tell you roughly what you're owed.