CIS Tax Return Guide UK for Subcontractors

A CIS deduction is not the same as your final tax bill. That is the point many subcontractors miss when they see 20% taken from a payment and assume the job is done. This CIS tax return guide UK explains how deductions, expenses and Self Assessment work together, so you can report the right figures to HMRC and claim back any overpaid tax.

For construction workers, paperwork often comes after a long day on site, not before it. But a few organised records can make the difference between a straightforward tax return and a refund being left sitting with HMRC.

What CIS means for your tax return

The Construction Industry Scheme, usually called CIS, applies when contractors pay subcontractors for construction work. Before paying you, the contractor should verify you with HMRC and deduct tax at the correct rate. For most verified subcontractors, this is 20%. If you are not verified, deductions can be made at 30%.

The contractor is responsible for sending those deductions to HMRC and providing you with a monthly CIS deduction statement. This should show the gross amount paid, the cost of materials where relevant, and the tax deducted. Keep every statement. They are the evidence needed to claim credit for the tax already paid on your behalf.

CIS tax is deducted from your labour element, not normally from materials you have supplied. It is also not a separate final tax. It is treated as advance payment towards your Income Tax and National Insurance position for the tax year. Once your income and allowable business costs are included on a Self Assessment return, HMRC works out whether you owe more or are due a repayment.

CIS tax return guide UK: who needs to file?

Most self-employed subcontractors need to complete a Self Assessment tax return each year, even where CIS deductions have been made from every payment. You must register for Self Assessment if you have not done so before, then report your self-employment income for the tax year ending 5 April.

The usual online deadline for submitting the return and paying any tax due is 31 January following the end of the tax year. For example, the 2025/26 tax return is generally due online by 31 January 2027. If you submit a paper return, the deadline is earlier, normally 31 October.

You may also need a return if you have other income, such as property rental income, a limited company directorship, or work outside CIS. The figures need to be considered together. A CIS refund cannot be calculated properly by looking at deductions alone.

If you work through your own limited company, the position is different. CIS deductions made from the company’s income are dealt with through the company’s payroll and corporation tax processes, rather than claimed on a personal sole trader return. This is an area where getting the treatment wrong can create delays, so take advice before submitting figures.

Get your income records in order

Start with the monthly deduction statements from every contractor you worked for. Check that your name, Unique Taxpayer Reference and the deduction amounts are correct. Add up the gross payments and CIS tax deducted for the tax year, rather than relying on what reached your bank account.

Your bank statements, invoices and job records are useful cross-checks. They can help identify missing statements, late payments or income from work where no deduction was made. If a contractor has not supplied a statement, ask for it promptly. Do not simply estimate the tax deducted, as HMRC may not give credit where the information does not match its records.

It is worth separating money received for labour from materials whenever possible. Good invoices make this easier for you and your contractor, particularly where you supply materials as part of the job.

Claim the expenses that genuinely belong to the business

Your taxable profit is broadly your business income minus allowable business expenses. A lower profit can reduce the final tax due, but only if the costs are wholly and exclusively for the trade. Personal spending cannot be put through simply because you are self-employed.

For a typical construction subcontractor, valid costs may include tools and protective clothing, vehicle running costs for business journeys, public transport to temporary workplaces, insurance, accountancy fees, mobile phone use, advertising, training that maintains existing skills, and a proportion of home costs where you genuinely work from home.

The detail matters. Ordinary clothing is not usually allowable, even if you only wear it to work, but protective boots, hard hats and branded workwear may be. Travel from home to a regular, permanent site is normally commuting and is not an allowable expense. Travel to temporary sites can be treated differently. If you use your own car or van, you may claim actual business costs or use an approved mileage method, depending on which approach suits your circumstances.

Keep receipts, invoices and a clear record of business mileage. A photograph or scanned copy is often easier to store than a glovebox full of fading receipts, provided the record is legible and complete.

Enter CIS deductions correctly on Self Assessment

When completing the self-employment part of the return, enter your turnover and expenses first. There is then a section to enter CIS tax deducted from your payments. This is where the monthly statements do the heavy lifting.

The deduction is not an expense. Recording it as an expense as well as entering it as CIS tax deducted can distort the return. HMRC should set the deductions against your final liability after calculating the tax due on your profit.

A refund is common where deductions exceed the total tax and National Insurance due, particularly if you have had a quiet period, significant valid expenses, or started work part-way through the year. It is not guaranteed, though. Higher earnings, income from other sources or payments on account can reduce or remove a refund.

Do not overlook payments on account

A surprise for many first-time filers is that the January bill may include a payment on account towards the following year. HMRC normally asks for these advance payments where your Self Assessment tax bill is over £1,000 and less than 80% of that tax was collected at source.

Payments on account are generally due on 31 January and 31 July. They are estimates based on the previous year’s bill, not an extra tax charge. If your income has fallen, you can apply to reduce them, but do so with care. Reducing them too far can lead to interest if your eventual bill is higher.

CIS deductions can affect this calculation, which is another reason not to assume that a 20% deduction means there will be nothing else to pay.

Common CIS return mistakes that cost time

The most avoidable mistake is filing without all monthly deduction statements. This can mean missing tax credits and a smaller refund than you are entitled to. The opposite problem is claiming deductions that are not supported by HMRC records, which can hold up repayment processing.

Other common issues include using banked amounts instead of gross income, claiming personal travel, mixing up materials and labour, and forgetting non-CIS income. Late filing is costly too. A return submitted after the deadline can attract an initial £100 penalty even where no tax is owed.

Give yourself time before January. Chasing contractors for missing documents on the last working day of the month is rarely the quickest route to a clean return.

Make next year easier from the first invoice

Set aside a regular time each month to save CIS statements, photograph receipts and update your income and expense records. This is far less painful than rebuilding a whole year from bank statements. It also gives you a better view of cash flow, particularly when tax, insurance and vehicle costs are all due at once.

Making Tax Digital for Income Tax will bring more frequent digital reporting for many self-employed people and landlords, with phased introduction based on qualifying income. Keeping digital records now is a sensible working habit, not just an HMRC requirement waiting in the background.

If your records are incomplete, you are unsure which expenses are allowable, or your CIS deduction figures do not look right, get the issue sorted before filing. A good accountant should speak plainly, understand the pressure of working job to job, and help make sure the tax you have already paid is properly recognised.