To file a tax return as a CIS subcontractor, you submit a Self Assessment return (form SA100 plus the self-employment pages) reporting your total turnover, materials, and expenses, and enter the tax your contractors already deducted under the Construction Industry Scheme in the dedicated CIS deductions box so HMRC credits it against what you owe. Do this online by 31 January following the end of the tax year. Most subcontractors taxed at 20% at source end up owed a refund, but only if the return is filed and the CIS figure is right.
Who Has to File
If you worked as a sole trader or partner in construction and were paid through CIS at any point during the tax year, you need to file. That is true whether contractors deducted 20%, 30%, or nothing because you hold gross payment status. It is also true if your total earnings sat below the personal allowance and you owe no tax at all. The return is the mechanism HMRC uses to reconcile what was withheld against what you actually owe, and without it any overpayment stays with the government.
Registering as a CIS subcontractor with HMRC is a separate step that sets your deduction rate; you can register for Self Assessment and CIS together through the HMRC online service by selecting “working as a subcontractor” when prompted.1GOV.UK. What You Must Do As a Construction Industry Scheme (CIS) Subcontractor – How to Register You will need your Unique Taxpayer Reference (10 or 13 digits) and your National Insurance number to log in and file.
What to Gather Before You Start
The UK tax year runs 6 April to 5 April.2GOV.UK. Self Assessment Tax Returns – Deadlines Pull together everything covering that window before you open the return.
The most important documents are the monthly payment and deduction statements your contractors should have issued throughout the year. Each one shows three figures: the gross amount invoiced, the cost of materials, and the CIS tax deducted. Materials matter because tax is only withheld on the labour portion, not on materials you supplied. If any statements are missing, chase the contractor directly. Without them you cannot prove tax was already withheld, and HMRC has no reason to credit those deductions.
Alongside the CIS statements, collect receipts for every business expense you plan to claim. Common allowable costs for subcontractors include tools and equipment, protective clothing and safety gear, public liability insurance, the business-use portion of your phone bill, accountancy fees, and travel to job sites. Only costs incurred “wholly and exclusively” for the business qualify.
For vehicle travel, you can claim HMRC’s approved mileage rate instead of tracking fuel and maintenance. From 6 April 2026, cars and vans are 55p per mile for the first 10,000 business miles and 25p per mile after that; motorcycles are 24p per mile throughout. Carrying a colleague to the same site adds 5p per mile per passenger. These rates cover all running costs, so if you claim mileage you cannot also claim fuel, insurance, or repairs on the same vehicle. Travel between home and a regular permanent workplace is not deductible, but travel to temporary sites is, which covers most CIS work. Keep a simple log: date, destination, purpose, miles.
Keep all these records for at least five years after the 31 January submission deadline for that tax year.3GOV.UK. Business Records if You’re Self-Employed – How Long to Keep Your Records
Filling In the Return
The return is the main SA100 form plus supplementary self-employment pages.4GOV.UK. Self Assessment Tax Return Forms If your annual business turnover was below the £90,000 VAT threshold, use the short self-employment version (SA103S). At or above that level, use the full version (SA103F).5HM Revenue & Customs. Self Assessment – Self-Employment (Short) (SA103S)
Enter your total turnover first. This is the full gross amount you invoiced during the year, before any CIS deductions or material costs were taken off. Then enter the total materials figure from your CIS statements so materials are excluded from taxable profit. Enter your allowable expenses in the sections provided. The system calculates net profit from these figures and works out the income tax and Class 4 National Insurance due.
The single most important field on the self-employment pages is the CIS deductions box. Enter the total tax already withheld by all your contractors across the year. HMRC uses this figure to offset the tax already paid against the tax your return calculates. Leave it blank or understate it and you lose credit for tax you have already handed over, and any refund shrinks accordingly. Cross-check the total against the sum of every monthly CIS statement. If the numbers do not match, find the discrepancy before you submit.
Deadlines and How to Submit
For the 2025/26 tax year:2GOV.UK. Self Assessment Tax Returns – Deadlines
- Paper returns must reach HMRC by 31 October 2026.
- Online returns must be submitted by 31 January 2027.
Filing online is the practical choice. You get immediate confirmation of receipt, automatic calculation of your tax position, and three extra months compared with paper. Any tax owed that is not covered by CIS deductions must also be paid by 31 January, so the filing deadline and the payment deadline are the same day. Leaving both to the last minute risks a late filing penalty and interest on unpaid tax at the same time.
Getting Your Refund
If your CIS deductions exceed the income tax and Class 4 NIC you owe, HMRC issues a refund. Nominate a bank account within the return by entering your sort code and account number. Online Self Assessment refunds are typically processed within five to ten working days, though complex returns or peak-season filing can run longer. You can track the status through your HMRC personal tax account.
Payments on Account and the 80% Rule
Payments on account catch many subcontractors off guard. If your Self Assessment bill (income tax plus Class 4 NIC) is over £1,000 after deducting tax already collected at source, HMRC normally requires advance payments toward next year’s bill, each equal to half the previous year’s liability, due on 31 January during the tax year and 31 July after it ends.6GOV.UK. Understand Your Self Assessment Tax Bill – Payments on Account
Most CIS subcontractors escape this. If more than 80% of your income tax was already collected through CIS deductions, you do not need to make payments on account.6GOV.UK. Understand Your Self Assessment Tax Bill – Payments on Account If you had significant non-CIS income, or your CIS deductions covered less than 80% of the total tax bill, budget for those advance payments.
Late Filing, Interest, and Appeals
Miss the filing deadline and penalties stack up quickly:7GOV.UK. Self Assessment Tax Returns – Penalties
- £100 fixed penalty straight away, even if you owe no tax.
- After 3 months: £10 per day for up to 90 days (maximum £900).
- After 6 months: 5% of the tax due or £300, whichever is greater.
- After 12 months: a further 5% of the tax due or £300, whichever is greater.
A return a full year late can cost £1,600 or more in penalties on top of the tax itself. Tax not paid by 31 January also accrues interest at 7.75% per year as of January 2026. HMRC charges this as simple interest, calculated daily from the day after the deadline. The rate tracks the Bank of England base rate plus 4% and moves when the base rate moves. Interest runs on top of penalties, not instead of them.
If you had a genuine reason for missing the deadline, you can appeal. HMRC accepts a “reasonable excuse” such as serious illness, a hospital stay, bereavement of a close relative, a fire or flood destroying records, or a failure of HMRC’s own online systems.8GOV.UK. Disagree With a Tax Decision or Penalty – Reasonable Excuses Finding the HMRC system confusing, not receiving a reminder, or not having the money do not count. You have 30 days from the date on the penalty notice to appeal, and you must still file or pay as soon as you are able.
Making Tax Digital from April 2026
A significant change lands on 6 April 2026. Making Tax Digital for Income Tax becomes mandatory for sole traders and landlords with total gross income from self-employment and property above £50,000.9GOV.UK. Making Tax Digital for Income Tax for Sole Traders and Landlords Plenty of CIS subcontractors will be over that line.
If it applies to you, the annual Self Assessment return is replaced by quarterly digital updates plus a final declaration. You must keep digital records using HMRC-compatible software and submit summaries four times a year.10GOV.UK. Choose the Right Software for Making Tax Digital for Income Tax Quarterly deadlines for 2026/27 are:
- Quarter 1 (6 April to 5 July): due 7 August 2026.
- Quarter 2 (6 July to 5 October): due 7 November 2026.
- Quarter 3 (6 October to 5 January): due 7 February 2027.
- Quarter 4 (6 January to 5 April): due 7 May 2027.
A final declaration confirming your full tax position for the year is still due by 31 January 2028, replacing the old return. Late quarterly submissions carry a points-based penalty: each late update earns one point, and a £200 penalty is triggered once you accumulate four points. You can use accounting software that creates digital records directly, or bridging software that connects to existing spreadsheets, provided it can submit updates and the final declaration to HMRC. If your income is below £50,000, the traditional Self Assessment process continues for now, though the threshold is expected to drop to £30,000 from April 2027.
A Note on VAT and the Reverse Charge
If your turnover is below the £90,000 VAT registration threshold, this section does not apply. If you are VAT-registered, the domestic reverse charge changes how you invoice contractors. When both you and your contractor are VAT-registered and the work falls within CIS, you do not charge VAT on your invoice; your contractor accounts for the VAT themselves, and your invoice must state that the reverse charge applies.11GOV.UK. VAT Domestic Reverse Charge Technical Guide
The reverse charge does not apply to zero-rated work such as new residential construction, or when the customer is an “end user” who has told you so in writing. Getting this wrong creates real problems: charging VAT when the reverse charge should apply leaves your contractor unable to reclaim it normally, and HMRC actively pursues these errors. From April 2026, HMRC has expanded powers to cancel gross payment status immediately and impose penalties of up to 30% of lost tax on businesses involved in fraudulent transactions.