Do I still need to complete a Self Assessment if I am under CIS?

Learn whether you still need to complete a Self Assessment tax return if you work under the Construction Industry Scheme (CIS). Understand how CIS deductions work and why filing is essential to claim your refund or settle your tax bill.

If you work in construction as a subcontractor, your contractor usually deducts tax from your payments under the Construction Industry Scheme (CIS). Because tax is already being taken, many workers assume they do not need to file a Self Assessment tax return.

However, this is not the case. Even if you are under CIS and tax is deducted automatically, you still need to complete a Self Assessment tax return each year. Filing your return allows you to calculate your total income, claim expenses, and find out whether you have overpaid or underpaid tax.

In most cases, CIS subcontractors are due a tax refund after completing their return.

Why CIS subcontractors must file a Self Assessment

Under CIS, contractors deduct 20% (or 30% if you are unregistered) from your payments and send it directly to HMRC. These deductions are not final—they are advance payments towards your overall tax bill.

As a self employed subcontractor, you are responsible for:

  • Reporting your total income and expenses for the year.

  • Calculating your profit.

  • Paying the correct amount of tax and National Insurance.

Your CIS deductions are then offset against your tax bill. If you have paid too much, HMRC issues a refund. If you have not paid enough, you will owe the difference.

How CIS and Self Assessment work together

CIS and Self Assessment are two separate systems that work alongside each other:

  • CIS: Contractors deduct tax at source from your payments.

  • Self Assessment: You declare your full earnings, expenses, and deductions to HMRC each year.

Self Assessment ensures your final tax position is accurate based on your total profit. The deductions made by contractors are taken into account when calculating whether you owe more tax or are due a refund.

Example

Let’s say during the tax year you earn £40,000 under CIS. Your contractor deducts 20% tax (£8,000) and pays it to HMRC.

At the end of the year, your allowable expenses (tools, travel, insurance, etc.) total £5,000.

Your taxable profit is £35,000. The actual tax you owe on this is £7,000.

Because your contractor has already deducted £8,000, you have overpaid by £1,000. When you complete your Self Assessment tax return, HMRC will refund you the £1,000.

What happens if you don’t file a Self Assessment

If you do not file your Self Assessment tax return:

  • HMRC will not know your total income or expenses.

  • You will not receive any refund of overpaid CIS tax.

  • You may receive automatic penalties for missing the filing deadline.

HMRC can also estimate your income and issue a determination (an estimated tax bill), which could be higher than what you actually owe. You must then submit a return to correct it.

How to file your Self Assessment tax return under CIS

You can file your return online through the HMRC website or use an accountant to handle it for you. The process typically involves:

  1. Registering for Self Assessment: If you are new to self employment, you must register with HMRC before 5 October following the end of the tax year.

  2. Gathering your CIS statements: Your contractor must provide a payment and deduction statement each month showing how much tax has been deducted.

  3. Recording your income and expenses: Include all payments received and any allowable business expenses such as tools, materials, and mileage.

  4. Completing your tax return: Report your income, expenses, and CIS deductions in the self employment section of your return.

  5. Submitting and paying: File your return online by 31 January following the end of the tax year and pay any balance owed.

Claiming a CIS tax refund

Most subcontractors overpay tax during the year because the 20% deduction does not account for allowable expenses. When you submit your Self Assessment, HMRC will:

  • Offset your CIS deductions against your final tax bill.

  • Issue a refund if you have overpaid.

Refunds are usually paid directly to your bank account within two to four weeks of submission, provided your return is accurate and up to date.

Deadlines to remember

For the 2024/25 tax year (ending 5 April 2025):

  • 5 October 2025: Deadline to register for Self Assessment if you are new to CIS.

  • 31 January 2026: Deadline to file your online tax return and pay any outstanding tax.

Missing these deadlines can result in penalties, interest, and delayed refunds.

What you can claim as expenses under CIS

When completing your Self Assessment, you can claim any costs that are wholly and exclusively for business purposes. Common CIS expenses include:

  • Tools and equipment.

  • Work clothing and protective gear.

  • Materials and supplies.

  • Fuel, mileage, and vehicle costs.

  • Public liability insurance.

  • Accountancy fees.

  • Training or certification costs.

Claiming all allowable expenses ensures you do not overpay tax and maximises your refund.

When you might not need to file a Self Assessment

The only time you may not need to complete a Self Assessment is if:

  • You work solely as an employee under PAYE (not as a subcontractor).

  • You have no self employed income or other untaxed earnings.

If you are paid under CIS as a self employed worker, you must file a Self Assessment, even if you only worked for part of the year.

How an accountant can help

An accountant experienced in CIS can help you:

  • Register for Self Assessment.

  • Accurately calculate your income, expenses, and deductions.

  • File your tax return on time.

  • Claim all allowable expenses.

  • Maximise your CIS refund.

Professional support ensures your return is accurate and compliant with HMRC, helping you avoid penalties and get your refund faster.

The bottom line

Even if you are paid under the Construction Industry Scheme and tax is deducted at source, you still need to complete a Self Assessment tax return. Filing your return ensures your total income, expenses, and CIS deductions are correctly recorded.

In most cases, completing your Self Assessment leads to a refund of overpaid tax. By keeping good records, claiming legitimate expenses, and filing on time, you can stay compliant with HMRC and keep more of your hard-earned money.