Self Assessment for CIS workers is how subcontractors in the construction industry report their income, claim expenses, and get credit for the CIS deductions their contractors have already paid to HMRC on their behalf. Filing a tax return is the only way to reconcile what's been deducted throughout the year with what you actually owe — and for many subcontractors, it results in a refund.

This guide walks through the full Self Assessment process for CIS subcontractors: what you need before you start, how to complete each section of the return, how CIS deductions are credited against your tax bill, and what happens after you file.

What You Need Before Filing a CIS Tax Return

To file your CIS Self Assessment return, you need your CIS payment statements, income and expense records, your UTR, and Government Gateway login details. Here is what to gather before you start:

CIS payment statements:

Monthly statements from every contractor you worked for during the tax year
Each statement shows: gross amount, materials deducted, CIS deduction (20% or 30%), and net payment
If you're missing statements, ask your contractor for replacements — they're required to provide them
See our guide on CIS payment statements for what each field means

Income records:

Total of all invoices raised during the tax year
Bank statements showing payments received
If you use cash basis accounting (the default), count income when received, not when invoiced

Expense records:

Receipts and invoices for all allowable business expenses — tools, travel, protective clothing, phone, insurance
Mileage log if claiming vehicle costs
Material costs that were deducted on your CIS statements (you don't claim these again as expenses)

Personal details:

Your Unique Taxpayer Reference (UTR) — a 10-digit number
Your National Insurance number
Government Gateway login details
Bank details (for any refund)

How to Complete Your CIS Tax Return

Step 1: Log In and Start Your Return

Go to HMRC online services and sign in with your Government Gateway credentials
Select "Self Assessment" from your account homepage
Select "Complete your tax return" for the relevant tax year

Step 2: Answer the Tailoring Questions

The return asks about your circumstances to show only relevant sections. For a CIS subcontractor, you'll typically answer "yes" to:

Were you self-employed? (your CIS work counts as self-employment)
Did you receive CIS deductions? (this triggers the CIS deductions field)

If you also had employment income, rental income, or other sources, answer those questions too.

Step 3: Complete the Self-Employment Pages

This is where you report your CIS income and expenses. You'll use either:

SA103S (short version) — for straightforward self-employment with turnover under £85,000
SA103F (full version) — for turnover over £85,000 or more complex affairs

Key fields to complete:

Field	What to enter
Business description	Your trade (e.g., "bricklayer", "electrician", "painter and decorator")
Turnover / income	Your total gross income from all CIS work — the full invoice amounts before any deductions
Expenses	Your allowable business expenses by category
Net profit	Income minus expenses (calculated automatically online)

Important: Enter your gross income (the full amount invoiced), not the net amount you received after CIS deductions. The deductions are entered separately and credited against your tax bill — don't reduce your income figure by the amount deducted.

Step 4: Enter Your CIS Deductions

There is a dedicated field on the return for CIS deductions. Enter the total of all CIS deductions shown on your payment statements for the tax year.

This is the total amount your contractors withheld and paid to HMRC on your behalf. HMRC uses this figure to credit your account — reducing your tax bill or generating a refund.

How to calculate the total:

Gather all payment statements for the tax year (6 April to 5 April)
Add up the "CIS deduction" amount from each statement
Enter this total in the CIS deductions field

Example: James worked for three contractors during 2025/26. His total CIS deductions across all statements were:

Contractor A: £4,800
Contractor B: £2,400
Contractor C: £1,200
Total CIS deductions: £8,400

He enters £8,400 in the CIS deductions field.

Step 5: Review and Submit

Before submitting:

Check your income matches your bank statements and invoices
Verify expenses are realistic and supported by receipts
Confirm CIS deductions match your payment statements
Review the tax calculation HMRC generates

Submit your return and save the confirmation receipt.

How CIS Deductions Offset Your Tax Bill

After you file, HMRC calculates your actual tax liability and compares it to the CIS deductions already paid. There are three possible outcomes:

Scenario	What happens
Deductions > tax owed	HMRC refunds the difference to you
Deductions < tax owed	You pay the remaining balance by 31 January
Deductions = tax owed	Nothing more to pay or claim

Worked Example: CIS Refund

Sarah is a self-employed plumber. In the 2025/26 tax year:

Item	Amount
Total gross CIS income	£48,000
Allowable expenses (tools, travel, insurance, phone)	£8,000
Taxable profit	£40,000

Her tax bill:

Tax	Calculation	Amount
Income Tax (basic rate)	(£40,000 − £12,570) × 20%	£5,486
Class 4 NI (main rate)	(£40,000 − £12,570) × 6%	£1,646
Total liability		£7,132
CIS deducted during the year	(£48,000 − £8,000 materials) × 20%	£8,000
Refund due	£8,000 − £7,132	£868

Sarah's contractors deducted £8,000 over the year, but she only owed £7,132. HMRC refunds her £868.

Why CIS workers often get refunds:

CIS deductions are calculated on gross labour income (before expenses)
Your actual tax is calculated on profit (after expenses)
The bigger your expenses, the bigger the gap — and the bigger your refund

For more on the refund process, see our guide on claiming back CIS deductions.

Key Deadlines for CIS Subcontractors

Date	What to do
5 April	Tax year ends — your final CIS statements should arrive shortly after
5 October	Register for Self Assessment if this is your first year (you need a UTR)
31 October	Paper return deadline (if filing on paper)
31 January	Online filing deadline + payment deadline for any tax owed
31 July	Second payment on account (if applicable)

Filing early is worth it. You can file from 6 April — you don't need to wait until January. Filing early means:

You find out sooner if you're owed a refund
Refunds are processed faster (typically 4-6 weeks)
You have more time to arrange payment if you owe extra

CIS Self Assessment and Making Tax Digital

From 6 April 2026, CIS subcontractors with qualifying income over £50,000 (gross self-employment plus property income) need to use Making Tax Digital for Income Tax. This means:

Keeping digital records in MTD-compatible software
Submitting quarterly updates to HMRC (instead of one annual return)
Filing a final declaration by 31 January (replaces the current Self Assessment return)

Your CIS deductions are still credited against your tax bill under MTD — the way deductions work doesn't change. What changes is the reporting frequency (quarterly instead of annual) and the requirement for digital records.

If your income is between £30,000 and £50,000, MTD applies from April 2027.

Common CIS Tax Return Mistakes to Avoid

Entering net income instead of gross. Your income figure should be the full amount invoiced (before CIS deductions), not the amount you received. The deductions are entered separately.

Forgetting to enter CIS deductions. If you don't enter your deductions, HMRC won't credit them against your tax bill — and you'll appear to owe the full amount.

Missing payment statements. Without statements, you can't prove how much was deducted. Chase contractors for any missing statements well before filing.

Claiming materials as both a CIS deduction and an expense. Materials deducted on your CIS payment statement (reducing the amount CIS is calculated on) are not the same as business expenses. Don't claim the same materials cost twice.

Not claiming enough expenses. Many CIS workers only claim obvious costs like tools and forget travel, phone, protective clothing, insurance, and training. Review the full list of CIS subcontractor expenses.

CIS Self Assessment: Frequently Asked Questions

Do all CIS subcontractors need to file a Self Assessment return?

If you're a CIS subcontractor and your gross income exceeds £1,000 per tax year (2025/26), you need to file a Self Assessment return. Even if your CIS deductions cover your entire tax bill, filing is the only way to get a refund of any overpaid amount.

How long does a CIS refund take?

If you file online, HMRC typically processes refunds within 4-6 weeks. Filing early in the tax year (April-June) often results in faster processing than filing close to the January deadline. Refunds are paid directly to your bank account if you've provided your details.

Can I file my CIS tax return myself or do I need an accountant?

Many CIS subcontractors file their own returns, especially if their affairs are straightforward (single trade, clear expenses, payment statements from contractors). The HMRC online system guides you through each section. Consider professional help if you have multiple income sources, complex expenses, or aren't confident with the numbers.

What if my contractor didn't give me CIS payment statements?

Contact your contractor and request the statements — they're legally required to provide them. If a contractor has ceased trading or can't be reached, contact HMRC's CIS helpline on 0300 200 3210. HMRC holds records of CIS deductions reported by contractors and can help verify your figures.

This guide is for informational purposes only and does not constitute tax, legal, or financial advice. Tax rules change frequently. Always verify current requirements on GOV.UK or consult a qualified accountant for your specific situation.

Official Sources

CIS: check your payments and deductions - GOV.UK
Self Assessment tax returns - GOV.UK
File your Self Assessment tax return online - GOV.UK
Construction Industry Scheme (CIS) - GOV.UK

Self Assessment for CIS Workers: How to File Your Tax Return

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