It is not a legal requirement, but CIS makes an accountant worth having. Contractors deduct 20% or 30% from your pay, so you are often owed a refund, and it only comes back correctly if every monthly deduction statement is tracked and claimed on your Self Assessment return.
Why the trades are different
Most self-employed people pay tax after the year ends. Under the Construction Industry Scheme, tax is taken at source from your pay throughout the year. Those deductions count as advance payments towards your bill, so what matters at year end is whether your bill is smaller or bigger than what has already been taken.
Worked example
A Brighton plasterer is paid £42,000 by contractors in 2025/26 before deductions, and has £8,400 taken under CIS at 20%. Materials, tools, van and insurance come to £11,000, so his profit is £31,000.
His income tax is £3,686 and his Class 4 National Insurance is £1,106. Total £4,792. He has already paid £8,400, so HMRC owes him about £3,608.
If he loses statements worth £800 of deductions, or forgets a claim, the refund is smaller by the same amount.
What an accountant does for a tradesperson
- Asks for your CIS statements as a matter of routine, and checks them against what the contractor reported.
- Knows which costs are genuinely deductible: tools, van, materials, protective clothing.
- Files early, so any refund comes back sooner.
- Handles Making Tax Digital quarterly reporting when it applies to you.
What to do
- Keep every monthly payment and deduction statement, or ask contractors for them.
- Log expenses as you go, not at year end.
- File as soon as the tax year ends, since you can send a return after 6 April.
- If you have never checked your refund against what you were owed, have it reviewed.
Common mistakes
- Losing deduction statements.
- Not registering for CIS, so 30% is taken instead of 20%.
- Over-claiming van costs without a mileage log.
Frequently asked questions
Speak to James Fitzpatrick — free consultation
Doing this yourself and not sure your CIS refund matches what you're owed? Let's take a second look.
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