CIS Tax Rebate Calculator

What is the CIS scheme?

The Construction Industry Scheme (CIS) is HMRC's system for taxing payments to sub-contractors in construction. Contractors deduct 20% from sub-contractor payments before passing them on (or 30% if the sub-contractor isn't verified with HMRC).

The deduction is treated as tax already paid against your eventual Self Assessment liability. Most CIS workers are over-deducted because the 20% doesn't account for allowable expenses.

How the refund is calculated

Three steps:

1. Calculate profit = gross CIS income − allowable expenses
2. Calculate tax + Class 4 NI on that profit (using normal Self Assessment rates)
3. Compare against CIS already deducted — overpayment is your refund

Allowable expenses commonly missed by DIY filers:

• Mileage at 45p/mile (first 10,000 miles) for travel between sites
• Tools, equipment, PPE (boots, hi-vis, hard hats)
• CSCS card, ticket renewals, training courses
• Use of home (if you do paperwork at home)
• Phone, accountancy fees, professional subscriptions

When to claim

You can go back 4 tax years from the current year. So in 2025/26, you can still file or amend:

• 2024/25 (still recent)
• 2023/24
• 2022/23
• 2021/22 (deadline 5 April 2026)

Multi-year refund claims are common in CIS. We routinely see £2,000–£5,000+ refunds when catching up on 3 years.

Maximising your CIS refund

• Track every business mile — site travel adds up fast at 45p/mile
• Keep receipts for tools/PPE — even fuel-stop snacks if working away from base
• Apply for Gross Payment Status — if turnover is high enough, contractors stop deducting 20%
• Use a van — better than a car (full capital allowance, no BIK)
• Consider Ltd company — once turnover is £50k+, going Ltd often saves more than CIS's reduced upfront cashflow