An SA302 is HMRC's official summary of your tax calculation for a Self Assessment year, showing your total income, allowable deductions, the tax and National Insurance due, and, for CIS subcontractors, the deductions already made under the scheme.
For a CIS sole-trader subcontractor the SA302 is the document that proves how much tax has been prepaid via CIS deductions and whether a refund is owed or a balance is payable. The calculation works as follows:
- Your gross CIS income (the full amount before any deductions) is the starting figure. Never use your net receipts: HMRC tests MTD ITSA thresholds, refund calculations and your tax liability all against the gross.
- Allowable expenses are deducted to arrive at your taxable profit.
- Income tax (20% / 40% / 45% in 2026/27) and Class 4 National Insurance (6% between £12,570 and £50,270, 2% above) are calculated on that profit, after your £12,570 personal allowance.
- The total CIS deductions shown on your payment and deduction statements are then set off against the tax and NIC due. If the deductions exceed the liability, the difference is your refund.
HMRC generates the SA302 automatically once a Self Assessment return is filed. You can download it from your Personal Tax Account or ask HMRC to send a paper copy. Mortgage lenders and landlords often request it as proof of income, making it useful beyond its immediate tax purpose.
If the SA302 shows a larger liability than expected, the usual reasons are missing expenses, incorrect gross income figures, or CIS deductions not matching the statements. A CIS accountant will reconcile the statements against invoices before filing to prevent these discrepancies.
For a step-by-step guide to filing the return that produces the SA302, see CIS Self Assessment: the complete guide for subcontractors.