UK Self-Employed — SA100 / SA103 Self Assessment Return

Establish the client's trading status, accounting basis election, and registration position. Confirm UTR number, Self Assessment registration with HMRC, and whether the client is within the Making Tax Digital for Income Tax (MTD ITSA) pilot. Identify any change of accounting date that would affect the basis period.

Gather and reconcile all trading income for the tax year (6 April to 5 April). Under cash basis, record receipts when received; under traditional accounting, recognise income when invoiced. Cross-check against bank statements, invoicing software (Xero, QuickBooks, FreeAgent, etc.), and any HMRC pre-populated data in the SA return. Identify any non-trading income (employment, rental, dividends, interest) that will appear on other SA100 pages.

Classify and substantiate all business expenses. Under cash basis, capital expenditure on plant and equipment is deducted in the year of purchase (no separate capital allowances computation). Under traditional accounting, claim Annual Investment Allowance (AIA — £1,000,000 per year) or Writing Down Allowances (WDA) on a capital allowances pool. Check eligibility for the simplified expenses flat rates (mileage at 45p/mile first 10,000 miles then 25p/mile; use of home at £10/month for up to 25 hours/month). Identify use-of-home costs, vehicle expenses, and any private-use adjustments.

Compute net trading profit or loss. Apply the £1,000 trading allowance if gross income is below this threshold (no expenses claimable). Calculate Class 2 NIC (£3.45/week for 2024-25, collected via SA) and Class 4 NIC (9% on profits between £12,570 and £50,270; 2% above £50,270). If a loss arises, identify the most beneficial relief: carry forward against future trading profits, sideways relief against other income in the same or prior year (subject to the £25,000 cap for uncommercial trades), or terminal loss relief.

Combine self-employment profit with any other income sources to calculate total taxable income. Apply the personal allowance (£12,570 for 2024-25, tapered at £1 for every £2 of adjusted net income above £100,000). Calculate income tax at 20% (basic rate, up to £50,270), 40% (higher rate, £50,271–£125,140), and 45% (additional rate, above £125,140). Check eligibility for the Marriage Allowance transfer (up to £1,260), pension relief (net pay or relief at source), Gift Aid donations, and any Blind Person's Allowance. Include student loan repayments (Plan 1/2/4/5 as applicable) calculated on SA.

Calculate the balancing payment due for the tax year (31 January following the year end) and the two payments on account for the following year (31 January and 31 July). Each payment on account is 50% of the prior year's SA liability (excluding CGT and student loan). Check whether a claim to reduce payments on account is appropriate (SA303) if current-year profits are expected to be materially lower. Confirm HMRC's online account balance, any earlier payments made, and any late filing or late payment penalties already issued.

Verify whether the client has crossed or is approaching the VAT registration threshold (£90,000 rolling 12-month taxable turnover for 2024-25 onwards). If registered, confirm the VAT scheme (standard quarterly, flat rate, cash accounting, or annual accounting) and ensure VAT returns (VAT100) are filed and paid on time via Making Tax Digital for VAT. Flag deregistration eligibility if taxable turnover has fallen below £88,000.