Complete Guide to Self Assessment Tax Returns
Who needs Self Assessment, what records to prepare, common deductions, deadlines, and when an accountant is worth it.
Last reviewed: 2 June 2026
Who usually needs to file
Self Assessment is used when HMRC needs more information than PAYE can collect automatically. It commonly applies to sole traders, partners, landlords, directors with untaxed income, people with capital gains, higher earners, and people with foreign income.
Your situation can change from year to year, so it is worth checking early rather than waiting until January.
Records to prepare
Good records make the return cheaper and less stressful. Keep evidence for income, business expenses, mileage, pension contributions, charitable donations, rental property costs, savings income, dividends, and capital gains.
- Bank statements and bookkeeping exports
- Invoices and receipts
- P60, P45, P11D, and payslips where relevant
- Mortgage interest and rental statements for property income
- Share or crypto transaction summaries for gains
Common expense mistakes
Many errors come from mixing personal and business costs, claiming private-use portions incorrectly, or missing small recurring costs. If you are unsure, ask before filing. HMRC can charge penalties where a return is careless or inaccurate.
When an accountant is useful
A simple employment-only return may be manageable yourself. An accountant is more valuable when there are multiple income sources, property income, capital gains, foreign income, business losses, high income child benefit charge, or payments on account to plan for.