UK Self Assessment

Self Assessment collects tax on income that does not pass through PAYE. This module explains what the system is, then works through a distinction that catches people out: telling HMRC about income and having to file a return are two different obligations. Some untaxed income requires a return outright; in other cases HMRC can collect the tax through your tax code instead. It closes with the key dates, how to register, and what your UTR is for.

Everything taxable has to be reported, but different income goes in different places and some needs supplementary pages. This module covers self-employment income and the Trading Allowance, property income and the Property Allowance, the Rent a Room Scheme, dividends and savings interest with their respective allowances, and capital gains. It also flags the reliefs that a low earner can stack, and one reporting test that catches people even when no tax is due.

This module opens with a question that has to be settled first: which accounting basis your business uses. Since 2024/25 the cash basis is the default for most self-employed businesses, and the choice changes when income and expenses are counted and how equipment is relieved. From there it works through the wholly-and-exclusively test, premises, working from home, travel and vehicles, professional costs, and what cannot be claimed — including one item most people assume is allowable and is not.

Property businesses have their own rules, including their own cash basis, which is not the same as the trading one. This module covers what a landlord can deduct, the repairs-versus-improvements distinction that produces more errors than any other area of property tax, the Replacement of Domestic Items Relief, and the finance cost restriction on residential lettings — which is not simply twenty per cent of the interest paid.

Much of this module may not apply to you, and it says so at the top. Under the cash basis, most equipment is deducted as an ordinary expense and capital allowances do not come into it; cars are the main exception. For those using traditional accounting, the module covers the Annual Investment Allowance, writing down allowances at the rate that took effect in April 2026, the new 40% first year allowance, and the separate rules for cars.

The main return and its supplementary pages: which form covers which kind of income, what the SA100 itself captures, and how filing online compares with filing on paper. It also sets out how long to keep the return and the documents behind it, and why the period is longer than most people assume.

If your tax bill exceeds £1,000, HMRC will usually ask for two advance payments towards the following year. This module explains how they are calculated — which is narrower than "half of last year's bill" — when they apply, how to reduce them and the risk in doing so, and why the first January under Self Assessment produces a demand larger than the liability you were expecting.

MTD for Income Tax came into force in April 2026, and the thresholds tighten in 2027 and 2028. This module explains who is actually in scope — the test looks at an earlier year's return, which is the part most often misread — what the quarterly updates involve, and why they are cumulative rather than four separate quarters. It also covers record keeping, retention periods, and the penalty changes already legislated for later years.

A quick reference for 2026/27: Income Tax bands for England, Wales and Northern Ireland and, separately, the six Scottish bands with their own thresholds. Also National Insurance for the self-employed, the main allowances, dividend and capital gains rates, capital allowances, and simplified mileage rates. It closes with a list of what changed for 2026/27, so you can check any figure you have carried over from last year.

The practical review points, in three stages: what to gather before you start, what to check while completing the return, and what to do after submitting. Written as a working list rather than an explanation.

The HMRC pages and helplines you are most likely to need, with Reinza's contact details for anything the guide does not answer.