HMRC Self Assessment is the system through which millions of UK taxpayers declare income that is not taxed at source. If you are self-employed, a company director, a landlord, or someone with income above certain thresholds, it is likely to apply to you. Yet every year, people miss deadlines, underpay, or simply do not realise they need to file at all.

This guide covers the current position for 2026/27: who needs to complete a return, the tax bands and thresholds that apply right now, when payments are due, and what has changed with Making Tax Digital for Income Tax. We have also included the penalty structure, because understanding what HMRC charges for late filing or payment is often the best motivation to stay on top of things.

One important note: this article reflects the position as of June 2026. Tax rules change, and the figures here should be verified against your own circumstances before you act on them.

HMRC does not always write to tell you that you need to file. The responsibility sits with you to register if you meet the criteria. The most common reasons people need to complete a Self Assessment tax return are:

• You are self-employed or a sole trader with gross income above £1,000 in the tax year.
• You are a partner in a business partnership.
• You are a company director (with some exceptions for non-profit organisations).
• You have untaxed income from property, including furnished holiday lets.
• Your total income in 2025/26 was above £100,000, which removes your personal allowance gradually through the return.
• You received income from savings, investments, or dividends above the relevant allowances.
• You or your partner received Child Benefit and either of you earned more than £60,000 (the High Income Child Benefit Charge threshold, revised in April 2024 and still in force).
• You have foreign income or income that is complex enough that PAYE cannot tax it accurately.

If you are unsure whether you need to register, it is worth checking rather than assuming PAYE has caught everything. HMRC’s own tool on GOV.UK can confirm your position, or you can speak to an accountant who will tell you in a few minutes.

For the 2026/27 tax year, the main income tax bands in England and Wales are unchanged from recent years. The government has kept them frozen, which means more people are being pulled into higher brackets as earnings rise.

• Personal allowance: £12,570 (no tax on income up to this level, unless your total income exceeds £100,000).
• Basic rate (20%): on income between £12,571 and £50,270.
• Higher rate (40%): on income between £50,271 and £125,140.
• Additional rate (45%): on income above £125,140.

Scottish taxpayers have their own rates, which differ from the rest of the UK. If you are resident in Scotland, the bands above do not apply to your non-savings, non-dividend income.

In addition to income tax, self-employed people pay National Insurance. Class 4 contributions apply to profits between the lower profits limit and upper profits limit, and the rates are set each April. Class 2 contributions were abolished in April 2024, so the self-employed now only pay Class 4 on profits above the relevant threshold.

The dividend allowance remains at £500 for 2026/27. If you take income partly as dividends through a limited company, this is worth factoring in at the planning stage.

The 2025/26 tax year ended on 5 April 2026. If you need to file a return for that year, the deadlines are as follows:

• Paper return: 31 October 2026.
• Online return: 31 January 2027.
• Tax payment: 31 January 2027 (balancing payment plus the first payment on account for 2026/27, if applicable).
• Second payment on account: 31 July 2027.

Payments on account are something many first-time filers do not anticipate. If your Self Assessment bill is above £1,000, HMRC requires you to pay half of that year’s liability in advance on account for the following year. This can create a large first-year payment, so it is worth planning for it.

The penalty structure for late filing starts at £100 on the day after the deadline. After three months, HMRC can charge up to an additional £10 per day (up to 90 days), making the potential total £900 before further penalties at six and twelve months. Late payment attracts a 5% surcharge on tax unpaid at 30 days, six months, and twelve months, plus interest.

These charges accumulate quickly and are rarely reduced on appeal unless you have a genuinely exceptional reason. Filing on time, even if you cannot pay immediately, avoids the daily penalties.

Making Tax Digital for Income Tax (MTD for IT) is no longer something on the horizon. It launched in April 2026 for self-employed individuals and landlords with qualifying income above £50,000. If you fall into that group, you are now required to use MTD-compatible software, keep digital records, and submit quarterly updates to HMRC, with a final declaration replacing the traditional Self Assessment return.

The phased rollout continues as follows:

• April 2027: MTD for IT extends to those with qualifying income above £30,000.
• April 2028: the threshold drops to £20,000, bringing a further tranche of self-employed people and landlords into the regime.

If you are above £50,000 and have not yet set up MTD-compatible software or registered with HMRC for the new regime, you should address this as a priority. HMRC has been clear that the quarterly update requirement is mandatory, not optional, for those in scope.

For those below the current threshold, the traditional annual Self Assessment process continues as normal for now. However, with the threshold falling to £20,000 by 2028, most self-employed people and landlords will eventually need to make the switch. Getting your bookkeeping into a cloud accounting platform sooner rather than later makes that transition considerably easier. Our Making Tax Digital for Income Tax guide covers the practical steps in more detail.

After working through Self Assessment returns for clients across a range of industries, we see the same errors come up repeatedly. These are not complicated mistakes, they are usually oversights that cost people more than they need to pay.

Forgetting income sources

Bank interest, freelance income, rental income, and dividends all need to be declared, even if tax has already been partially deducted at source. A lot of people include their main employment income and nothing else, then receive an unexpected bill months later.

Missing allowable expenses

Self-employed people can deduct genuine business costs before calculating profit. This includes mileage (currently 45p per mile for the first 10,000 miles in a car, then 25p), use of home as office, professional subscriptions, and equipment. Leaving these out means paying tax on income that should not be taxed.

Ignoring payments on account

As noted above, if your bill exceeds £1,000, you are also paying in advance for the following year. First-time Self Assessment filers are often caught off guard by a bill that is one and a half times what they expected.

Not registering in time

You must register for Self Assessment by 5 October following the end of the tax year in which you first became liable. Miss that and penalties can apply before you have even filed a return.

Addressing these points with a bit of forward planning each year makes a meaningful difference to the overall bill and the stress involved.