Self Assessment glossary: 47 UK tax questions, answered

For the 2025/26 tax year, you must submit your online Self Assessment tax return and pay what you owe by 11:59pm on 31 January 2027.

Register online with HMRC by 5 October 2026 if you need to send a return for the 2025/26 tax year and have either never sent one before, or registered earlier but did not need to send one for 2024/25.

You must send a Self Assessment return if, in the last tax year, you were a sole trader earning more than £1,000 before expenses, a partner in a business partnership, or had Capital Gains Tax to pay.

A UTR, or Unique Taxpayer Reference, is a 10-digit number HMRC gives you when you register for Self Assessment or set up a limited company.

Sign in to your HMRC personal tax account or the HMRC app, or look at an old tax return or HMRC letter.

The UK tax year runs from 6 April to 5 April the following year.

You can correct a Self Assessment return within 12 months of the filing deadline, online or on paper, but you must wait 72 hours after filing before you update an online return.

Tell HMRC as soon as possible, using its online Self Assessment cessation service, or by phone or post to the Self Assessment general enquiries team.

If you are self-employed, keep your records for at least five years after the 31 January submission deadline for the tax year.

Payments on account are advance payments towards your next Self Assessment tax bill, including Class 4 National Insurance if you are self-employed.

A balancing payment is the difference between your total Self Assessment bill for a tax year and the payments on account you have already made towards it.

Yes.

A tax code is used by your employer or pension provider to work out how much Income Tax to take from your pay or pension.

Yes, if you owe less than £3,000 (you cannot part-pay to get under it), you already pay tax through PAYE, and you submitted your online return by 11:59pm on 30 December, or a paper return by 31 October.

Pay HMRC by the deadline using online bank approval, online or telephone banking, CHAPS, a debit or corporate credit card, or at your bank.

HMRC charges an initial £100 penalty for a late return.

HMRC charges penalties of 5% of the tax unpaid at 30 days, 6 months and 12 months after the payment deadline.

A reasonable excuse is something that stopped you meeting a tax obligation for a valid reason, such as a close relative's death shortly before the deadline, an unexpected hospital stay, a serious illness, a fire, flood or theft, or HMRC online services failing.

You may be able to set up a payment plan with HMRC to pay what you owe in instalments.

An SA302 is evidence of your earnings that you can get for the last 4 years once you have sent your Self Assessment return.

The SA100 is HMRC's main Self Assessment tax return.

Supplementary pages are extra forms sent with your SA100.

The SA102 is the set of supplementary pages used to record employment income on your SA100 tax return.

The SA103 is the supplementary form where a sole trader reports business income and expenses on the SA100.

The SA105 supplementary pages record UK property income on your SA100 tax return.

The SA108 supplementary pages record capital gains and losses on your SA100 tax return.

The trading allowance is up to £1,000 a year of tax-free trading income.

The property allowance is up to £1,000 a year tax-free from income from land or property.

The Rent a Room Scheme lets you earn up to £7,500 a year tax-free from letting furnished accommodation in your own home.

The standard Personal Allowance is £12,570 for both 2025/26 and 2026/27.

Marriage Allowance lets you transfer £1,260 of your Personal Allowance to your husband, wife or civil partner.

The High Income Child Benefit Charge is a tax charge on people with income over £60,000, for tax years starting from 2024 to 2025.

Adjusted net income is your total taxable income before any Personal Allowances, less certain tax reliefs.

Class 2 National Insurance is £3.50 a week for 2025/26 and £3.65 a week for 2026/27.

Class 4 National Insurance is 6% on self-employed profits between £12,570 and £50,270 and 2% on profits above £50,270.

In England, Wales and Northern Ireland, Income Tax is 20% on taxable income up to £37,700 above the Personal Allowance (£12,571 to £50,270), 40% from there to £125,140, and 45% above £125,140.

The dividend allowance is £500 a year, and you only pay tax on dividend income above it.

The Capital Gains Tax annual exempt amount is a tax-free allowance of £3,000 for individuals and £1,500 for trusts, according to gov.uk.

If you are self-employed, HMRC calculates your student loan repayment from your Self Assessment return, using your income for the whole year.

Allowable expenses are business costs you deduct from your income to work out taxable profit before paying Income Tax.

Cash basis is the standard way for sole traders and partnerships without corporate partners to record income and expenses.

Under simplified expenses, a self-employed driver can claim a flat rate per business mile.

Under simplified expenses, a self-employed person who works at least 25 hours a month from home can claim a flat rate: £10 a month for 25 to 50 hours, £18 for 51 to 100 hours, and £26 for 101 hours or more.

Basis period reform moved sole traders and partners to a tax year basis from 2024 to 2025, with 2023 to 2024 as the transition year.

It depends on your qualifying income from self-employment and property.

Under the Construction Industry Scheme, a contractor deducts 20% from a registered subcontractor's payments and passes it to HMRC, or 30% if you are not registered.

Section 24 limits tax relief for residential landlords' finance costs, such as mortgage interest.