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Written by Brad Askew
Legal Tech Founder
Civil & Commercial Law background · Founder of LegalDocuments.co.uk
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Updated April 2026 · England & Wales
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BA
Written by Brad Askew Legal Tech Founder
Civil & Commercial Law background · Founder of LegalDocuments.co.uk
Updated May 2026
·
England & Wales
If your income does not flow neatly through PAYE, HMRC will usually expect you to account for it through Self Assessment. That covers a wide mix of people: sole traders, landlords, company directors, those with sizeable investment income, and anyone whose tax affairs fall outside the standard employee setup.
The return itself is how you tell HMRC what you earned in the tax year, what you can deduct, and what tax is due as a result. Done properly, it is manageable. Done in a rush at the end of January, it tends to go wrong.
This guide walks through who falls within Self Assessment, what the return actually asks for, the common pitfalls, and where the deadlines bite. The rules change at the edges every few years, so always cross-check current thresholds on gov.uk before you file.
Overview
A Self Assessment return is the mechanism HMRC uses to collect Income Tax (and, where relevant, Class 2 and Class 4 National Insurance, student loan repayments and the High Income Child Benefit Charge) from people whose tax cannot be worked out automatically by their employer or pension provider. The main form is the SA100, supported by supplementary pages depending on your sources of income: SA103 for self-employment, SA105 for UK property, SA106 for foreign income, SA108 for capital gains, and so on.
You report your gross income for the tax year (6 April to 5 April), claim any allowable expenses and reliefs, and the system calculates the tax liability. Most people now file online through their HMRC Personal Tax Account or Government Gateway, though a paper route still exists with an earlier deadline.
Filing does not just settle what you owe: it is also how you claim refunds, certain reliefs, and losses you want to carry forward.
Key steps
01
Register with HMRC in good time. If this is your first return, you need to register for Self Assessment before you can file. The deadline is 5 October following the end of the tax year in which the income arose. HMRC will issue a Unique Taxpayer Reference (UTR), which you need to file. Leaving this until December is a common mistake that causes real stress.
02
Gather your figures before you start. Pull together everything that affects your tax position: self-employment income and business expenses, rental income and property costs, dividend vouchers, interest statements, P60 and P45 forms, pension contributions, Gift Aid donations, and records of any capital disposals. Good bookkeeping throughout the year makes this step short. Poor records make it painful.
03
Complete the main return and any supplementary pages. Log into your Government Gateway account and work through the sections that apply. The online system only shows you the pages relevant to the answers you give, which helps. Declare all taxable income, including smaller amounts that might feel trivial. Omissions are the single biggest cause of HMRC enquiries.
04
Claim allowable expenses and reliefs accurately. Deduct only costs that genuinely qualify for your situation: wholly and exclusively for trade for the self-employed, specific allowable property costs for landlords, and so on. Keep receipts and records for at least the period HMRC requires. If something is borderline, it is safer to leave it out or seek guidance than to guess in your favour.
05
Submit, pay, and keep proof. File before midnight on 31 January following the tax year end if you are filing online. Pay the balancing amount and, where applicable, your first payment on account by the same date. Save the submission receipt and your calculation. Payments on account for the next year are due 31 January and 31 July, which catches a lot of first-time filers off guard.
Common questions
QWho actually needs to file a Self Assessment return?
HMRC expects a return from the self-employed above the trading allowance, most landlords with rental profits, company directors with untaxed income, partners in partnerships, people with significant savings, dividend or foreign income, those liable to the High Income Child Benefit Charge, and anyone with capital gains to declare. If HMRC has sent you a notice to file, you must file even if you believe nothing is owed. Check the current criteria on gov.uk.
QWhat are the key Self Assessment deadlines?
The tax year runs 6 April to 5 April. Register by 5 October after the tax year ends if it is your first return. Paper returns are due by 31 October. Online returns and any balancing payment are due by 31 January. Payments on account, where they apply, fall on 31 January and 31 July. Missing these dates triggers automatic penalties and interest, so diarise them early.
QWhat happens if I file late or pay late?
HMRC applies an automatic penalty for a late return, with further penalties building up the longer it remains outstanding. Late payment attracts interest and, after set periods, additional surcharges. Penalties can apply even when no tax is due. If you have a genuine reasonable excuse, you can appeal. For current penalty amounts and interest rates, check gov.uk rather than relying on older figures.
QCan I amend my Self Assessment return after filing?
Yes. You generally have 12 months from the 31 January filing deadline to amend an online return, done by logging back into your account and resubmitting. After that window, corrections usually need to be made by writing to HMRC. If an error means you underpaid tax, disclose it promptly: unprompted disclosures normally attract lower penalties than mistakes HMRC finds during a later enquiry.
QDo I need an accountant to complete Self Assessment?
Not legally. Many people with straightforward affairs file their own return without difficulty. An accountant tends to be worth it once your situation becomes more complex: multiple income sources, property portfolios, foreign income, capital gains, or if you simply want peace of mind that reliefs and expenses are being handled correctly. The cost is often offset by tax saved and penalties avoided.
QHow long should I keep my Self Assessment records?
If you are self-employed or in a partnership, HMRC expects records to be kept for at least five years after the 31 January submission deadline of the relevant tax year. For individuals who are not in business, the requirement is shorter, usually 22 months after the tax year end. Keeping records longer is sensible if there is any prospect of an enquiry or ongoing disputes.
QWhat if I made a loss in my self-employment or rental business?
Losses still need to be reported on the relevant supplementary pages. Depending on the type of loss and the rules in force, you may be able to set it against other income in the same year, carry it back to an earlier year, or carry it forward against future profits of the same trade or property business. The options differ for self-employment and property, so check the current rules carefully.
BA
Brad Askew Legal Tech Founder
Brad has a background in civil and commercial law and founded LegalDocuments.co.uk to make clear, reliable legal information accessible to everyone. This site is not a law firm and does not provide regulated legal advice.
Legal disclaimer
This article is for general information only and does not constitute legal advice. We are not solicitors. For advice on your specific situation, please consult a qualified solicitor.
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