Self Assessment Basics
Start here if you are new to Self Assessment. Learn what it is, who needs to file, and what counts as taxable income.
Articles
What is Self Assessment?
Self Assessment is the system HMRC uses to collect income tax from people whose income is not taxed at source. Here is what it means and how it works.
Who needs to file Self Assessment?
Sole traders, landlords, company directors, and high earners. A clear checklist to see if you are in scope.
What does tax code 0T mean? (UK 2026)
0T means no Personal Allowance is being applied at that income source. Learn what it means, why it can be used and how to check it with HMRC.
What does tax code 1257L mean? (UK 2026/27)
1257L is the usual 2026/27 code for most people with one job or pension. Learn what the number and letter mean and when to check it.
UK tax thresholds 2026/27: the complete reference
Every threshold that decides how much tax you pay in the 2026/27 UK tax year. Income tax bands, National Insurance, tax-free allowances, with the figures HMRC uses.
The 40% tax bracket: when you pay it and how to plan around it (UK 2026/27)
40% income tax starts at £50,270 of taxable income in 2026/27. What that actually means for your take-home, and the legal ways to keep more of your money.
Emergency tax code: W1, M1, X and NONCUM (UK 2026/27)
Emergency tax-code markers W1, M1, X and NONCUM explained: what they mean, where to check your code and what to do after a job change.
Self Assessment deadlines 2026/27: every key date and what happens if you miss them
The full Self Assessment deadline calendar for the 2025/26 tax year (filed in 2026/27): registration by 5 October 2026, paper returns by 31 October 2026, online filing and payment by 31 January 2027.
The UK personal allowance 2026/27: what it is, what you keep tax-free, and when you lose it
The personal allowance is £12,570 for 2026/27. Income up to this amount is tax-free. The allowance tapers away above £100,000 and disappears entirely at £125,140.
How much tax do you pay in the UK? Take-home pay at every salary (2026/27)
Exactly how much income tax and National Insurance you pay on £20k, £30k, £50k, £60k, £80k, £100k and £150k salaries in 2026/27, with monthly take-home pay numbers.
UK National Insurance rates and thresholds 2026/27: a complete guide for employees, the self-employed, and employers
Employee NI is 8% above £12,570 and 2% above £50,270. Self-employed Class 4 is 6% / 2% on the same bands. Class 2 is voluntary in some cases. Full rate table and worked examples.
UK dividend tax 2026/27: rates, allowance, and how directors should take income
Dividend tax rates for 2026/27 are 10.75%, 35.75% and 39.35% above the £500 dividend allowance (basic and higher rates raised 2pp from 6 April 2026). Worked examples for limited company directors, the optimal salary vs dividend split, and why dividends still beat salary.
UK Capital Gains Tax 2026/27: rates, the £3,000 allowance, and the 60-day property reporting trap
Capital Gains Tax rates for 2026/27: 18% / 24% on residential property and other assets (rates aligned from 30 October 2024), with a £3,000 annual allowance. Plus Business Asset Disposal Relief at 18% and the 60-day property reporting rule.
IR35 explained for UK contractors: inside vs outside, CEST, and what 'inside' actually costs you
IR35 (off-payroll working) decides whether a contractor working through a PSC is treated as employed or self-employed for tax. The rules, who decides, what 'inside IR35' costs in pounds, and the small company exemption.
Who actually needs to file a Self Assessment return?
Anyone whose tax cannot be fully collected through PAYE. The common triggers: self-employment, rental income, dividends beyond the allowance, a company directorship with untaxed income, the High Income Child Benefit Charge, Capital Gains to report, or foreign income. Employment income alone, taxed through PAYE, does not put you in Self Assessment.
The test is untaxed income, not size of income. A £3,000 rental profit puts you in; a £300,000 salary on PAYE alone does not.
What the process looks like
Register once with HMRC — by 5 October after the end of the first tax year you need to file for — and you receive a Unique Taxpayer Reference. Each year you then report all your income for the tax year, 6 April to 5 April: employment from your P60, plus every untaxed source. HMRC's calculation nets off tax already paid and produces the bill.
File online by 31 January, pay by the same date. That is the whole machine; everything else is detail.
What to declare — everything, then let the reliefs work
The return wants all of it: employment, self-employment, rent, dividends, interest, pensions, gains, foreign income. Declaring income that turns out to be covered by an allowance costs nothing; omitting income that was not covered is where penalties live.
The allowances do real work — personal allowance, dividend allowance, personal savings allowance, trading and property allowances — but they apply through the return, not instead of it.
Where it goes wrong, and our standard
The classic failures are administrative: registering late, filing in January from a shoebox, and discovering payments on account the day the bill arrives. None of them are tax problems; all of them are record problems.
RR clients run through Compliance Vault™: records maintained year-round, the return prepared early from the Evidence Pack, and the January date reduced to a payment reminder from Deadline Lock.
Frequently asked questions
I'm employed and do a little freelancing. Do I need to file?
Once gross self-employment income passes the £1,000 trading allowance, yes — register and file, even though your job is fully taxed under PAYE. Below £1,000 the trading allowance usually covers it with no return needed for that alone.
Do company directors automatically have to file?
No — directorship alone does not require a return if all your income is taxed at source. In practice most director-shareholders take dividends, which do trigger filing once they pass the allowance. If HMRC has issued you a notice to file, you file regardless.
What is a UTR and where do I find it?
Your Unique Taxpayer Reference — a ten-digit number issued when you register for Self Assessment. It is on HMRC letters and in your online tax account, and you need it to file. Lost it? It is retrievable through your HMRC account rather than by re-registering.
Can HMRC just take the tax through my tax code instead?
Sometimes — smaller Self Assessment balances can be collected through a PAYE tax code if you file by 30 December and meet the conditions. It spreads the cost interest-free across the following year, which for employed filers is often worth having.