The Self Assessment walkthrough for 2026/27 — every box explained
The SA100 main return plus supplementary pages cover up to 80 different income and deduction boxes. Most filers only need 8-12 of them. This walkthrough explains which pages apply to your situation, the boxes that catch people out, and how Payments on Account work so the bill doesn't double-up.
The UK Self Assessment return for 2026/27 (tax year ended 5 April 2026) is filed online between 6 April 2026 and 31 January 2027. The SA100 covers the basics (personal details, allowances, gift aid, marriage allowance). Supplementary pages cover specific income types: SA102 employment, SA103S/F self-employment, SA105 UK property, SA106 foreign income, SA108 capital gains. Filing online is free at gov.uk; you can save progress and return later. The hardest part for most filers is getting Payments on Account right — your first bill includes 100% of 2025/26 tax plus 50% advance for 2026/27.
Before you start — what you need to gather
UTR (10-digit Unique Taxpayer Reference)
National Insurance number
P60 from each employer (year-end pay summary)
P45 if you left a job in the tax year
P11D for any benefits-in-kind (company car, private medical, etc.)
Self-employment income and expense records
Bank/building society interest summaries (the £1 if you exceeded PSA)
Dividend vouchers from each platform/company
Pension contribution certificates (especially personal pensions where you want higher-rate relief)
Gift Aid donation receipts
Rental income statements + allowable expense records
Capital gains records (purchase price, sale price, dates, costs)
Charity donations and other reliefs
SA100 — the main return
The SA100 has 8 pages. Most people complete pages 1, 2, 3 and 8 — the rest are supplementary triggers.
Page 1 — Personal details
Name, address, date of birth, NI number, UTR. HMRC pre-populates this — check it carefully and update if wrong (especially address).
Page 2 — Tax-paid sources of income
Tick boxes confirming which supplementary pages apply: employment (triggers SA102), self-employment (SA103), partnerships (SA104), UK property (SA105), foreign (SA106), trusts (SA107), capital gains (SA108), residence/domicile (SA109).
Page 3 — Income from interest, dividends and pensions
Box 1: UK interest — total interest received from UK accounts. Gross figures (banks now pay gross since 2016). Includes ISA interest? No — ISA interest is tax-free, don't declare.
Box 2: UK dividends — total UK dividends received. Includes from ISAs? No — ISA dividends tax-free.
Box 3: Foreign dividends — yes if any UK-resident foreign holdings (uses SA106 page instead if substantial)
Box 4: State Pension — annual State Pension received
Box 5-7: Other pensions — workplace and personal pension income
Page 4 — Allowances, reliefs, donations
Marriage Allowance — if you want to claim or have claimed
Gift Aid donations — total of net donations made in tax year
Pension contributions to relief-at-source schemes — total of net contributions. HMRC adds the basic-rate relief automatically. Higher-rate taxpayers get extra 20-25% relief through this box.
Annuity payments
Subscriptions to qualifying body — some professional fees deductible (look up at gov.uk: List 3)
Page 5-7 — Tax-free income, blind person's allowance, etc.
Most people leave blank.
Page 8 — Tax calculation
HMRC calculates this automatically online. On paper, you do it yourself with the HMRC Tax Calculation summary (SA110).
SA102 — Employment
One SA102 per employment. You need: employer's name and PAYE reference, total pay and tax from P60, any benefits from P11D.
High-risk SA102 boxes
Box 1: Pay from this employment — gross pay before tax (figure on P60 box 1). Most common mistake: confusing taxable pay with gross pay (they differ if you have a pension salary sacrifice).
Box 2: UK tax taken off — total tax deducted (P60 box 2)
Box 3: Tips and other payments — only if not in box 1
Box 9: Company car — taxable amount of company car BIK (from P11D)
Box 17: Professional fees — yes if member of a List 3 body (e.g. RIBA, ACA, ACCA)
Box 20: Other expenses — travel/subsistence not reimbursed by employer
SA103 — Self-employment
SA103S (short form, turnover below £85,000) or SA103F (full form). One per business.
High-risk SA103 boxes
Turnover — total income from the business before any deductions
Allowable expenses — sum of allowed business expenses (HMRC's "wholly and exclusively" test). Common: cost of stock, vehicle expenses, premises, equipment depreciation, professional fees, marketing, postage, training
Capital allowances — special tax relief on assets bought for the business (e.g. £3,000 laptop). Annual Investment Allowance allows 100% first-year relief up to £1m.
Use of home as office — flat rate £6/wk (simplified) OR actual costs apportioned
Class 4 NICs — auto-calculated at 6% (£12,570-£50,270) / 2% above. Don't enter manually.
Trading allowance trap. If your gross turnover is below £1,000 you can claim the trading allowance instead of expenses (one or the other, not both). If turnover is above £1,000 but expenses exceed turnover, you have a loss — declare it; can be carried forward or sideways.
SA105 — UK property
For all UK rental income. One SA105 per person (joint owners file half each, but on their own SA105).
High-risk SA105 boxes
Total rents — gross rent for the year. Include rent owing if billed before 5 April even if not yet paid.
Property income allowance — £1,000 flat alternative to expenses. Useful for low-yield properties.
Allowable expenses — repairs (NOT capital improvements), insurance, agent fees, mortgage interest (basic-rate credit only from 2020/21), accountant fees, advertising, ground rent, service charges, repairs.
Mortgage interest restriction — since 2020/21, you cannot deduct mortgage interest from rental profit. You get a 20% tax credit instead. SA105 has a separate box for this.
Rent-a-room scheme — £7,500 tax-free if you let a furnished room in your only home. Declare income but claim relief.
SA108 — Capital Gains
For disposals (sales) of assets where total gains exceed the example-box">
Cryptoasset disposals — yes, every sale or swap. Each swap (BTC → ETH) is a disposal.
Other assets — collectibles, foreign property, business assets.
Annual exempt amount — auto-deducted by HMRC online. Don't enter twice.
Losses — separately notified within 4 years; can be carried forward.
30/60-day reporting trap. Residential property gains must also be reported separately within 60 days of completion (UK Property Disposal return), and CGT paid then. The SA108 is the year-end reconciliation. If you missed the 60-day deadline, declare the gain on SA108 and a late-payment penalty letter usually follows.
Payments on Account — the trap that catches new filers
Once your annual SA tax bill exceeds £1,000, HMRC requires Payments on Account (POA) — advance payments toward the following year's bill.
How POAs workYour first tax bill includes: (a) 100% of the tax owed for the year just ended, PLUS (b) 50% advance payment toward the following year (1st POA, due same day, 31 Jan), PLUS (c) another 50% due 31 July (2nd POA). So your first SA bill is typically 200% of the actual tax owed.
Example: £4,000 tax bill for 2025/26
31 Jan 2027 payment: £4,000 (2025/26 tax) + £2,000 (1st POA for 2026/27) = £6,000
31 Jul 2027 payment: £2,000 (2nd POA for 2026/27)
When you file 2026/27 (by Jan 2028), you've already paid £4,000 in POAs. If 2026/27 tax is £4,000, you owe nothing. If it's £5,000, you owe £1,000 balancing payment + £2,500 new POA. If it's £3,000, you get a £1,000 refund.
If income drops, claim a reduction.You can apply to reduce your POAs if you know next year's tax will be lower (e.g. left high-paying job). Form SA303 or via online account. Be cautious: if you under-reduce, HMRC charges interest.
Most common errors that trigger HMRC enquiries
Error 1: Pension contributions in the wrong box.Auto-enrolment workplace pensions are usually relief-at-source — declared in SA100 page 4, NOT in SA102 employment. Salary sacrifice schemes don't go anywhere — the tax saving is already in your gross pay figure.
Error 2: Forgetting to declare bank interest.Even if PSA covers it, HMRC expects to see the interest figure for cross-checking against bank records. Putting £0 when £400 was received triggers an enquiry letter.
Error 3: Crypto missing.HMRC has data-sharing agreements with major exchanges (Coinbase, Binance, Kraken). They cross-check disposals against SA returns. Undeclared crypto gains are an enquiry magnet.
Error 4: Rental losses not declared.If your rental made a loss, declare it on SA105 — this allows carry-forward against future rental profits. Many filers skip it and lose the future relief.
Error 5: Joint accounts split incorrectly.HMRC defaults to 50/50 for joint accounts. If you have a Form 17 election (legal property declaration of unequal beneficial interest), declare the split explicitly.
Get the January checklist
The January Self Assessment checklist runs through everything you need ready before the 31 January deadline — documents, payments, supplementary pages, and how to file from start to finish.
UK Tax Drag is not authorised by the Financial Conduct Authority and does not provide regulated financial or tax advice — see the content disclaimer for the full position. The methodology page documents how every guide is built and reviewed.
Four common Self Assessment scenarios — who needs to file, what to declare, and the impact on the 31 January deadline.
Lucy — full-time employee with a side hustle
Situation: PAYE salary £35,000 plus £6,500 freelance graphic design through Fiverr/Upwork.
Question: Does she need to file Self Assessment?
What to do: Yes — gross side-hustle income above £1,000 triggers Self Assessment. She registers by 5 October 2026 for the 2025/26 tax year, files online by 31 January 2027. Trading allowance £1,000 OR actual expenses (whichever is larger). On £6,500 minus expenses of £800: taxable profit £5,500. Income tax 20% = £1,100. Class 4 NI 6% = £330. Total bill: £1,430 + Class 2 NI £179 = £1,609.
James — director of own limited company
Situation: Salary £12,570 + dividends £35,000 from his Ltd company.
Question: Does he need to file Self Assessment?
What to do: Yes — all directors of UK companies must file Self Assessment regardless of income level. He declares the £12,570 PAYE salary plus £35,000 dividends. £500 dividend allowance covers the first chunk; remaining £34,500 taxed at 8.75% basic (up to £50,270 total income). Total tax owed: ~£3,019 dividend tax. Filed by 31 January 2027.
What to do: Yes — rental income over £2,500 triggers SA. Mortgage interest relief is now capped at 20% basic-rate credit (post-Section 24). Allowable expenses: maintenance, insurance, agent fees. Taxable rental profit: £14,400 - £1,200 maintenance = £13,200. Tax on profit at her band, then 20% credit on the £4,800 mortgage interest reduces final tax. Files by 31 Jan.
David — higher-earner with foreign dividends
Situation: PAYE £75,000 plus £4,000 foreign dividends from US shares with 15% withholding tax.
Question: Does he need Self Assessment?
What to do: Yes — foreign income above £2,000 triggers SA even if remitted. Declare £4,000 gross dividends. £500 covered by dividend allowance. £3,500 × 33.75% higher-rate = £1,181 UK liability. Foreign tax credit of 15% × £4,000 = £600 reduces this to £581. Net additional tax: £581. Keep contract notes and US 1099 forms in case HMRC asks for evidence.
Marcus — pension drawdown taker
Situation: Retired at 62, drawing £40,000 from pension flexi-access drawdown, plus £8,000 dividends from previously-held shares.
Question: Self Assessment needed?
What to do: Yes — total income above £100k untaxed by PAYE OR £10k+ dividend income triggers SA. Drawdown income is taxed at source by pension provider (emergency code initially, often over-taxed in first year). Marcus reconciles via SA to claim refund of over-deducted tax. Plus dividend tax on £8,000 - £500 allowance × 8.75% basic = ~£656. Refund expected if pension drawdown was over-coded.
Scenarios use 2026/27 UK tax-year rates. Personas are illustrative — verify your own situation against current HMRC guidance.
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