Self Assessment before it bites.
Every deadline, allowance, and MTD-for-ITSA pitfall UK sole traders, landlords, and directors hit, in the order they hit them, with the numbers that matter in 2026.
Who has to file
Sole traders, landlords, directors with untaxed income, high earners, CGT disposals, foreign income, six distinct triggers, one shared return.
Jump to section 02The four dates
Registration by 5 Oct, paper return 31 Oct, online return + balancing payment 31 Jan, second Payment on Account 31 Jul.
Jump to section 03What MTD changes
Quarterly digital updates now apply above £50k gross income, dropping to £30k in 2027. Spreadsheets-only no longer enough.
Jump to section 04Where filers lose money
Under-claimed expenses, forgotten Payments on Account, missed higher-rate relief on pensions and Gift Aid, late-filing penalties from day one.
Jump to sectionWho Has To File a Return
Self Assessment catches a broader group than most UK taxpayers realise. You must register with HMRC and file a return if any of the following apply during a tax year (6 April to 5 April):
- Sole trader profit over £1,000, the trading allowance covers small side incomes; anything above triggers full reporting.
- Rental income over £1,000, after the property allowance, landlords must declare rental profits regardless of scale.
- Company directors with untaxed income, dividends, benefits in kind, or other extractions not handled through payroll.
- High earners, anyone with adjusted net income over £150,000, or with the High Income Child Benefit Charge.
- Capital gains, disposals of residential property, shares, or business assets above the £3,000 annual exempt amount.
- Foreign income, overseas rental, dividends, or employment income of any amount.
The Self Assessment Calendar
Four dates matter. Missing any of them costs money. The tax year runs 6 April to the following 5 April. Registration with HMRC (if new) must happen by 5 October after the tax year ends. Paper returns are due 31 October. Online returns and the balancing payment are due 31 January. If you pay Payments on Account, a second instalment is due 31 July.
Run the numbers on your own position with our Self Assessment Tax Calculator, it models Income Tax, Class 2 and Class 4 NIC, the balancing payment and both Payments on Account in one view.
What 2026 Changes, MTD for ITSA
Sole traders and landlords with combined gross income over £50,000 must now keep digital records and submit quarterly updates to HMRC through approved software. The annual Self Assessment return remains, but is replaced by a "Final Declaration" that consolidates the four quarters plus any adjustments. From April 2027 the threshold drops to £30,000.
The shift is not cosmetic. Spreadsheets on their own no longer satisfy record-keeping rules, digital links are required, and bridging software is mandatory if you continue using Excel. The time to migrate to a compliant bookkeeping setup is before the threshold catches you, not after.
Where Most Filers Lose Money
- Under-claiming expenses, particularly home-office proportioning, vehicle mileage, and pre-trading costs.
- Forgetting Payments on Account, HMRC demands 50% of next year's expected tax in advance each January and July if your liability exceeded £1,000.
- Late filing penalties, the first £100 hits the day after the deadline regardless of whether tax is owed.
- Reliefs that slip through, pension contributions at higher-rate, Gift Aid, marriage allowance transfers, and SEIS/EIS claims all require active submission.
Go Deeper
Making Tax Digital for Income Tax Guide
Making Tax Digital for Income Tax explained for sole traders and landlords, including current start dates, thresholds, quarterly updates and software.
BriefingPayments on Account Explained
How Self Assessment payments on account work, when they apply, how much is due and when reducing them can create interest.
ComplianceSelf Assessment Deadlines and Penalties
Key Self Assessment deadlines, late filing penalties, late payment interest and what to check before submitting your tax return.
Related Tools & Services
Sort your Self Assessment before the January rush.
We close out the prior year's Self Assessment, file the return ahead of the 31 January deadline, and surface the planning moves that are still available before the new tax year starts.
File your Self Assessment with a Chartered Management Accountant