The 6 October tax deadline checklist for Isle of Man sole traders
TL;DR — Work through this checklist a few weeks before 6 October and filing day becomes a review-and-submit, not a scramble. Sections cover income, expenses, categorisation, the paperwork you'll need, and what to hand over if you're using an accountant. Miss 6 October and it's a £100 penalty, plus £200 more after six months.
Your self-assessment return for the 2025/26 tax year is due with the Assessor of Income Tax by 6 October 2026. This checklist is what to have in front of you before you start.
Income
- Every invoice raised between 6 April 2025 and 5 April 2026
- Every payment received in the same period, however it came in
- Any income received through online platforms, with the platform's own statement
- Bank interest received
- Any other business income, however small
- Invoices raised in March 2026 and April 2026 double-checked against the correct tax year
Expenses
- Receipts and invoices for every business expense across the year
- Bank and card statements cross-checked against your records
- Mileage log for any personal-vehicle business use
- Capital purchases (equipment, tools, IT kit, vehicles) with date and cost, so capital allowances can be claimed
Categorisation
- Personal costs kept out of business records
- VAT treatment correct on each transaction (if VAT-registered)
- Rechargeable expenses flagged separately
Paperwork ready to hand
- Tax reference number
- T14 (Isle of Man ITIP and NI Deduction Card) for any employed income alongside self-employment
- T21 leaving certificate if you left a job partway through the tax year
- Details of any tax deducted at source (bank interest, subcontractor deductions)
- Bank statements for the full tax year
- Final income and expense totals, split by category
If you're using an accountant
- Everything above sent at least four weeks before 6 October
- Short note flagging anything unusual (big one-off, new income stream, change in trading pattern)
If you're filing yourself
- Assessor's online service login working (allow days, not hours, for a new account)
- Return read through once before you start filling it in
What Tally Up does for the deadline
If you've been recording in Tally Up through the year, most of this checklist is already done. Income and expense totals are split by category, with a running estimate of your tax and National Insurance so you know roughly what you'll owe before you file.
Not on Tally Up yet? Spreadsheet mode and CSV import can bring a year's worth of records in quickly. For the underlying dates and penalties, see what is the Isle of Man tax year and what are the key dates?.