Self Assessment Made Simple
Understand deadlines, who needs to file, and how to report your income correctly. Get expert guidance on deductions, penalties, and everything you need to know about UK Self Assessment.
Do You Need to File Self Assessment?
You MUST file if:
- ✓ Self-employed (turnover > £1,000)
- ✓ Taxable income > £100,000
- ✓ Rental income (£1,000+)
- ✓ Dividend income > £1,000
- ✓ Untaxed interest (after £1k allowance)
- ✓ Trustees or beneficiaries
You DON'T need to file if:
- ✗ Employee earning < £100k
- ✗ Tax paid via PAYE entirely
- ✗ All income in ISAs
- ✗ No trading income
- ✗ HMRC hasn't sent notice
- ✗ Only receive employment income
Important Deadlines
5 April
End of UK tax year
New tax year begins 6 April. Last day to make contributions to previous year's pension.
31 October
Paper return deadline
Last day to submit paper Self Assessment return (6 months after tax year end)
31 January
Online deadline & tax due
Last day to file online return and pay tax in full. Second payment on account due.
Common Allowable Business Expenses
Home Office
- • Rent/mortgage proportion
- • Council tax share
- • Utilities
- • Internet
Travel
- • Business mileage (45p/mile)
- • Trains/flights
- • Parking & tolls
- • Vehicle insurance
Equipment & Materials
- • Computer/laptop
- • Software & apps
- • Tools & machinery
- • Raw materials
Professional Fees
- • Accountant/bookkeeper
- • Legal advice
- • Professional memberships
- • Training courses
General Expenses
- • Stationery
- • Phone & broadband
- • Client entertainment
- • Subscriptions
Stock & Inventory
- • Goods for resale
- • Stock purchased
- • Opening & closing values
- • Depreciation
⚠️ Keep records: You must keep invoices and receipts for at least 5 years. HMRC can ask for evidence at any time.
Common Self Assessment Mistakes
Missing the deadline
Late filing penalties start at £100 immediately after 31 Jan. After 3 months, it's 5% of the tax owed.
Not keeping records
HMRC can request evidence up to 5 years later. Without documentation, you can't claim business expenses.
Forgetting to declare all income
Including small amounts of interest, dividends, and freelance work. HMRC can see bank deposits.
Over-claiming expenses
Only claim expenses that are wholly and exclusively for the business. Personal expenses are not allowable.
Not updating address
If HMRC sends notices to the wrong address and you miss deadlines, penalties still apply.
Mixing personal and business accounts
Commingling makes it harder to prove what was business expense vs personal spending.
Self Assessment Penalties Timeline
£100 late filing penalty
Applies automatically after 31 January deadline
5% of tax owed
If still not filed after 3 months
Another 5% of tax owed
Total penalty now 10% of tax owed
Further £300 or 5% (whichever is higher)
Plus interest on any unpaid tax from due date
How to Make Self Assessment Easier
📱 Use Tax Software
Use HMRC-approved software (many free options available). They guide you through step-by-step.
📊 Keep Digital Records
Use accounting software to categorize expenses throughout the year, not just at deadline.
🗂️ Organize Documents
Scan receipts and invoices into folders. Use a simple spreadsheet if software seems too much.
⏰ File Early
File by 31 December if you want a refund for that tax year. But 31 January is the main deadline.