UK Sole Trader First Tax Return Mistakes to Avoid 2026/27

Becoming a sole trader is exciting -- but your first Self Assessment tax return can be stressful, especially if you discover expensive surprises too late. Here are the ten most common mistakes that trip up first-time filers in 2026/27.

Mistake 1: Getting the Accounting Period Wrong

Your Self Assessment covers the tax year, which runs from 6 April to 5 April -- not the calendar year, not your business anniversary. If you started trading on 1 July 2025, your first return (for 2025/26) covers 1 July 2025 to 5 April 2026. Only profits earned in that period are taxable for that return, not a full year's figures.

Mistake 2: Missing Allowable Expenses

HMRC allows you to deduct costs that are wholly and exclusively for business purposes. First-timers often miss legitimate deductions including:

• Home office costs (using the flat rate of £6/week, or a proportion of actual bills).
• Business mileage at AMAP rates: 45p per mile for the first 10,000 miles; 25p per mile above that.
• Phone and broadband -- the business-use proportion.
• Professional subscriptions and memberships relevant to your trade.
• Accountancy fees for preparing the return itself.
• Bank charges on a business account.

Underclaimng expenses is one of the most common errors -- and entirely avoidable.

Mistake 3: The Payments on Account Shock

This is the most frequently cited nasty surprise for new sole traders. If your tax bill exceeds £1,000, HMRC requires you to make payments on account -- advance payments towards next year's tax.

On your first return, you pay:

• The full tax for the year just ended.
• Plus 50% of that amount as a first payment on account for the following year.
• Plus another 50% as a second payment on account, due 31 July.

If your first tax bill is £5,000, you pay £7,500 by 31 January (£5,000 + £2,500), then £2,500 more by 31 July. New traders are frequently unprepared for this -- budget for it from day one by saving 25--30% of net income throughout the year.

Mistake 4: Not Registering for Self Assessment on Time

You must register with HMRC for Self Assessment by 5 October following the end of the tax year in which you started self-employment. If you started trading in 2025/26, you must register by 5 October 2026. Late registration can trigger penalties even before you file.

Mistake 5: Claiming Personal Costs as Business Expenses

Buying a new laptop used for both personal streaming and work? Only the business-use proportion is claimable. The test is strict: costs must be wholly and exclusively for business. Mixed-use items must be apportioned, and purely personal costs are never allowable. HMRC enquiries often focus on travel, meals, clothing and equipment -- keep a clear audit trail.

Mistake 6: Getting the NI Class Wrong

As a sole trader with profits above the Small Profits Threshold (£7,105 in 2026/27), you pay Class 4 NI: 6% on profits between £12,570 and £50,270, then 2% above. Class 2 NI was abolished from April 2024, so there is no separate weekly stamp to pay.

However, if your profits are below £7,105 you may need to pay voluntary Class 3 NI (£18.40/week) if you want to protect your State Pension record -- Class 4 does not automatically give you qualifying years at very low profit levels.

Mistake 7: No Record of Receipts

HMRC can ask for evidence of claimed expenses going back several years. Shoebox receipts, unclear bank statements and "I remember buying it" are not adequate records. Use accounting software or at minimum a spreadsheet with photos of receipts. HMRC's free Basic PAYE Tools does not cover sole trader records -- consider something like FreeAgent, QuickBooks, or even a well-organised spreadsheet.

Mistake 8: Forgetting Cash and Non-Invoice Income

All income must be declared -- not just amounts invoiced or received by bank transfer. Cash-in-hand payments, tips, barter arrangements, and income from side platforms (Etsy, Fiverr, Airbnb) all count. The Trading Allowance gives you £1,000 of miscellaneous income tax-free, but above that it is all taxable.

Mistake 9: Missing the Deadline and Paying Late Penalties

The online filing deadline is 31 January after the end of the tax year. The tax payment deadline is the same date. Late filing incurs an automatic £100 penalty, then additional daily penalties after 3 months. Late payment incurs interest (currently above 7% per annum) plus surcharges. There are almost no good reasons to miss these dates -- set calendar reminders for October (to gather documents) and January (to file and pay).

Mistake 10: Not Using the Cash Basis

Most sole traders with turnover under £150,000 can use the cash basis of accounting -- recording income when received and expenses when paid, rather than when invoiced. This is simpler, avoids counting unpaid invoices as income, and is now the default for most small businesses. If you want to use the accruals basis instead, you must opt out explicitly. Make sure your accountant or software is using the method that suits your cashflow best.

One Final Tip: Register for Making Tax Digital Early

HMRC is rolling out Making Tax Digital for Income Tax (MTD ITSA) to sole traders with income above £50,000 from April 2026, and above £30,000 from April 2027. This requires quarterly submissions using compatible software. If your income is near these thresholds, start preparing now rather than facing a systems change at the same time as a tax deadline.

Getting your first tax return right sets up good habits for every year that follows. If in doubt, a one-hour session with an accountant is almost always worth the cost.