First Sole Trader Tax Return 2026/27: 10 Common Self Assessment Mistakes to Avoid

Filing your first Self Assessment tax return as a sole trader involves dozens of deductions, allowances, and compliance rules. Many new traders leave thousands of pounds in tax relief on the table or incur penalties by missing deadlines. This guide covers 10 critical mistakes to avoid on your 2026/27 return.

Mistake 1: Getting Your Accounting Basis Wrong

You must choose either cash basis or accruals basis accounting. This choice massively affects your reported profit and tax liability.

Cash basis: Record income and expenses only when money actually changes hands.

Example: You invoice a client GBP 5,000 in February 2026 but don't receive payment until April 2026.

• Cash basis: GBP 0 income in 2026; GBP 5,000 income in 2027 (when received)
• Accruals basis: GBP 5,000 income in 2026 (when invoiced)

Accruals basis: Record income when earned (invoiced) and expenses when incurred (regardless of payment timing).

Which to choose:

• Cash basis (simpler, often better for first-year traders):

  • Easier administration (just track bank transactions)
  • Better cash flow management (claim expenses when you actually pay them, not when invoiced)
  • Limited to: GBP 1.5M turnover; no partnerships
  • Can claim 100% home-as-office cost (see mistake 6)

• Accruals basis (required for larger businesses):

  • More complex (need to track invoices and accrued expenses)
  • Better matches income to expenses (more accurate profit picture)
  • Required if turnover likely to exceed GBP 1.5M within 5 years
  • Can only claim proportionate home-as-office cost (not 100%)

Common mistake: New traders choose accruals basis because they think it's "more professional," then struggle with record-keeping. For most first-year traders, cash basis is simpler and often more tax-efficient.

Mistake 2: Forgetting Pre-Trading Expenses

Expenses you incur before you officially start trading are still deductible against your first year's profit.

Example: You spend GBP 1,200 on a laptop, GBP 800 on website design, and GBP 400 on stationery in January 2026, then officially start trading in March 2026.

• These are pre-trading expenses (incurred before first income)
• Fully deductible against your March-April trading profit
• Many new traders forget these entirely and miss GBP 2,400 in deductions

What qualifies:

• Equipment and software purchases
• Website and branding costs
• Professional advice (accountancy, legal)
• Training courses relevant to your trade
• Market research
• Travel to scout premises or suppliers

What doesn't:

• Rent or mortgage interest on premises before you use it for trading
• Loan interest if funds were borrowed before trading started (some exceptions apply)

Mistake 3: Missing the January Payment on Account Deadline

New traders often overlook the payment on account system.

If your tax bill (Self Assessment) exceeds GBP 1,000, HMRC assumes you'll owe similar in the next tax year. They request two "payments on account" (typically 50% of your expected tax due):

• First payment: 31 January following the end of the tax year
• Second payment: 31 July

Example: Your 2025/26 tax bill is GBP 2,000.

• HMRC will request 2 payments on account of GBP 1,000 each in January 2026 and July 2026
• These are estimates for your 2026/27 liability
• If you don't pay by 31 January, interest and penalties apply

Mistake: Many first-year traders don't expect this payment and miss the January deadline.

Solution:

• Budget for the January payment (usually matching or exceeding your previous year's bill)
• You can appeal if circumstances change significantly (e.g., business closed down)
• Request a reduced payment if you believe your 2026/27 profit will be lower

Mistake 4: Not Claiming Mileage Relief (45p/Mile)

If you use your car for business purposes, you can claim mileage relief at 45p per mile for the first 10,000 miles, then 25p per mile thereafter.

Example: You drive 8,000 miles for business in 2026/27.

• Claim: 8,000 miles × 45p = GBP 3,600
• Tax saving (at 20%): GBP 720

What qualifies:

• Driving to client premises
• Driving to suppliers
• Driving to business meetings (not commuting to a permanent office)
• Driving to exhibitions or trade shows

What doesn't qualify:

• Commuting from home to a permanent business premises
• Private use (socialising, holidays)
• Driving between multiple jobs on the same day (usually no relief)

Record: Keep a simple log:

• Date of journey
• Business purpose
• Mileage (or use estimates if you didn't track every journey)

Simplified approach: If you drive, estimate miles conservatively (e.g., 6,000-8,000 annually) and claim 45p per mile. Most HMRC audits don't challenge reasonable mileage claims for new traders.

Mistake 5: Neglecting Capital Allowances

Capital allowances allow you to deduct the cost of business equipment and machinery over time (rather than claiming the full cost immediately).

Plant and machinery allowances (2026/27):

• Annual Investment Allowance (AIA): GBP 1,000,000 per year (relief for most equipment under this threshold)
• First Year Allowance (FYA): 100% for green/low-emission vehicles
• Balancing Allowances: For items sold or scrapped (calculate relief based on actual sale price vs book value)

What qualifies:

• Machinery and equipment
• Computers and office equipment
• Furniture and fixtures (some restrictions apply)
• Vehicles (with restrictions for cars above GBP 45,000)

What doesn't:

• Land and buildings
• Tools costing under GBP 500 (treated as small tools, may be expensed immediately or written down)
• Improvements to existing buildings (likely building allowance, not plant)

Example: You buy office furniture for GBP 3,000, a computer for GBP 2,000, and a van for GBP 15,000 in your first year.

• Total: GBP 20,000 in capital items
• AIA available: GBP 1,000,000
• You claim the full GBP 20,000 as a deduction in year 1 (under AIA)
• No need to write down over multiple years

Mistake: Treating equipment as an expense rather than claiming capital allowances. The maths is usually better with AIA (full relief year 1) than writing down gradually.

Mistake 6: Claiming Home-as-Office Incorrectly

If you work from home, you can claim a proportion of your household costs.

Two methods:

Method 1: Simplified rate (easiest)

• GBP 5 per month if 1-25 hours per week home-working
• GBP 10 per month if 25-50+ hours per week home-working
• No need for detailed records; accepted by HMRC

Example: You work from home 30 hours per week.

• Claim: GBP 10 per month × 12 = GBP 120 annually
• No records needed; instant HMRC acceptance

Method 2: Proportionate method (more detailed)

• Calculate percentage of home used for business (e.g., 1 room out of 5 = 20%)
• Claim percentage of:
  • Rent/mortgage interest (not capital)
  • Council tax
  • Utilities
  • Building insurance
  • Maintenance and repairs

Example: Home-office is 10% of your 3-bedroom house.

• Annual mortgage interest: GBP 4,000
• Utilities: GBP 1,200
• Building insurance: GBP 600
• Maintenance (estimated): GBP 1,000
• Total household costs: GBP 6,800
• Claim 10%: GBP 680

Mistake: Claiming more than your actual home-business proportion (e.g., claiming 20% when office is 10% of home). HMRC may challenge if your claim exceeds the actual space used.

Cash basis advantage: On cash basis, you can claim 100% of "eligible household expenses" even if shared (e.g., 100% of internet if used for business). On accruals, you must proportion.

Mistake 7: Misunderstanding Phone, Internet, and Broadband Deductions

If you use your personal phone or broadband for business, you can claim a proportion.

Approach:

• Estimate what percentage is business use (e.g., 50% if you're home-working)
• Claim that percentage of the monthly cost

Example: Your broadband costs GBP 30/month; 50% business use.

• Claim: GBP 30 × 50% × 12 = GBP 180 annually

Mistake: Claiming 100% of internet or phone bills for part-time home-working (HMRC expects proportioning). Be reasonable: 50-75% is defensible for most home-based traders.

Mistake 8: Wrong Start Date of Trade

Your start date of trade determines which tax year your income falls into and affects your first-year tax bill calculations.

HMRC definition of "start of trade":

• First invoice issued
• First business transaction completed
• Not when you formally registered or planned to start

Example: You register as self-employed in June 2026 but don't issue a first invoice until September 2026.

• Start date: September 2026 (not June)
• 2026/27 return includes only September-March income (7 months)
• Next full-year return is 2027/28

Mistake: Reporting the wrong start date (e.g., claiming June start when actual trading began September). Check your Self Assessment return carefully.

Mistake 9: Not Keeping Records for 5 Years

HMRC requires you to retain business records for 5 years after the year-end to which they relate.

What to keep:

• Invoices issued to clients
• Receipts and invoices from suppliers
• Bank statements
• Expense records (mileage log, home-office calculations)
• Payroll records (if you employ anyone)
• Equipment purchase receipts (for capital allowances)

Retention period:

• 2026/27 tax year ends 5 April 2027
• Keep records until 5 April 2032 (5 years after year-end)

Mistake: Discarding receipts after 1 year or failing to back up digital records. HMRC can issue fines (GBP 3,000+) if you cannot produce records when requested.

Solution: Scan receipts or use accounting software that auto-backs up (e.g., FreeAgent, Xero).

Mistake 10: Late Registration Penalty

You must register as self-employed by the 5 October following the end of the tax year in which you first trade.

Example: You start trading in July 2026.

• You must register by 5 October 2026 (self-employed status)
• Self Assessment return due: 31 January 2027
• If you register after 5 October 2026, you may incur a GBP 100-GBP 500 penalty

Mistake: Delaying registration. Even if you're uncertain about income, register immediately. You can file a return showing minimal profit.

Critical Deadlines for 2026/27

Key Deductions Summary

Conclusion

Your first Self Assessment return sets the pattern for subsequent years. Mistakes on issue 1 (accounting basis, start date) cascade into future years. Prioritise claiming allowed relief (mileage, home-as-office, capital allowances) correctly and avoid missing deadlines (January payment on account, 5 October registration). Use accounting software to track expenses, keep records for 5 years, and consider hiring an accountant for year 1 (cost typically GBP 200-600; often saves more in tax relief than it costs).