Common Self Assessment Mistakes That Cost You Money

Every year, thousands of taxpayers make avoidable mistakes on their Self Assessment returns. Some result in overpaying tax, others trigger penalties or HMRC enquiries, and many cause unnecessary stress and wasted time. Today, we're highlighting the most common errors so you can avoid them.

Mistake 1: Missing Allowable Expenses

The Error: Many self-employed people fail to claim all their allowable business expenses, resulting in paying more tax than necessary.

The Cost: If you're a higher-rate taxpayer and miss £5,000 of legitimate expenses, you'll overpay tax by £2,000.

How to Avoid It: Keep meticulous records throughout the year. Common overlooked expenses include:

• Professional subscriptions and memberships
• Business insurance
• Bank charges and interest on business loans
• Professional development and training
• Proportion of home costs if you work from home
• Business mileage (45p per mile for the first 10,000 miles)
• Equipment and software
• Marketing and advertising costs
• Accountancy fees

Remember, expenses must be "wholly and exclusively" for business purposes. If something has both personal and business use (like your mobile phone), you can only claim the business proportion.

Mistake 2: Incorrect Classification of Capital vs. Revenue Expenses

The Error: Confusing capital expenditure (buying assets) with revenue expenditure (running costs). These are treated differently for tax purposes.

The Cost: Capital items aren't immediately deductible against your profits. Instead, they qualify for capital allowances, which are claimed differently. Getting this wrong could mean missing out on valuable tax relief or claiming relief you're not entitled to.

How to Avoid It: Generally speaking:

• Revenue expenses: Day-to-day running costs (stationery, fuel, repairs)
• Capital expenses: Items you keep and use in the business (computers, vehicles, machinery)

If you've purchased significant equipment or assets, ensure you're claiming capital allowances correctly rather than treating them as ordinary expenses.

Mistake 3: Failing to Declare All Income

The Error: Not declaring all sources of income, whether through oversight or misunderstanding what needs to be declared.

The Cost: HMRC receives information from many sources (banks, employers, investment platforms). If they spot undeclared income, you'll face penalties, interest charges, and potentially an investigation. Penalties for carelessness start at 15% of the tax owed.

How to Avoid It: Declare everything, including:

• All self-employment income (even small amounts)
• Interest from savings accounts (even if it's below the Personal Savings Allowance)
• Dividends from shares
• Rental income
• Income from online platforms (eBay, Etsy, Airbnb, YouTube, etc.)
• Tips and gratuities
• Income from overseas sources
• Cryptocurrency gains

Even if no tax is due on these income sources, you must still declare them if HMRC has asked you to complete a return.

Mistake 4: Getting the Dates Wrong

The Error: Reporting income or expenses in the wrong tax year.

The Cost: This can significantly affect your tax bill and may trigger an HMRC enquiry. It can also create problems with payments on account.

How to Avoid It: Remember the tax year runs from 6th April to 5th April. Only include income and expenses that fall within these dates:

• If you received payment on 7th April 2024, it's in the 2024/25 tax year
• If you received payment on 4th April 2024, it's in the 2023/24 tax year

For accrual-basis accounting (required if your turnover is over £150,000), it's the date of the invoice that matters, not when payment was received.

Mistake 5: Incorrect National Insurance Calculations

The Error: Misunderstanding how National Insurance works for self-employed people, particularly when income fluctuates.

The Cost: Underpaying National Insurance can affect your State Pension entitlement. Overpaying is simply money wasted.

How to Avoid It: Understand that self-employed people pay two classes of National Insurance:

• Class 2: Fixed weekly amount (currently £3.45 per week) if profits exceed £12,570
• Class 4: Percentage of profits between £12,570 and £50,270 (currently 6%), then 2% on profits above that

The online system calculates this automatically, but it's worth understanding how it works.

Mistake 6: Not Claiming Work from Home Allowances

The Error: If you work from home (whether self-employed or employed), you may be able to claim tax relief on household expenses, but many people miss this.

The Cost: For self-employed people, you could miss out on hundreds of pounds of allowable expenses. For employees, you could miss the simplified flat rate allowance of £6 per week (worth £125 tax relief for a basic-rate taxpayer).

How to Avoid It: If you're self-employed, calculate the proportion of your home used for business and claim appropriate costs for:

• Heating and electricity
• Internet and phone costs
• Council Tax (business proportion)
• Rent or mortgage interest (business proportion)

Alternatively, you can use HMRC's simplified expenses allowance (£10-£26 per month depending on hours worked from home).

Mistake 7: Overlooking Pension Contribution Relief

The Error: Forgetting to claim higher-rate tax relief on pension contributions, or not realising that contributions can reduce your adjusted net income.

The Cost: Higher-rate taxpayers can claim an additional 20% tax relief through Self Assessment. On a £10,000 pension contribution, that's £2,000 of unclaimed tax relief. Additionally, pension contributions can help you avoid the High Income Child Benefit Charge or keep you below the £100,000 threshold where personal allowance starts to taper.

How to Avoid It: Always check the "tax reliefs" section of your return and ensure pension contributions are correctly recorded. Remember to include both your personal contributions and any employer contributions.

Mistake 8: Errors with Capital Gains Tax

The Error: Incorrectly calculating capital gains, not claiming the annual exemption, or forgetting about allowable costs.

The Cost: Capital Gains Tax rates are 10% or 20% (or 18%/24% for property). On a £50,000 gain, an error could cost you thousands.

How to Avoid It: Remember:

• You have an annual Capital Gains Tax allowance (£3,000 for 2024/25)
• You can deduct purchase costs, sale costs, and enhancement costs
• You may be able to claim reliefs like Private Residence Relief or Business Asset Disposal Relief
• Losses can be carried forward to offset future gains

If you've made significant capital gains, professional advice is strongly recommended.

Mistake 9: Rushing at the Last Minute

The Error: Leaving your tax return until late January and rushing through it.

The Cost: Rushed returns are far more likely to contain errors. You're also at risk of technical issues, missing documents, or discovering you can't access your Government Gateway account.

How to Avoid It: Start now. Even if you can't complete your return immediately, gathering information and making a start reduces stress and improves accuracy.

Mistake 10: Not Keeping Adequate Records

The Error: Failing to keep comprehensive records of income and expenses.

The Cost: If HMRC opens an enquiry and you can't provide evidence for figures on your return, they may disallow expenses or estimate income, resulting in a higher tax bill plus penalties.

How to Avoid It:

• Keep all invoices and receipts (digital copies are fine)
• Maintain a mileage log if claiming vehicle expenses
• Keep bank statements showing business transactions
• Photograph receipts on your phone immediately to prevent them fading
• Use accounting software or spreadsheets to track income and expenses throughout the year

You must keep records for at least five years after the 31st January submission deadline.

Mistake 11: Paying Tax Late

The Error: Submitting your return on time but forgetting that payment is also due by 31st January.

The Cost: Interest is charged daily on late payments from 1st February, plus penalties if payment is more than 30 days late (5% of the outstanding tax). Six months late? Another 5%. Twelve months late? A further 5%.

How to Avoid It: Check your payment methods and how long they take to clear:

• Faster Payments: Usually instant
• Debit/credit card payments: Clear within 3 working days
• Bank transfers: Can take 3-5 working days
• Cheques: Take at least a week

Build in buffer time and don't leave payment until the final day.

Mistake 12: Not Understanding Payments on Account

The Error: Being surprised by payments on account – advance payments towards next year's tax bill.

The Cost: On 31st January, if you owe tax of £5,000 for 2024/25, you may also need to make the first payment on account of £2,500 towards 2025/26, meaning you pay £7,500 in total. Many people aren't prepared for this.

How to Avoid It: Understand that payments on account are required if your Self Assessment tax bill is over £1,000. You'll make two payments:

• First payment on account: 31st January (50% of last year's bill)
• Second payment on account: 31st July (the remaining 50%)

If your income has dropped significantly this year, you can reduce your payments on account, but be cautious – if you reduce them too much, you'll face interest charges.

Double-Check Everything

Before hitting submit:

• Review every figure against your source documents
• Check you've included all income sources
• Verify you've claimed all eligible expenses
• Ensure dates are in the correct tax year
• Confirm your bank details are correct if you're expecting a refund
• Check you've answered all questions fully

Even better, have someone else review your return with fresh eyes.

When Professional Help Makes Sense

If you're finding any of these areas complex, or if your tax affairs involve multiple income streams, property, investments, or capital gains, professional help can save you far more than it costs. An experienced accountant will:

• Identify allowable expenses you've missed
• Ensure you're claiming all reliefs available to you
• Structure your affairs tax-efficiently
• Provide peace of mind that everything is correct
• Deal with HMRC on your behalf if any queries arise

Concerned about making costly mistakes on your tax return? Let MCC Partners review your tax affairs and ensure you're not paying more than necessary. Our experienced team at our Gravesend office provides comprehensive Self Assessment services for individuals and businesses throughout Kent. Contact us on 01474 619 990 or email This email address is being protected from spambots. You need JavaScript enabled to view it..