Why Self Assessment Tax Return Mistakes Matter
You are responsible for ensuring that the information included in your tax return is complete and accurate. HMRC may compare the figures you report with information received from employers, banks, pension providers, investment platforms, online marketplaces and other organisations.
A genuine error will not automatically mean that you have acted dishonestly. However, an inaccurate return may still result in additional tax, interest and a penalty where HMRC considers that reasonable care was not taken. Correcting an error promptly and cooperating with HMRC can affect how the matter is treated.
10 Self Assessment Mistakes Taxpayers Commonly Make
- Forgetting about tax-free allowances
- Reporting PAYE salary or employment benefits incorrectly
- Claiming expenses that are not allowable
- Failing to check an incorrect tax code
- Not declaring every source of taxable income
- Missing required supplementary information
- Making calculation or data-entry errors
- Submitting a return when one is not required
- Missing registration, filing or payment deadlines
- Failing to plan for the tax bill and Payments on Account
1. Forgetting About Tax-Free Allowances
One of the most common tax return mistakes is failing to use an allowance or relief that is available. An allowance does not always remove the requirement to report income, and eligibility depends on your individual circumstances, so each claim should be checked carefully.
Main Tax-Free Allowances for 2026/27
| Allowance | 2026/27 amount | What it means |
|---|---|---|
| Personal Allowance | £12,570 | The standard amount most people can receive before paying Income Tax. It is reduced where adjusted net income exceeds £100,000. |
| Marriage Allowance | Transfer £1,260 | An eligible spouse or civil partner can transfer £1,260 of unused Personal Allowance, reducing the recipient’s tax by up to £252. |
| Personal Savings Allowance | £1,000, £500 or £0 | Basic-rate taxpayers may receive up to £1,000 of savings interest tax-free, higher-rate taxpayers £500 and additional-rate taxpayers receive no allowance. |
| Dividend Allowance | £500 | Dividend income within the allowance is taxed at 0%, although it still counts towards the taxpayer’s Income Tax band. |
| Blind Person’s Allowance | £3,250 | An additional allowance available to eligible individuals, added to their Personal Allowance. |
| Trading Allowance | Up to £1,000 | May cover qualifying gross trading or miscellaneous income, subject to the rules and exclusions. |
| Property Allowance | Up to £1,000 | May cover qualifying gross property income, but it cannot normally be used alongside actual property expenses for the same income. |
The correct treatment depends on the type and amount of income you receive. For example, choosing the £1,000 trading allowance may be less beneficial than claiming actual allowable expenses where business costs exceed the allowance.
2. Reporting PAYE Salary and Employment Benefits Incorrectly
Employees and company directors may need to include employment income on their return. A common error is entering the wrong salary, omitting a previous employment or failing to report taxable benefits.
Documents that may be needed include:
- A P60 for employment held at the end of the tax year
- A P45 for an employment that ended during the tax year
- A P11D or equivalent details for taxable benefits and expenses
- Payslips or an HMRC employment record where figures need to be checked
If you had more than one job during the tax year, each employment may need to be reported separately. Directors should also check whether company benefits, reimbursed personal costs or benefits provided to family members create reporting obligations.
3. Claiming Ineligible or Unsupported Expenses
Claiming an expense does not automatically make it tax deductible. The rules depend on whether you are self-employed, employed, a landlord or receiving another type of income.
Expenses for Sole Traders and Self-Employed Individuals
Self-employed expenses must generally be incurred wholly and exclusively for the purposes of the trade. Depending on the business, eligible costs may include:
- Office, software and stationery costs
- Business premises and utility costs
- Stock and materials purchased for resale
- Business travel and vehicle costs
- Employee wages and subcontractor costs
- Professional fees, insurance and subscriptions
- Advertising and marketing expenditure
- Relevant training that updates existing business skills
Private expenditure is not allowable. Where an expense has both business and personal use, only the identifiable business proportion should normally be claimed.
Employment Expenses
Employees face stricter rules. Relief may be available for qualifying professional subscriptions, necessary business travel, uniforms, tools or additional household costs where the statutory conditions are met. Ordinary commuting and general clothing are normally not deductible.
Personal Tax Reliefs
Relief may also be available for eligible pension contributions, Gift Aid donations and certain other payments. These are not business expenses and must be entered in the correct part of the return.
Keep invoices, receipts, mileage records and calculations supporting every material claim. Unsupported or excessive expenses are among the top Self Assessment mistakes likely to attract further questions.
4. Failing to Check the Correct Tax Code
A PAYE tax code tells an employer or pension provider how much Income Tax to deduct. It is not normally entered as the basis for calculating Self Assessment, but an incorrect code can mean too much or too little tax has already been collected.
Your code may need checking after:
- Starting or leaving a job
- Receiving taxable employment benefits
- Beginning to receive a pension
- Having more than one job or pension
- A change in untaxed income
- HMRC attempting to collect an earlier underpayment through PAYE
Sole traders do not have a tax code for their self-employed profits. They use a ten-digit Unique Taxpayer Reference for Self Assessment, although they may still have a PAYE code for separate employment or pension income.
5. Not Declaring All Taxable Income
Failing to include every relevant source of income is one of the most serious mistakes of Self Assessment. HMRC receives information from many third parties and may compare it with the figures shown on your return.
Depending on your circumstances, you may need to report:
- Employment and director’s income
- Self-employment, freelance and side-hustle income
- UK property and holiday letting income
- Foreign income and gains
- Savings interest
- Dividends and other investment income
- Pension income
- Income from online selling or content creation where it amounts to trading
- Taxable state benefits
- Capital gains on property, shares, cryptocurrency or other chargeable assets
Investments in schemes such as the Enterprise Investment Scheme or Seed Enterprise Investment Scheme are not themselves income, but qualifying investments and relief claims may need to be entered correctly. Do not confuse investment subscriptions with dividends, disposals or capital gains arising from those investments.
6. Missing Supplementary Information
A Self Assessment return may require additional sections or supplementary pages according to the taxpayer’s income and circumstances. Online software usually presents relevant sections after you answer its initial questions, while paper filers may need to obtain and complete separate pages.
| Form or section | Main purpose |
|---|---|
| SA101 | Additional information, including less common income, deductions and reliefs |
| SA102 | Employment or directorship income |
| SA103S / SA103F | Short or full self-employment information |
| SA104S / SA104F | Short or full partnership information |
| SA105 | UK property income |
| SA106 | Foreign income |
| SA107 | Trust, settlement and estate income |
| SA108 | Capital gains and losses |
| SA109 | Residence and related claims |
The exact forms and questions can change between tax years. Use the return and guidance for the relevant year rather than copying information from a previous submission.
7. Making Calculation and Data-Entry Errors
Simple human error remains a frequent cause of inaccurate returns. Examples include:
- Transposing digits
- Entering gross income where net income is requested, or vice versa
- Using information from the wrong tax year
- Duplicating an income source
- Leaving a required field blank
- Selecting an incorrect answer to an eligibility question
- Failing to carry forward losses correctly
- Entering Payments on Account as business expenses
Compare the completed return with your source documents before submission. Review HMRC’s calculation, but remember that software cannot identify income or claims that were omitted entirely.
8. Submitting Self Assessment When You Do Not Need To
Not everyone with additional income must submit a return. Some liabilities can be dealt with through PAYE or another reporting process, while income may be covered by an allowance. However, you should not simply stop filing because you believe a return is no longer needed.
If HMRC has issued a notice requiring a tax return, the return normally needs to be submitted unless HMRC agrees to withdraw the notice. You may need to contact HMRC and explain why you no longer meet the criteria.
Equally, failing to register because you assume no return is necessary can create penalties where you have an undeclared liability. Check your full circumstances, including income, gains, High Income Child Benefit Charge and relief claims.
9. Missing Self Assessment Deadlines
Missing registration, filing or payment dates is one of the most expensive Self Assessment tax return mistakes. For a return covering the 2025/26 tax year, which ended on 5 April 2026, the main deadlines are:
| Date | Requirement |
|---|---|
| 5 October 2026 | Notify HMRC if you need to register for Self Assessment for the first time, subject to the applicable rules |
| 31 October 2026 | HMRC must receive a paper tax return by 11:59pm |
| 30 December 2026 | Online filing deadline if you want HMRC to consider collecting eligible tax through your PAYE code |
| 31 January 2027 | Online filing deadline and payment deadline for the balancing payment and first Payment on Account, where applicable |
| 31 July 2027 | Second Payment on Account, where applicable |
Late Filing Penalties
Under the standard Self Assessment late-filing rules, an online or paper return submitted after its deadline can lead to:
- An initial £100 fixed penalty, even where no tax is due
- Daily penalties of £10 after three months, for up to 90 days
- A further penalty after six months of 5% of the tax due or £300, whichever is greater
- Another penalty after twelve months of 5% of the tax due or £300, whichever is greater
More serious treatment can apply in some twelve-month cases. Late payment penalties and interest are separate from late-filing penalties.
Late Payment Penalties
Under the standard rules, late payment penalties may be charged at 5% of the unpaid tax at 30 days, six months and twelve months. Interest normally runs from the original payment deadline until the liability is settled. The applicable interest rate can change over time in line with Bank of England base rate movements, so it’s worth checking the current late tax payment interest rate rise if you have an outstanding balance.
Penalty rules are changing for taxpayers brought into the Making Tax Digital for Income Tax regime, so the rules applying to a particular obligation and tax year should be checked carefully.
10. Failing to Plan for the Tax Bill
Submitting an accurate return is only part of the process. A taxpayer can file on time and still face interest or penalties if the tax is paid late.
A common surprise is the first Payment on Account. Where Payments on Account apply, the amount due on 31 January can include:
- The balancing payment for the tax year just ended
- The first advance payment towards the following year’s bill
- Class 2 National Insurance or other amounts where relevant
- Any Capital Gains Tax or Student Loan amount included in the calculation
A second Payment on Account is normally due on 31 July. Filing early gives you more time to understand these amounts, reserve funds and consider whether a claim to reduce Payments on Account is justified. Reducing them without a reasonable basis can result in interest if the final liability is higher.
If you cannot pay in full, contact HMRC as early as possible. A Time to Pay arrangement may be available depending on your circumstances, but interest may continue while the debt remains outstanding.
Making Tax Digital and Future Self Assessment Reporting
Making Tax Digital for Income Tax applies from 6 April 2026 to qualifying sole traders and landlords whose total gross income from self-employment and property exceeded £50,000 in 2024/25. The threshold falls to more than £30,000 from 6 April 2027 and more than £20,000 from 6 April 2028.
Those within the regime must use compatible software to maintain digital records, send quarterly updates and complete their year-end tax reporting. Making Tax Digital can improve record visibility, but it does not remove the need to review income, expenses, allowances and the final tax calculation carefully.
How to Avoid the Top Self Assessment Mistakes
- Keep records throughout the tax year rather than collecting them in January
- Reconcile bank accounts, invoices and platform statements
- Gather P60, P45, P11D, pension, savings and investment documents
- Check every income source against the previous year’s return
- Confirm that claims and expenses meet the relevant tax rules
- Use the correct supplementary sections
- Review the tax calculation and Payments on Account
- File early enough to correct missing information
- Retain evidence supporting the return
- Seek professional advice where the position is uncertain or complex
Correcting a Self Assessment Mistake
If you discover an error after filing, do not ignore it. An online return can usually be amended within twelve months of the normal filing deadline. After that period, a different correction or disclosure process may be required.
The appropriate action depends on the tax year, the nature of the error and whether additional tax is due. A voluntary and prompt correction may reduce the risk of a higher penalty compared with waiting for HMRC to identify the issue. If you’re contacted about a correction or amendment, it’s also worth staying alert, as fraudsters sometimes exploit this exact situation our self-assessment scam warning explains how to distinguish genuine HMRC contact from a scam.
Professional Help With Self Assessment Tax Return Mistakes
At CIGMA Accounting, we help individuals, sole traders, landlords and company directors prepare tax returns, review supporting records and correct earlier submissions. If you’d like a closer look at why these Self Assessment mistakes raise HMRC eyebrows in the first place, our companion guide covers the underlying reasons and how HMRC identifies inconsistencies.
Our accountants work through offices across London, including Farringdon and Hammersmith, and support clients in nearby areas such as Smithfield, Hatton Garden, Brackenbury Village and Ravenscourt Park.
Professional support can be particularly valuable where you have several income streams, overseas matters, property income, capital gains, loss relief claims or uncertainty about whether a return is required.
Final Thoughts on Avoiding a Self Assessment Mistake
Most top Self Assessment mistakes arise from incomplete records, misunderstood tax rules or leaving the return until the deadline. Reviewing all income sources, checking available allowances, claiming only eligible expenses and planning for the final payment can make the process more manageable.
A single Self Assessment mistake can affect the tax calculation or lead to avoidable HMRC contact, but careful preparation and an early review can significantly reduce that risk.
Self Assessment Mistakes Case Study
James, a freelance IT consultant, visited our Farringdon office after receiving a letter from HMRC asking for further information about his recently submitted tax return. Although he had completed the return himself, he later realised he had overlooked dividend income from a small investment portfolio and had claimed an expense without keeping the supporting receipt.
We explained that the top Self Assessment mistakes are often caused by incomplete records, forgotten income sources or rushing to meet the filing deadline. Together, we reviewed his return, checked his employment income, self-employment records, dividend statements and allowable expenses to ensure the figures were complete and accurately supported.
During our meeting, James mentioned that he normally started preparing his return in January each year. We discussed how filing Self Assessment well before the deadline gives taxpayers time to identify missing information, understand any Payments on Account and budget for the tax due. Early preparation also makes it much easier to correct genuine errors before they become more costly or lead to unnecessary HMRC enquiries.
By the end of the meeting, James understood that avoiding the top Self Assessment mistakes is not about completing the return quickly but about maintaining accurate records throughout the year, reviewing every source of taxable income and allowing enough time to submit a complete and accurate return with confidence.
Avoid the Most Common Self Assessment Mistakes With Expert Support From Cigma Accounting in London
Making a self assessment mistake can result in HMRC enquiries, financial penalties, or unnecessary delays in processing your tax return. Cigma Accounting supports clients across the Fulham, including individuals and businesses in Fulham Palace Road and Bishop’s Park, helping taxpayers prepare accurate returns and avoid common filing errors before submission.
Whether you’re completing your first return or have filed for years, understanding common tax return mistakes can help you stay compliant with HMRC rules. Learning about the mistakes of self assessment, recognising the 10 self assessment mistakes that frequently affect taxpayers, and avoiding the top self assessment mistakes can improve the accuracy of your return and reduce the risk of costly corrections or penalties.
