5 Common Mistakes in Self‑Assessment Filing—and How to Avoid Them

Navigating the UK Self Assessment process can be challenging, especially for landlords, independent contractors, and individuals with multiple sources of income. Unfortunately, making common filing errors for the Self Assessment can lead to severe penalties, HMRC investigations, or delays. Let’s examine the most common mistakes made by filers and, most importantly, how to prevent them if you want to keep your tax affairs organized for 2025.

1. Missing the Self Assessment Deadline

Missing the deadline is one of the most costly mistakes in self-assessment filing. Whether or not you owe taxes, you may be subject to automatic penalties for missing the self-assessment deadline.

What are the deadlines?

• Paper tax return: October 31st, following the fiscal year’s conclusion.
• Online return deadline: January 31st, following the tax year’s conclusion.
• Tax due date: January 31st.

Penalties for missing deadlines:

• Even if you have no taxes to pay, a £100 fixed penalty applies for being even one day late.
• A fine of £10 per day (up to £900) after three months.
• An additional £300 or 5% of the original amount, whichever is higher, is owed after six months.
• Another £300 or 5% after a year.

How to avoid it:

Avoid last-minute filing by setting up automated reminders and recording deadlines in multiple calendars. If you’re feeling overburdened, consider seeking professional assistance, such as QuickTaxReturns Self Assessment services, which ensure that deadlines are never missed.

2. Incorrectly Declaring Income

Ignoring or underreporting taxable income is a serious HMRC Self Assessment mistakes. People who work for themselves, have investments, rental or property income, or work several part-time jobs are most likely to make this mistake.

What are the Common self-assessment errors here?

• Failing to disclose freelance or gig income.
• Failing to report earnings from investments or overseas sources.
• Failing to disclose capital gains or underreporting real estate income.

Penalties: 

Penalties for inaccurate declarations range from 0% of the tax owed (for factual errors) to 100% of the tax owed (for intentional or concealed errors).

How to prevent making false income declarations:

• Verify each payslip, invoice, and bank statement twice to ensure accuracy.
• Don’t rely on memory; ensure that you include all sources and forms of income.
• When in doubt, use reliable software or speak with a tax advisor.

3. Mishandling Allowable Expenses

One standard allowable expense Self Assessment error is making an incorrect claim, either by overclaiming or underclaiming. Many filers underreport, and some make the mistake of claiming expenses that are not allowed, which puts them at risk of an HMRC challenge.

What counts as allowable business expenses?

• Equipment and supplies for offices.
• Travel for work (not including commuting).
• Accounting, professional fees, and insurance.
• Subscriptions, marketing, and web expenses.
• A percentage for working from home.

Ways to prevent this error:

• Only “wholly and exclusively” business-related expenses should be claimed.
• Maintain thorough records and invoices for a minimum of six years.
• If in doubt, take the precautionary route or consult QuickTaxReturns Self Assessment services for guidance.

4. Capital Gains Declaration Errors

Capital gains declaration errors are easy to make, particularly after selling shares, valuable personal belongings, or real estate. These errors can result in investigations or additional tax charges and range from failing to report gains to entering information in the incorrect section.

Key pitfalls:

• Failing to disclose sales of assets or real estate.
• Utilizing an incorrect annual exempt amount (for example, £3,000 for 2024–2025).
• Filling out the incorrect section of the gains form (wrong supplemental pages).

How do I properly include capital gains on my tax return?

• List every asset that was sold, along with the accurate date of sale.
• For the tax year, use the appropriate CGT allowance.
• If in doubt, have a tax professional or consult official HMRC guidelines when reviewing your return.

5. Administrative Errors: UTR/NI Numbers and Box-Ticking

Simple mistakes, such as checking the wrong boxes or entering the wrong UTR (Unique Taxpayer Reference), can result in delays, inaccurate calculations, and inquiries from HMRC.

• UTR number errors: Your tax payment may be rejected or misattributed if your UTR is incorrect or missing.
• Ticking the wrong boxes on a tax return: Penalties or investigations may result from inaccurate declarations or missing checks (such as claiming a relief when ineligible).
• UK Self Assessment supplementary pages: One common mistake is not filing the necessary supplementary pages to declare certain types of income (such as CGT, partnerships, or self-employment).

How to avoid these mistakes:

• Verify all personal data and UTR/NI entries again.
• Carefully read the form’s instructions, then reread them before submitt
• Examine the supplemental pages that pertain to your particular circumstance.
• Use verified accounting software or have a QuickTaxReturns tax expert review your return.

FAQs

1. What are the penalties for missing the Self-Assessment deadline?

£100 in penalties for missing the deadline, which increases to daily fines, additional fixed fines, and interest if the deadline is missed by more than three, six, or twelve months.

2. How do I avoid incorrectly declaring income?

Make use of a comprehensive list of all potential sources of income, keep thorough records, and ask for help when you’re unsure.

3. What counts as allowable business expenses?

Any expenditure that is “wholly and exclusively” related to your business, including marketing, insurance, professional fees, office expenses, and travel. All costs, whether for personal or professional purposes, must be proportionately calculated.

4. How can I correctly report capital gains in my return?

Always report disposals, use the appropriate CGT allowance for the tax year, check the requirements for supplementary pages, and declare within 60 days for residential property.

5. What happens if I tick the wrong boxes or use incorrect UTR/NI numbers?

Your tax may be incorrectly applied, your return may be delayed or questioned, and HMRC may launch an investigation. To prevent these problems, take your time, review your work before submitting, and consider seeking professional assistance.

The Role of QuickTaxReturns Self Assessment Services

When income streams, expenses, or investments become complex, professional advice can save time, money, and stress. In simple cases, DIY tax returns may be sufficient. The services offered by QuickTaxReturns Self Assessment are designed to:

• By warning you before the missing self-assessment deadline, you can stay on track.
• Before they occur, mark all possible errors in the self-assessment filing.
• Utilize expert software and human oversight to identify and correct Common Self Assessment errors.
• Assist you in accurately declaring all of your income sources, permitted expenses, and gains so that you never overpay or run the risk of fines.

Our knowledgeable staff takes care of everything, from compiling documentation and creating forms to communicating directly with HMRC on your behalf, enabling you to file with confidence every year. With the assurance that your tax return is correct, in compliance, and filed on time, let QuickTax Returns handle the hassle of self-assessment so you can concentrate on your strengths.