In the UK, His Majesty’s Revenue and Customs (HMRC) is the authoritative body responsible for collecting income tax from individuals’ wages, savings, and pensions.
The HMRC Self Assessment system requires business owners, self-employed individuals, and company directors to report their total earnings for each tax year along with all income sources. Returns can be submitted online using the HMRC Self Assessment form.
It is called Self Assessment because it’s the taxpayer’s responsibility to declare their own income and ensure accuracy.
While filing Self Assessment tax returns online, individuals must disclose a wide range of information including salary, dividends, savings, rental income, and overseas income.
Those who receive directors’ income must also file a Self Assessment and report any PAYE already deducted. However, the process can be challenging and complex, especially for those new to tax filing.
There are strict rules and deadlines, and missing or misunderstanding them can lead to costly penalties.
Listed below are the most common issues individuals might face when filing their HMRC Self Assessment tax returns online.
You can also read our detailed Self Assessment Tax Mistakes Guide for more filing tips and solutions.
1. Missing the Deadline
It is essential to follow every HMRC deadline and ensure that all information is submitted on time.
- The online filing deadline for the 2023–2024 tax year is midnight on 31 January 2025.
- Paper returns are due by 31 October 2024.
Penalties start at £100 even for being one day late, and additional fines apply for further delays.
Always track important dates such as the end of the tax year, Self Assessment registration deadlines, and balancing payment due dates.
2. Not Registering with HMRC
To file online, you must first register for Self Assessment. Many business owners don’t realise that registration can take time.
HMRC sends your activation code (PIN) by post, and it usually takes 10–14 working days. Without registration, online filing isn’t possible.
Always register well before the deadline to avoid late penalties.
3. Making Simple Errors
It may sound trivial, but many Self Assessment rejections happen due to small oversights such as forgetting to sign, date, or double-check entries.
Review each section carefully before submitting to ensure all fields are complete and accurate.
4. Skipping the Details
HMRC expects a full and transparent picture of your income and expenses. Provide exact figures wherever possible avoid rounding or estimates.
If you miss details, HMRC may query your return, causing unnecessary delays or corrections later.
5. Getting the Sums Wrong
Double-check your math. Errors in totals or calculations are among the most common reasons for amended returns.
If you’re unsure, use accounting software or a professional service to confirm your figures. Accuracy here directly affects your final tax due.
6. Mistaking Important Details
Your National Insurance (NI) number and Unique Taxpayer Reference (UTR) must be entered correctly.
Any mistake here can make your entire return invalid.
If you’ve misplaced your UTR, you can retrieve it from your Personal Tax Account or previous HMRC letters.
7. Making Incorrect Expense Claims
Claiming expenses correctly is key. HMRC only allows costs that are wholly and exclusively for business use.
Claiming personal items like holidays or home entertainment can trigger scrutiny.
At the same time, failing to claim legitimate expenses means paying more tax than necessary.
8. Not Filling In All the Details
The Self Assessment form is lengthy, and it’s easy to miss boxes but every detail matters.
Take time to fill every relevant section, and use the ‘Additional Information’ box to clarify anything unusual.
This helps HMRC understand your situation and can sometimes result in tax reliefs or adjustments in your favour.
9. Not Being Prepared
Filing a Self Assessment isn’t a one-day task. It requires thorough preparation and record-keeping.
Keep all receipts, invoices, mileage logs, and income records for at least five years after the filing deadline (for the self-employed).
HMRC may request these as evidence later.
Being organised ensures accuracy and reduces the stress of last-minute filing.
If You’ve Made a Mistake
If you realise you’ve made an error after submission, HMRC usually allows up to 12 months from the original filing deadline to amend your return.
However, if you’re unsure how to correct it, it’s best to consult a professional.
At Outbooks, we specialise in Self Assessment Tax Return Outsourcing Services helping you correct, file, or optimise your returns accurately.
Come have a chat with us, and we’ll review your financials to offer better solutions that improve compliance and accuracy.
Let us handle the HMRC, while you focus on growing your business.
We aim to save your time, reduce your stress, and ensure that your tax submissions are always right the first time.
FAQs
1. What are the most common issues when filing Self Assessment tax returns online?
Common issues include missing the HMRC deadline, incorrect UTR or NI details, not registering in time, calculation errors, and skipping required information fields.
2. What happens if I file my Self Assessment tax return late?
HMRC charges an automatic £100 penalty for late filing, with additional daily fines and interest if your return or payment remains outstanding beyond 3 months.
3. How can I avoid mistakes when filing Self Assessment online?
Start early, register with HMRC well in advance, double-check your figures, and consider outsourcing your Self Assessment filing to professionals like Outbooks to ensure accuracy and compliance.
Parul is a dedicated writer and expert in the accounting industry, known for her insightful and well researched content. Her writing covers a wide range of topics, including tax regulations, financial reporting standards, and best practices for compliance. She is committed to producing content that not only informs but also empowers readers to make informed decisions.