If you’re required to submit a self-assessment tax return, you’ll know it can be a bit of a minefield—especially if you’re juggling business, family, and everything in between. Every year, thousands of people make avoidable mistakes that lead to penalties, overpayments, or unnecessary stress. In this blog, we’ll walk you through the most common self-assessment errors—and how to make sure they don’t happen to you.
1. Missing the Deadline
The deadline for online self-assessment submissions is 31st January each year. HMRC is strict—miss it by even a day, and you’ll face an automatic £100 fine.
How to avoid it:
Set a reminder in your calendar for early January (or better yet, December).
Aim to file well before the deadline—you can submit any time from 6th April.
Work with your accountant early to avoid the last-minute rush.
2. Not Registering in Time
If you're new to self-assessment (e.g. newly self-employed), you must register with HMRC by 5th October following the end of the tax year.
How to avoid it:
As soon as you start earning untaxed income, check whether self-assessment applies to you.
Register online with HMRC as soon as possible to receive your UTR (Unique Taxpayer Reference).
3. Incorrect or Missing Information
Typos, missed income, or claiming incorrect reliefs can trigger HMRC queries—or worse, penalties.
How to avoid it:
Keep detailed records of income, expenses, pensions, savings interest, and dividends.
Double-check figures before submission, or ask your accountant to review your return.
4. Claiming Ineligible Expenses
Claiming for personal expenses (like meals, clothing, or holidays) under business costs is a common error.
How to avoid it:
Only claim expenses that are “wholly and exclusively” for business use.
If something is partially for business (e.g. phone bills), only claim the business portion.
5. Forgetting Additional Income
Self-assessment isn’t just for the self-employed. You must declare:
Rental income
Dividends or interest on savings
Income from side hustles or online sales
Overseas income
How to avoid it:
Review all sources of income, not just your main job or freelance work.
If unsure, speak to your accountant—it’s better to declare too much than miss something.
6. Paying the Wrong Amount of Tax
Incorrect figures can lead to overpayment or underpayment—either way, you lose out.
How to avoid it:
Use HMRC’s online calculation tools or, ideally, let your accountant calculate your tax.
Understand Payments on Account—many are surprised by these and underpay.
7. Not Keeping Proper Records
HMRC requires you to keep records for at least 5 years after the 31 January submission deadline.
How to avoid it:
Use digital accounting software to keep everything in one place.
Keep receipts, invoices, and bank statements organised and backed up.
Final Thoughts
Self-assessment doesn’t have to be stressful. By avoiding these common mistakes and staying organised, you’ll save time, money, and hassle. And remember—if in doubt, ask an expert. An experienced accountant can make sure your return is correct, compliant, and often more tax-efficient than if you go it alone.
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