Self Assessment is one of those things almost no one looks forward to, and it’s one of the easiest places to pay more tax than you actually owe. Not through cheating the system. Just through small mistakes and missed claims that quietly add up.
Here are the most common Self Assessment tax return mistakes we see, in roughly the order they cost people money.
1. Filing late
The Self Assessment deadline is 31 January for online returns covering the previous tax year (so the 2025/26 tax year is due by 31 January 2027). The penalty for missing it starts at £100, even if you owe no tax. Then it climbs.
Worse, late filing brings HMRC’s attention, and that interest tends to last. The simplest fix is the most boring: don’t leave it until January. The people who file in summer or autumn pay less, sleep more, and never miss the deadline.
2. Forgetting payments on account
This one catches new self-employed people out every year. Your Self Assessment bill isn’t just the tax for the year you’ve just had. If your bill is over £1,000, HMRC asks you to pay half of next year’s expected tax in advance, split into two payments: one on 31 January with your main bill, and one on 31 July.
So your first “normal” year of Self Assessment can feel brutal: you pay last year’s tax plus half of next year’s, all at once. Build it into your budgeting from year one, or it will absolutely surprise you.
3. Not claiming everything you’re entitled to
This is the single biggest reason people overpay. Self-employed expenses can be claimed against your income to reduce the tax you owe, but only if you actually claim them.
Commonly missed claims include:
- Home office use. If you work from home, a proportion of your bills (electricity, internet, even mortgage interest in some cases) can be claimed.
- Use of your car. Either through HMRC’s simplified mileage rate or by claiming actual costs.
- Subscriptions and professional memberships relevant to your work.
- Training costs that maintain or update your existing skills.
- Equipment and software used for the business.
- Bank charges and interest on business accounts and borrowing.
- Pension contributions, which reduce your taxable income.
Each one might be small. Together they often save hundreds of pounds. The trick is keeping a record through the year, not trying to remember in January.
4. Mixing personal and business expenses
HMRC’s general rule on expenses is that they must be “wholly and exclusively” for business purposes. Mixing personal and business spending is one of the fastest ways to invalidate a claim or invite a question during a tax investigation.
The practical fix is the simplest one: separate bank accounts. A dedicated business account, used only for business income and expenses, makes everything cleaner and far easier to defend if HMRC ever asks.
5. Forgetting other sources of income
Self Assessment is for declaring all your income, not just self-employed earnings. Things people regularly forget:
- Rental income from any property you let out.
- Dividends from limited companies you own.
- Interest above the personal savings allowance.
- Foreign income (yes, even small amounts).
- Capital gains from selling assets above the annual allowance.
- Pensions drawn down, including state pension.
HMRC sees most of this anyway, so missing it on your return isn’t a way to hide it. It’s just a way to attract questions.
6. Not budgeting for the tax bill
This isn’t a Self Assessment mistake exactly, but it’s the one that hurts most: people who earn well through the year, spend everything, and then can’t pay the tax bill in January.
The fix is a separate account and a simple habit. Move 25-30% of every payment you receive into a tax savings account the moment it lands. The exact percentage depends on your situation (and a good accountant can help you work out the right number), but the principle is universal: tax money is not your money.
7. Doing it alone when you shouldn’t
HMRC’s online system is genuinely usable, and plenty of people with straightforward affairs file their own returns happily. But there’s a tipping point where doing it yourself starts to cost more than it saves.
If you’re juggling multiple income sources, rental properties, capital gains, foreign earnings, or anything where the rules get complicated, the cost of an accountant is usually substantially less than the tax they’ll save you, plus the time and stress.
And if you’re self-employed and growing, the conversation with a good accountant before the year ends is worth far more than the return-filing service itself.
Want your Self Assessment sorted properly this year? Send us a message and we’ll take a look at your situation. The earlier we start, the more we can save you.
