TaxCalc Blog

Your Self Assessment Survival Guide: 12 common mistakes to avoid in 2025

Completing your own Self Assessment tax return is absolutely doable but, without the help of an accountant, it’s easy to fall into traps that could cost you time, money or even trigger unwanted attention from HMRC.

Whether you’re a freelancer, landlord, side hustler or someone with additional income or gains outside of PAYE, here are 12 of the most common mistakes people make when filing their Self Assessment tax return and how you can avoid them in 2025.

Reminder: The online filing deadline for the 2024/25 tax year (which runs 6 April 2024 to 5 April 2025) is 31 January 2026.

1. Missing the filing and payment deadline

Over a million taxpayers missed the January 31^st filing deadline last year and were immediately hit with a £100 late filing penalty, even if they owed no tax. Late payment charges and daily fines can follow shortly thereafter.

Avoid it: Note the deadline in your diary, set reminders - and don’t leave it until the last minute when HMRC systems are busiest. The tax return often takes longer to complete than you think.

2. Not registering in time or waiting too late to get your UTR

If you’re new to Self Assessment (i.e. you started freelancing or had taxable rental income for the first time), you must register with HMRC. It can take up to 10 working days to receive your Unique Taxpayer Reference (UTR) number and without it you cannot file your Self Assessment tax return.

Avoid it: Register for Self Assessment by 5 October following the end of the tax year in which you started receiving untaxed income. If you haven’t registered yet, do it now!

3. Forgetting income - especially side hustles, cryptoassets or temporary employments

It’s not just self-employment or business income that needs reporting. HMRC expects you to include all taxable income for the year, such as:

• Freelance or side-gig income (e.g. Etsy, Upwork, Deliveroo, tutoring)
• Rental income (including Airbnb)
• Interest, dividends and any other investment income
• Cryptoasset gains and staking income
• Capital gains from selling shares, property or digital assets
• Part-time or temporary employment

Avoid it: Review all income sources early, download platform statements, check bank statements, and don’t assume HMRC “already know”.

4. Claiming the wrong expenses or missing valuable reliefs

Some people underclaim because they’re unsure what’s allowed. Others overclaim and risk penalties by including personal or non-business costs.

Commonly missed reliefs include:
 - Business use of your home
 - Mileage allowance for business journeys
 - Pension contributions
 - Gift Aid donations
 - Marriage Allowance transfer (if eligible)
 - Trading and property allowance (if income is under £1,000)

Avoid it: Only claim genuine business or qualifying expenses. Check HMRC guidance or use software with built-in prompts to ensure you’re claiming correctly.

5. Overlooking capital gains or thinking HMRC won’t notice

Selling a second property, shares or cryptoassets may create a liability even if your gain is small. The CGT annual exempt amount for 2024/25 was only £3,000, meaning more people now fall within the reporting requirements.

Avoid it: Keep records of any investment activities and, if you're unsure, seek advice from an accountant or tax adviser.

6. Failing to set aside money for tax (and being surprised by payments on account)

If this is your first year with significant self-employed, freelance or rental income, you may be caught out when HMRC asks for payments on account - advance payments towards next year’s tax bill, due in January and July each year.

Avoid it: Estimate your bill during the year and set aside funds regularly. Don’t let your tax bill catch you unprepared after spending the income.

7. Not keeping proper digital records (and falling behind ahead of Making Tax Digital)

From April 2026, many self-employed individuals will have to comply with Making Tax Digital for Income Tax (MTD ITSA), meaning digital records and quarterly submissions.

Avoid it: Start keeping organised digital records now, either using approved software or a simple bookkeeping spreadsheet.

8. Relying on estimates and memory

Trying to reconstruct your tax year from random receipts, bank transactions or guesswork increases the risk of errors and missed claims.

Avoid it: Maintain organised records throughout the year. Store receipts digitally, keep spreadsheets or use bookkeeping software.

9. Typing errors or entering figures in the wrong sections

Common errors include entering gross income instead of net, misreporting tax deducted at source, or putting employment income under self-employment by accident. Even ticking (or not ticking) the wrong box can trigger miscalculations.

Avoid it: Double-check every entry. Use HMRC-recognised tax software that validates figures, performs automatic calculations and flags inconsistencies.

10. Ignoring your PAYE tax code or assuming employment income is “already sorted”

If you're employed as well as self-employed, your PAYE code may have already factored in benefits, prior underpayments or adjustments. If you don’t understand your tax code, you may overpay or underpay without realising.

Avoid it: Review your P60s, P45s, P11Ds and tax codes before filing. Make sure all income lines up.

11. Falling for tax scams, phishing emails and fake 'HMRC refunds'

Filing season is peak time for scams. Fraudsters mimic HMRC to request payments or promise refunds. Responding could lead to identity theft or bank fraud.

Avoid it: HMRC will never text or email you demanding immediate payment or sending clickable refund links. Always access your account via GOV.UK directly or using the HMRC app.

12. Not reviewing or correcting your return when something changes

You can amend a submitted return for up to 12 months after the filing deadline. Many forget this window exists or panic after spotting a mistake too late.

Avoid it: Review your return again after filing. If something changes - such as finding an expense you missed - amend it sooner rather than later.

Final thoughts: filing smarter, not harder

Filing your Self Assessment doesn’t have to be stressful. The key is staying organised, using reliable tools and allowing yourself enough time to get it right.

• Start early
• Gather records as you go
• Use trusted tax return software
• Review before submitting
• Correct mistakes promptly

With the right approach, you can stay on the right side of HMRC, avoid penalties and feel confident your return reflects your true tax position.