Self Assessment is the system HMRC uses to collect tax on income that isn’t taxed at source. If you’re self-employed, a company director, a landlord, or you have income that doesn’t pass through PAYE, this is how you report it and pay what you owe.
It affects more people than you’d think. Around 12 million people file Self Assessment returns each year, and over a million miss the deadline – triggering automatic penalties that start at £100 and escalate quickly.
The system is also changing. From April 2026, Making Tax Digital for Income Tax has begun replacing the traditional annual return for sole traders and landlords earning over £50,000, introducing quarterly digital reporting instead. For everyone else, the existing Self Assessment process continues as normal.
Here’s who needs to file, when the deadlines fall, and how to avoid the mistakes that cost people money every year.
Who Needs to File?
Self Assessment isn’t just for the self-employed. The list of people required to file is broader than most expect.
The Common Reasons
You’ll need to file a return if any of the following apply:
- Self-employed sole traders: if your gross self-employment income exceeds the £1,000 trading allowance
- Business partners: each partner files their own return to report their share of the partnership’s profits
- Company directors: not automatic, but most directors need to file if they receive dividends or have other untaxed income not fully dealt with through PAYE
- Landlords: the first £1,000 of property income is covered by the property income allowance. If your rental income is between £1,000 and £2,500, contact HMRC. A Self Assessment return is required if it exceeds £2,500 after allowable expenses or £10,000 before allowable expenses.
- Untaxed income over £2,500: from any source, including savings interest or investment returns
- Investment income: if you receive £10,000 or more from untaxed savings, investments, or dividends
- Capital gains: if you’ve disposed of assets and owe CGT (separate from the 60-day property reporting rule)
- Foreign income: any overseas income taxable in the UK must be declared
What About High Earners on PAYE?
This is one that’s changed recently. It used to be that anyone earning over £100,000 had to file. That’s no longer the case.
From the 2024/25 tax year onwards, HMRC has removed the income threshold for PAYE-only taxpayers entirely. If all your income is taxed through PAYE and you have no other filing triggers – no dividends, no rental income, no untaxed savings interest, no other complications – you don’t need to file a Self Assessment return, regardless of how much you earn. HMRC will use its own systems (including Simple Assessment) to collect any additional tax, such as reclaiming the personal allowance taper.
However, the moment you have any additional income or trigger, you’re back in Self Assessment. And in practice, most people earning over £100,000 do have at least one trigger – dividends from shareholdings, savings interest above the £500 higher rate allowance, or rental income.
If you’re unsure, HMRC’s check if you need to file tool is a good starting point.
The Less Obvious Reasons
These catch people out more often than the main categories:
- High Income Child Benefit Charge: if you or your partner receive Child Benefit and either of you has adjusted net income over £60,000, you must file to repay some of the benefit. It’s fully clawed back at £80,000.
- Pension tax relief: if you’re a higher or additional rate taxpayer and want to claim the full relief on pension contributions (your provider only claims basic rate automatically)
- Trustees: if you’re a trustee of a trust or registered pension scheme
- Trust income: if you receive income from a trust or settlement
- Specific tax reliefs: claiming EIS relief, Venture Capital Trust relief, or similar schemes requires a return
Key Deadlines
Self Assessment deadlines follow a fixed annual cycle. For the 2025/26 tax year (the return you’ll file during 2026/27), the dates are:
- 5 October 2026: deadline to register for Self Assessment if you’re filing for the first time
- 31 October 2026: deadline for paper returns
- 30 December 2026: deadline to file online if you want HMRC to collect underpaid tax (up to £3,000) through your PAYE code
- 31 January 2027: deadline for online returns and for paying any tax owed for 2025/26. Also the deadline for your first payment on account for 2026/27.
- 31 July 2027: second payment on account for 2026/27
The 31 January deadline is the one that catches the most people. It’s both a filing and a payment deadline, and missing it triggers an automatic £100 penalty for late filing plus interest on any unpaid tax.
Payments on Account
If your Self Assessment tax bill exceeds £1,000, and less than 80% of your total tax was collected at source (through PAYE), HMRC will require payments on account – advance payments towards next year’s bill.
Each payment is half of your previous year’s Self Assessment liability. They’re due on 31 January and 31 July.
This catches new filers off guard every year. In your second year of filing, you could face your full tax bill for the year just ended plus half of next year’s estimated bill – all on the same 31 January deadline. That’s potentially 150% of what you were expecting.
If your income has dropped and you expect next year’s bill to be lower, you can apply to reduce your payments on account. But be cautious – if you reduce them too much and it turns out you owed more, HMRC will charge interest on the shortfall.
What Happens If You’re Late
Late Filing Penalties
The penalty structure escalates the longer you leave it:
- 1 day late: automatic £100 penalty (even if you owe no tax)
- 3 months late: £10 per day for up to 90 days (maximum £900), on top of the initial £100
- 6 months late: a further £300 or 5% of the tax due, whichever is higher
- 12 months late: another £300 or 5% of the tax due, whichever is higher
A return that’s over 12 months late can attract penalties of £1,600 or more before interest is added. In serious cases involving deliberate withholding of information, penalties can reach 100% of the tax owed.
Late Payment Penalties
Separate from the filing penalties, HMRC charges penalties for late payment of tax:
- 30 days late: 5% of the unpaid tax
- 6 months late: a further 5%
- 12 months late: a further 5%
Interest also accrues daily on unpaid tax from 1 February (the day after the deadline) until the balance is cleared. The current rate is the Bank of England base rate plus 4%.
Filing and payment penalties stack. You can be hit with both at the same time, and neither cancels out the other.
Making Tax Digital: What’s Changing
From April 2026, sole traders and landlords with qualifying income over £50,000 are required to use Making Tax Digital for Income Tax instead of the traditional Self Assessment process. The threshold drops to £30,000 from April 2027, and to £20,000 from April 2028.
In practice, this means:
- Digital record-keeping: using MTD-compatible software (spreadsheets alone won’t work unless linked to bridging software)
- Quarterly updates: submitting summary totals of income and expenses to HMRC every three months
- Final declaration: replacing the traditional Self Assessment return, due by 31 January as usual
MTD also introduces a new points-based penalty system. Instead of an immediate £100 fine for a late submission, you accumulate penalty points – one for each missed quarterly deadline. Once you reach the threshold (four points for annual filers), a £200 penalty is triggered for that and each subsequent late submission. Points expire after 24 months of full compliance.
For sole traders and landlords not yet within MTD scope (income under £50,000 for now), the existing Self Assessment process and penalty regime continue unchanged.
Our article on Making Tax Digital for ITSA covers the new system in full detail.
Common Mistakes That Cost Money
Self Assessment errors are more common than most people realise, and they tend to fall into the same categories year after year.
Not Claiming All Allowable Expenses
This is the biggest one. Many sole traders and landlords pay more tax than they need to because they don’t track expenses properly or don’t know what’s deductible. Allowable expenses must be “wholly and exclusively” for business purposes, but the range is wider than most people think – from home office costs and professional subscriptions to mileage, software, and training.
Forgetting About Payments on Account
The January bill for a second-year filer can be a brutal surprise. Budget for it from the start – setting aside 25–30% of your profits throughout the year is a good rule of thumb.
Missing the Registration Deadline
If you need to file for the first time, you must register by 5 October after the end of the relevant tax year. Miss this and you risk a “failure to notify” penalty on top of any filing penalties.
Not Reporting All Income
HMRC cross-references Self Assessment returns against data from banks, employers, HMRC’s own records, and information from overseas tax authorities. If you forget to include a source of income – savings interest, a freelance payment, foreign earnings – HMRC will likely spot it. Voluntary disclosure before HMRC contacts you always results in a lower penalty than being caught.
Filing But Not Paying
Filing your return on time avoids the filing penalty, but it doesn’t stop the payment penalty. If you can’t pay the full amount by 31 January, file the return anyway and contact HMRC to set up a Time to Pay arrangement. This lets you spread the bill over monthly instalments and shows HMRC you’re engaging rather than ignoring the problem.
How to Prepare
Filing doesn’t have to be a last-minute scramble. A few habits throughout the year make the process far smoother:
- Track income and expenses as they happen. Don’t wait until January to open a carrier bag of receipts. Use accounting software or a simple spreadsheet and update it regularly.
- Reconcile against bank statements. Once a quarter, check that your records match what’s actually gone through your bank account. This catches missed transactions early.
- Keep all supporting documents. Invoices, receipts, bank statements, P60s, dividend vouchers, and any HMRC correspondence. You’ll need them if HMRC asks questions, and they make filling out the return much faster.
- Know your deadlines. Put 31 January and 31 July in your diary with reminders well in advance. If you use an accountant, agree a timeline for providing your information so they’re not working on your return at midnight on 30 January.
- Review the return before submitting. Check the figures twice. A simple transposition error can lead to HMRC queries, delays, or overpayment of tax.
How Double Point Can Help
Self Assessment can be straightforward if your affairs are simple. But for most people – particularly those with multiple income sources, rental properties, or a mix of employment and self-employment – the interactions between different types of income, reliefs, and deadlines make professional support worthwhile.
At Double Point, we handle Self Assessment returns for individuals, sole traders, landlords, and company directors. We make sure all allowable expenses are claimed, all income is reported correctly, and everything is filed well before the deadline. If you’re within scope for MTD, we can set you up with compatible software and manage the quarterly submissions on your behalf.
We also advise on the wider tax planning questions that Self Assessment raises – whether to adjust payments on account, how to structure income across a tax year, and when it makes sense to bring forward or defer certain transactions.
Book a free consultation with us today and let’s make sure your Self Assessment is handled properly.