Who has to use self assessment
HMRC expects you to file a self assessment return if you have income it cannot tax at source. The most common reasons are working for yourself, letting out property, being a company director who takes dividends, owing the High Income Child Benefit Charge, or having capital gains to report. Partners in a business partnership also file each year.
Gross self employed income of £1,000 or less, covered by the trading allowance, or gross rental income of £1,000 or less, covered by the property allowance, may not need reporting. Above those levels, you almost certainly need to register. Plenty of people in Fulham start a small side business, a market stall at Parsons Green or some freelance design work, and only find out about self assessment a year later.
Registering with HMRC
If you need to file for the first time, you must tell HMRC by 5/10 after the end of the tax year in which the new income started. So for income that began in the 2026/27 tax year, the registration deadline is 5/10/2027. HMRC then sends you a Unique Taxpayer Reference, a ten digit number you will use on every return.
Registration can take several weeks, and the letters arrive by post. Leaving it late puts pressure on the 31/1 filing deadline. We can register you, set up your HMRC online account and become your authorised agent, so HMRC letters and reminders also come to us.
What is included
- Registration for self assessment and agent authorisation
- Preparation and online filing of your return
- Tax calculation with payment dates
- Payments on account review
- Late return catch up and penalty appeals
- Time to Pay support
- Dealing with HMRC letters on your behalf
- Reminders before every deadline
The key self assessment dates
The tax year runs from 6/4 to 5/4. Deadlines are then set by the end of that year.
- 5/10: register for self assessment if you are new to it
- 31/10: deadline for paper tax returns
- 30/12: file online by this date if you want a small balance collected through your PAYE tax code
- 31/1: online return deadline, balancing payment for the last year and first payment on account for the current year
- 31/7: second payment on account
Payments on account explained
Payments on account are advance payments towards next year's tax bill. Each one is normally half of your previous year's self assessment bill. They catch many people out in their second year, because the payment on 31/1 can include last year's full balance plus half as much again in advance.
You do not have to make payments on account if last year's self assessment bill was under £1,000, or if more than 80% of your tax was already collected at source, for example through PAYE. If your income is falling, you can ask HMRC to reduce them. We review this every year, but we are careful, because reducing them too far leads to interest on the shortfall.
Late returns, penalties and catching up
Miss the 31/1 online deadline and HMRC charges an automatic £100 penalty, even if you owe no tax. Further penalties are added if the return is three, six and twelve months late. Interest also runs on any tax paid late, and there are separate penalties for late payment.
If you have fallen behind, the worst thing is to keep ignoring the letters. We gather what records you have, rebuild the missing figures where needed and file the outstanding returns. Where penalties were charged unfairly, for example because of illness or a bereavement, we can appeal on your behalf. If you cannot pay in one go, HMRC may agree a Time to Pay arrangement, and we can help you set one up.
Common self assessment mistakes
Small slips on a return can lead to HMRC enquiries or unexpected bills.
- Forgetting to register and assuming HMRC will send a return automatically
- Not putting money aside for payments on account
- Using the wrong tax year for income or expenses
- Filing at the last minute and paying from the wrong reference
- Missing the Making Tax Digital rules, which apply from 6/4/2026 to sole traders and landlords with qualifying income over £50,000
Getting your return done with us
Send us an enquiry and tell us what kinds of income you have. We review your situation for free and agree a fixed fee before any work starts. Have your Unique Taxpayer Reference, National Insurance number, last return if you have one, and records of income and expenses for the year ready.
We prepare a draft, go through it with you in person in Fulham or by video, and file once you approve it. You get a clear note of what to pay and when, so there are no surprises on 31/1 or 31/7. Clients in Hammersmith, Putney and West Brompton tend to send everything online and never need to visit at all.
Who this suits
- First time filers who have just started self employment or letting
- Company directors receiving dividends
- People who have missed one or more deadlines
- Anyone who received a penalty notice from HMRC
- Busy professionals in Fulham who would rather not do it themselves
Common questions
When is the self assessment deadline?
Online returns are due by 31/1 after the end of the tax year, and that is also when any tax owed must be paid. Paper returns are due earlier, by 31/10. A second payment on account, if you make them, is due by 31/7.
How do I register for self assessment?
You register online with HMRC by 5/10 after the end of the tax year in which you started receiving the income. HMRC then posts you a Unique Taxpayer Reference. We can do this for you and become your agent at the same time.
What is the penalty for a late tax return?
There is an automatic £100 penalty as soon as the online return is late, even if no tax is due. More penalties follow at three, six and twelve months, plus interest and penalties on any tax paid late.
Why is my tax bill on 31/1 so high?
Usually because of payments on account. In your second year of self assessment, the payment due on 31/1 includes the balance for the previous year and the first advance payment for the current year. It evens out after that if your income stays steady.
Can I reduce my payments on account?
Yes, if you expect your income to fall, you can apply to HMRC to reduce them. If you reduce them by too much, you pay interest on the difference, so it is worth getting the estimate right.
I have not filed for several years. What should I do?
Act now. The longer it goes on, the more penalties and interest build up. We help you gather records, file the missing returns and talk to HMRC, and coming forward yourself generally leads to better treatment.
Do I still need to file a tax return under Making Tax Digital?
Yes. Under Making Tax Digital for Income Tax you send quarterly updates and then file your tax return through the same software after the year end, still by 31/1. It applies from 6/4/2026 to those with qualifying income over £50,000.