by Asim Abbas
12 November 2026 
Not told HMRC about some income? What to do, and why acting first matters
Not told HMRC about some income? What to do, and why acting first matters
Side income from renting a room, selling online, freelancing or selling shares is easy to forget, until HMRC asks about it. The good news is that telling HMRC first usually means a better outcome.
- Tell HMRC as soon as possible about taxable income you have not declared.
- Already in Self Assessment? Correct your return. Not registered? Register and file a return for each year.
- Rental income, capital gains and self-employment, including regular buying and selling, all count.
- Other taxes may apply: Corporation Tax, VAT over £90,000 turnover, or PAYE if you employ people.
- Overseas income can carry penalties of up to double the tax.
GOV.UK says that contacting HMRC first may mean your case is considered more favourably.
This guide explains what counts as undeclared income, how to put it right and which route fits your situation.
Act as soon as possible
According to GOV.UK, you should tell HMRC as soon as possible if you have earned taxable money you have not declared, and contacting HMRC first may mean your case is considered more favourably.
What income needs declaring?
- rental income, including renting out a room or a whole property
- capital gains, for example from selling property, valuable items or shares
- self-employment, including regularly buying and selling items, which also means registering as self-employed
- foreign income, if you are UK resident
If this income takes your total over your Personal Allowance, you will owe tax on it.
How to put it right
If you already file Self Assessment
Correct your tax return. Returns can normally be amended within 12 months of the filing deadline; older years usually need a disclosure.
If you do not file Self Assessment
Register for Self Assessment and file a separate return for each year. GOV.UK says registering lets you declare untaxed income from the last four years.
Larger or older amounts
For more years, careless or deliberate errors, or several types of income, a formal disclosure is usually better. See our guide to choosing the right disclosure route.
Estimate your disclosure
Our free HMRC disclosure calculator works out tax, interest to today and penalties for each year, with PDF or CSV download.
Other taxes to check
- Corporation Tax, if you run a limited company
- VAT, if your taxable turnover went over the £90,000 registration threshold
- PAYE, if you employ people
Fixed-fee accountancy support
Transparent monthly packages for sole traders, landlords and limited companies. Fixed fees agreed upfront, VAT included.
Get a fixed-fee quoteCall 0161 531 4179Overseas income
If you are UK resident and have not declared foreign income, you may owe the unpaid tax plus a penalty of up to double the tax, and giving false information can lead to prosecution. Read our guide to foreign income and UK tax and the Worldwide Disclosure Facility.
Specific situations
- landlords: the Let Property Campaign
- crypto: our crypto disclosure guide
- a property sale or gain you never reported: unreported gains
- a letter from HMRC: what to do if HMRC has contacted you
Voluntary disclosure help near you
We help clients make disclosures across the UK, including London, Manchester, Birmingham, Leeds, Liverpool, Bristol, Newcastle, Cardiff and Wales, and in Scotland from Glasgow to Edinburgh. Start with our voluntary tax disclosure service.
What should I do next?
Each income source and each year.
For Self Assessment or as self-employed.
Return amendment, late returns or a disclosure.
We will look into your tax affairs and sort this out.
Frequently Asked Questions
Will HMRC go easier if I tell them first?
GOV.UK says contacting HMRC first may mean your case is considered more favourably, and penalties are usually lower.
How many years can I declare by registering for Self Assessment?
GOV.UK says registering lets you declare untaxed income from the last four years.
Does selling things online count as income?
Regularly buying and selling items can be self-employment, which needs registering and declaring.
Do I need to register for VAT?
If your taxable turnover goes over £90,000 in a rolling 12 months, you must register.
What if the income is from abroad?
UK residents normally pay UK tax on foreign income. Penalties for undeclared overseas income can be up to double the tax.
Need help with this? Wingate Accountants offers fixed-fee online accountancy support across the UK. Call 0161 531 4179 or request a quote at wingateaccountants.co.uk.
About the Author
Asim Abbas
I’m Asim Abbas, founder of Wingate Accountants and a Chartered Certified Accountant. I help sole traders, landlords and directors stay on top of their tax with fixed fees and plain-English advice.
