Important: This article provides general information about Capital Gains Tax in the UK. It is not financial or legal advice. Tax rules can change, and individual circumstances vary. Always consult a qualified tax professional or accountant for advice specific to your situation.
Capital gains tax (CGT) applies when you sell or dispose of an asset that has increased in value. It’s the gain you’ve made that’s taxed, not the total amount you receive. Many people wonder if they can avoid declaring these gains and whether HMRC will find out.
Having helped thousands of property sellers over the past 18 years—many of whom were navigating CGT obligations for the first time—we’ve seen first-hand how HMRC’s detection capabilities have transformed. This article explains the risks and realities of not declaring CGT, drawing on both official HMRC guidance and patterns we’ve observed from working with clients in these situations.
What counts as a capital gain?
You might need to pay CGT when you sell:
- Shares that aren’t in an ISA or PEP
- Most personal possessions worth £6,000 or more (apart from your car)
- Your main home if you’ve let it out, used it for business, or it’s very large
- Business assets
You don’t usually need to pay tax on gifts to your spouse, civil partner, or a charity. There’s also an annual tax-free allowance (the Annual Exempt Amount) which for the 2024/25 and 2025/26 tax years is £3,000. This is a significant reduction from £12,300 just two years ago, meaning more people than ever are now liable for CGT on relatively modest gains.
How HMRC tracks capital gains
HMRC has sophisticated systems to detect undeclared gains. According to HMRC’s own published guidance, they use:
- Land Registry data – Every property transaction is recorded and cross-referenced
- Information from banks and building societies – Large deposits trigger automatic reports
- Online marketplace data – Sales on platforms like eBay are monitored
- Social media activity – Lifestyle indicators that don’t match declared income
They can spot discrepancies between your declared income and your lifestyle. If you suddenly buy an expensive car or go on luxury holidays without a clear source of income, it might trigger an investigation.
What we’ve seen: A client came to us last year wanting to sell a buy-to-let property they’d inherited. They’d assumed that because inheritance tax had been paid on the estate, no further tax was due. In fact, CGT applied to the gain between the probate value and the sale price. Because we flagged this early, they were able to budget correctly and avoid an unexpected bill—or worse, an HMRC investigation.
What are the risks of not declaring?
Failing to declare capital gains is illegal. According to HMRC’s guidance on penalties for inaccuracies (CH82400), if caught you could face:
- Penalties of up to 100% of the tax due for deliberate and concealed inaccuracies
- Interest charges on top of the amount owed, compounding daily
- Criminal prosecution in serious cases of deliberate evasion
HMRC can go back up to 20 years to investigate tax affairs if they suspect deliberate non-compliance. Therefore, it’s essential to be transparent about any Capital Gains Tax you may owe.
Common excuses don’t work
Some people think they can avoid CGT by gifting assets to family members, selling assets in small chunks over time, or using offshore accounts. HMRC is wise to these tactics. They have international transparency agreements to share financial and banking information, making it harder to hide money abroad.
How likely is it that I will be caught?
While exact probabilities aren’t publicly available, HMRC has a substantial success rate when investigating potential evasion cases. HMRC’s 2023-24 Annual Report states they secured £34 billion from tackling tax avoidance, evasion, and non-compliance. The risk of being caught is significant, especially if your actions are substantial or span multiple years.
What you need to know about HMRC’s ‘Connect’ system
HMRC uses a powerful data analysis system called ‘Connect’. It cross-references billions of pieces of information from various sources, including:
- Banks and building societies
- DVLA
- Land Registry
- Companies House
- Social media platforms
The system can flag up inconsistencies that might indicate undeclared income or gains. If this happens, HMRC could begin an investigation into whether or not you’re liable for Capital Gains Tax.
What we’ve seen: We’ve worked with several clients who received HMRC enquiry letters after selling second properties. In most cases, they weren’t trying to evade tax—they simply didn’t realise the 60-day reporting requirement existed or miscalculated their gain. The Connect system had automatically flagged their Land Registry transaction against their tax records.
How to correct a non-declaration
If you’ve failed to declare capital gains, don’t panic. HMRC often looks more favourably on voluntary disclosures. According to HMRC’s guidance on voluntary disclosures, here’s what to do:
- Gather all relevant information about the asset sale and your gain
- Contact HMRC directly or use their Digital Disclosure Service (gov.uk/government/publications/hmrc-your-guide-to-making-a-disclosure)
- For recent undeclared gains, you might be able to amend your tax return online
- For older issues, consider writing to HMRC explaining the situation
Be prepared to pay the tax owed plus interest. Penalties may apply, but they’re often reduced for voluntary disclosures. If in doubt, seek advice from a qualified tax professional to navigate the process.
Capital Gains Tax for expats and non-residents
UK expats and non-residents may still be liable for CGT on UK property sales. Since April 2015, non-residents must pay CGT on UK residential property gains. They need to report and pay any CGT within 60 days of selling.
For other UK assets, the rules depend on your residency status. If you’re a UK expat, you might be liable for CGT on worldwide assets. Double taxation agreements can prevent you paying tax twice on the same gain—these vary by country, so check the specific treaty.
Capital Gains Tax and property
For most people, Capital Gains Tax becomes relevant when selling property. Here’s how it works:
You’ll pay CGT when selling a property that’s not your main home, like a buy-to-let or second home. You might also owe CGT if you’ve used part of your main home exclusively for business. No CGT is due on your main residence thanks to Private Residence Relief—this usually applies even if you’ve moved out, as long as you sell within nine months of leaving.
Remember, you only pay CGT on the profit—the difference between the purchase and sale prices, minus certain allowable expenses (including stamp duty paid on purchase, legal fees, and the cost of improvements that add value).
Current CGT rates for residential property (2024/25 and 2025/26)
| Taxpayer Type | CGT Rate |
| Basic rate taxpayers (income £12,571–£50,270) | 18% |
| Higher/additional rate taxpayers (income above £50,271) | 24% |
| Annual tax-free allowance (2024/25 & 2025/26) | £3,000 |
Source: GOV.UK Capital Gains Tax rates (gov.uk/capital-gains-tax/rates)
The 60-day property disposal return
Important: Since April 2020, you must report and pay CGT on UK residential property sales within 60 days of completion. This ‘property disposal return’ is separate from your annual self-assessment and makes it harder to avoid declaring gains on property sales. Missing this deadline can result in penalties even if you eventually pay the correct tax.
What we’ve seen: The 60-day deadline catches many sellers off guard. We recently helped a client who sold a rental property but didn’t realise the reporting requirement. By the time they contacted their accountant three months later, they faced late filing penalties on top of the CGT bill. We now make it a point to remind every client selling a second property about this deadline before completion.
Need to sell quickly to meet your CGT obligations?
If you need to pay Capital Gains Tax on a property and want to complete the sale quickly, Property Rescue can help. As cash buyers, we offer a fast, guaranteed transaction—which can be particularly useful when you need certainty for tax planning purposes.
- Exchange contracts in as little as 48 hours, or work to your preferred timescale
- No estate agent fees, EPC costs, or legal fees—we cover everything
- Properties purchased in any condition
- No chain, so no risk of delays affecting your tax position
Get a free, no-obligation cash offer to see how much your property is worth.
Voluntary disclosure
If you’ve failed to declare capital gains, it’s better to come clean before HMRC catches you. The ‘Let Property Campaign’ allows landlords to voluntarily disclose unpaid tax. While not specific to CGT, it shows that HMRC often offers chances to rectify mistakes.
When voluntary disclosure is made, penalties are usually lower than if HMRC discovers the underpayment themselves. HMRC’s penalty guidance states that penalties can be reduced by up to 100% for ‘unprompted’ disclosures made before an investigation begins.
The ‘nudge letter’ approach
HMRC often sends ‘nudge letters’ (officially called ‘one to many’ letters) to taxpayers they suspect might have undeclared income or gains. These letters prompt you to check your tax affairs and make any necessary disclosures.
Receiving a nudge letter doesn’t mean you’re under investigation, but ignoring it could lead to one. If you receive one, treat it seriously and review your tax position—ideally with professional help.
International information sharing
The Common Reporting Standard (CRS) allows tax authorities in over 100 countries to automatically exchange financial information. This means it’s much harder to hide money or assets abroad than it once was.
If you have overseas assets, HMRC likely knows about them. They might be waiting to see if you declare them correctly before taking action.
Time limits for investigations
According to HMRC’s Compliance Handbook, HMRC can normally go back:
- 4 years for unintentional errors
- 6 years if they suspect careless behaviour
- 20 years for deliberate non-compliance
Even if you’ve got away with not declaring gains in the past, you’re not necessarily in the clear.
The cost of getting caught
If HMRC discovers undeclared gains, you’ll have to pay:
- The original tax owed
- Interest on the unpaid tax (currently 7.5% per annum)
- Penalties, which can be up to 100% of the tax due
In severe cases, you could face criminal prosecution and even jail time. This is uncommon for straightforward non-compliance but remains a possibility for deliberate, large-scale evasion.
The importance of record keeping
Good record keeping is essential—and something we can’t emphasise enough based on our experience with clients. If HMRC investigates, you’ll need to prove your calculations. Poor record keeping can lead to penalties, even if you haven’t deliberately under-declared. According to HMRC’s guidance on record keeping, you should keep:
- Purchase and sale documents (including completion statements)
- Receipts for improvement costs (extensions, renovations—not routine maintenance)
- Incidental costs of purchase and sale (legal fees, stamp duty, surveyor fees)
- Any valuations (particularly important for inherited properties)
CGT Record-Keeping Checklist
☐ Original purchase contract and completion statement
☐ Stamp duty receipt
☐ Receipts for capital improvements (with dates)
☐ Legal fees for purchase and sale
☐ Probate valuation (for inherited properties)
☐ Sale completion statement
Summary
While it might be tempting not to declare capital gains, the risks far outweigh any potential benefits. HMRC has powerful tools at its disposal to detect undeclared income and gains. With the Connect system, international information sharing agreements, and sophisticated data analysis, the chances of getting caught are higher than ever.
If you’re unsure about your CGT liabilities, seek professional advice from a qualified accountant or tax advisor. It’s always better to be compliant from the start than to face the consequences of non-declaration later.
Key Takeaways
- The CGT annual allowance is £3,000 for 2024/25 and 2025/26
- Property CGT rates are 18% (basic rate) and 24% (higher rate)
- You must report and pay CGT on property sales within 60 days
- HMRC can investigate up to 20 years back for deliberate evasion
- Voluntary disclosure typically results in lower penalties
Official HMRC sources
For the most up-to-date information, refer to these official HMRC resources:
- Capital Gains Tax rates and allowances: gov.uk/guidance/capital-gains-tax-rates-and-allowances
- Report and pay CGT on property: gov.uk/report-and-pay-your-capital-gains-tax
- Making a voluntary disclosure: gov.uk/government/publications/hmrc-your-guide-to-making-a-disclosure
- Let Property Campaign: gov.uk/government/publications/let-property-campaign-your-guide-to-making-a-disclosure
