If you forgot to declare income to HMRC, the most important thing is to act now — not wait. Voluntary disclosure before HMRC contacts you results in lower penalties and reduces the risk of criminal prosecution. Here is exactly what to do in 2026/27.
Why You Must Act Quickly
HMRC’s penalty system rewards people who come forward voluntarily. The key distinction is:
- Unprompted disclosure — you contact HMRC before they contact you: lowest penalties
- Prompted disclosure — HMRC contacts you first: higher penalties, less negotiation room
- No disclosure — HMRC investigates and finds the income: highest penalties, potential prosecution
The longer undeclared income sits unaddressed, the more it accumulates in unpaid tax, interest (currently 7.75% per annum), and potential penalties.
What Income Needs to Be Declared?
Common types of income people forget to declare:
| Income type | Threshold before tax applies |
|---|---|
| Self-employment / trading income | Trading Allowance of £1,000/year |
| Rental income | Property Allowance of £1,000/year |
| Side income (freelance, eBay selling for profit) | Trading Allowance of £1,000/year |
| Bank interest | Personal Savings Allowance (£500–£1,000 depending on tax band) |
| Dividends | Dividend Allowance of £500/year |
| Capital gains (e.g. property, shares) | CGT Annual Exempt Amount of £3,000/year |
| Overseas income | Must be declared if you are UK resident for tax |
If your income from any source exceeds these allowances, you need to report it — either through Self Assessment or by contacting HMRC.
Step-by-Step: How to Disclose Undeclared Income
Option 1 — File or Amend a Self Assessment Return
If you are already registered for Self Assessment, you can:
- File a late return (for years you missed completely)
- Amend a return you already filed (within 12 months of the filing deadline)
Late returns trigger automatic penalties (£100 for missing the deadline, rising to £1,000+ after 12 months) — but the penalties for a late honest return are far lower than HMRC penalties for discovered evasion.
Option 2 — HMRC’s Digital Disclosure Service (DDS)
The DDS is designed for people who are not registered for Self Assessment and want to make a voluntary disclosure of undeclared income. Use it for:
- Rental income not previously declared
- Self-employment income from earlier years
- Overseas income or assets
- Capital gains not previously reported
Go to: gov.uk/guidance/admitting-tax-fraud-the-contractual-disclosure-facility-cdf and follow the process. You will be asked to estimate the tax owed. HMRC then issues a calculation, and you pay.
Option 3 — Register for Self Assessment and File
If you have ongoing untaxed income (self-employment, rental, etc.), register for Self Assessment at gov.uk/register-for-self-assessment. File returns for all outstanding years — HMRC will usually work with you on a payment plan if the bill is large.
Penalties for Undeclared Income
| Type of error | Unprompted disclosure penalty | Prompted disclosure penalty |
|---|---|---|
| Genuine mistake | 0% | 0–30% |
| Careless error | 0–30% | 15–30% |
| Deliberate (but not concealed) | 20–70% | 35–70% |
| Deliberate and concealed | 30–100% | 50–100% |
Penalties are calculated as a percentage of the Potential Lost Revenue (PLR) — the tax that should have been paid. On top of penalties, HMRC charges daily interest from the date the tax was originally due.
Worked Example: Sarah, Rental Income Not Declared for 3 Years
Sarah has been renting out a room for £650/month since April 2023. She did not declare it because she did not realise she needed to. Her total rental income over 3 years: £23,400. After deducting the Rent a Room Scheme allowance (£7,500/year), taxable income: £0 for years 1 and 2 (both under £7,500), and £2,400 in year 3 (if it exceeds the threshold).
Actually: £650/month = £7,800/year — above the £7,500 Rent a Room allowance by £300/year. Tax owed on £300/year at 20% = £60/year × 3 years = £180 total. Plus interest on the earliest year’s £60 at 7.75% = modest amount.
Sarah makes an unprompted disclosure via DDS. Because it was a genuine mistake (she did not know the allowance was £7,500), the penalty is 0%. She pays £180 tax plus modest interest — total bill under £250.
Without disclosing: If HMRC found the income (flagged via her bank), the same tax would be due — but with a 15–30% penalty added, and potentially interest on the penalty too.
What HMRC Can See
HMRC’s data-matching system receives information from:
- UK banks — interest and account data
- Online platforms — eBay, Etsy, Vinted, Airbnb, Uber (under UK rules, platforms now report seller income to HMRC)
- Land Registry — property sales and purchases
- Companies House — directors and shareholders
- Overseas tax authorities — via international exchange agreements
If you have income from any of these sources that you have not declared, the probability of HMRC identifying it is high and increasing each year.
See our Self Assessment guide, what happens if HMRC investigates you, and trading allowance guide.