Many landlords lose thousands of pounds each year simply because of avoidable errors on their tax returns. In the 2024/25 tax year alone, HMRC’s enforcement activity recovered over ÂŁ107 million in unpaid tax from landlords, averaging more than ÂŁ13,500 per case. This guide explains the most common property tax mistakes and how to avoid falling foul of HMRC.
Claiming Full Mortgage Interest as an Expense
One of the most persistent misunderstandings among landlords relates to mortgage interest. Since April 2020, individual landlords can no longer deduct mortgage interest as a straightforward expense against rental income. Instead, they receive a tax credit fixed at the basic rate of 20 percent, regardless of their actual tax bracket, applied to the lower of their finance costs, property profits or adjusted total income.
This change under Section 24 of the Finance Act has significantly increased tax bills for higher and additional rate taxpayers. Many landlords still enter their full mortgage interest figure incorrectly, or miscalculate the 20 percent reduction, when it should be reported separately in the finance costs section of the property pages on their tax return. Landlords also frequently forget that capital repayment amounts on a mortgage do not qualify for relief at all, only the interest portion does, meaning a breakdown from the lender is essential each year.
Forgetting to Carry Forward Unused Finance Costs
Where the tax reduction cannot be fully used in a given year because profits are too low, the unused finance costs can be carried forward to future tax years. Many landlords are unaware of this rule and simply lose the relief rather than rolling it forward on their next return.
Deducting Capital Expenses Instead of Revenue Expenses
Confusing capital expenditure with revenue expenditure is another area where landlords regularly go wrong. Revenue expenses, such as repairs, cover the day to day running and maintenance of a property and can be deducted directly against rental income. Capital expenses, such as installing a new extension or a significantly upgraded kitchen, must instead be set against Capital Gains Tax when the property is eventually sold.
The distinction matters because claiming capital costs as revenue expenses on a Self Assessment return can trigger an HMRC enquiry and repayment demand. Training costs for landlords are treated similarly, where courses that reinforce existing skills are allowable, but learning an entirely new skill is not.
Not Declaring All Rental Income
A property tax return must include all rental income, not just income from a main let. This includes income from any additional properties, and each landlord’s share of income from jointly owned property. HMRC increasingly uses bank data and other information sources to identify undeclared rental income, so gaps are difficult to hide.
Landlords also often misunderstand when they need to register for Self Assessment at all. If gross rental income is under 1,000 pounds it is generally exempt under the property allowance, income between 1,000 and 2,500 pounds should be reported directly to HMRC, and anything above 2,500 pounds in profit or 10,000 pounds in gross income requires a full Self Assessment return.
Misunderstanding Income Splits Between Partners
Couples who jointly own a rental property sometimes allocate more of the rental income to the lower earning partner to reduce their combined tax bill. This is a legitimate strategy, but the income split must match the legal ownership split. If one partner is to receive 70 percent of the rental income, they must legally own 70 percent of the property, and this should be formally documented.
Getting Capital Gains Tax Wrong on Sale
When a rental property is sold or transferred, Capital Gains Tax usually applies on the profit made. A frequent error, particularly among landlords who have remortgaged and released equity over time, is confusing the gain with the equity left after paying off the mortgage. The taxable gain is simply the difference between the original purchase price and the sale price, adjusted for allowable costs, not the cash received at completion.
Landlords must also report and pay Capital Gains Tax on UK residential property within 60 days of completion, using HMRC’s separate property reporting service. Missing this short deadline is a common and costly mistake that triggers automatic penalties.
Treating Holiday Lets Under Outdated Rules
The furnished holiday lettings regime, which previously offered more generous tax treatment, was abolished from 6 April 2025. Landlords who continue to apply the old furnished holiday let rules on their return are likely to miscalculate their tax position, since these properties are now treated the same as any other residential letting.
Not Seeking Professional Advice Before Investing
Several sources point to a lack of professional advice as the root cause behind many other mistakes on this list. Getting advice before purchasing a rental property helps ensure the business is structured tax efficiently from day one, and that landlords do not miss out on allowances or inadvertently trigger reliefs they cannot use. Understanding the full tax position in advance is particularly important given that mortgage interest is no longer a deductible expense, since some landlords end up with a taxable profit even when their mortgage payments absorb most of the rent received.
Choosing the Wrong Business Structure
Because Section 24 applies only to individual landlords and partnerships, incorporation has become a more popular strategy for landlords with larger portfolios, since limited companies can still deduct mortgage interest as a normal business expense. Deciding between operating as an individual landlord or through a limited company has significant tax consequences, and switching structures later can itself trigger Capital Gains Tax and Stamp Duty charges, making early advice valuable.
What Happens If You Have Underpaid Tax
If a landlord realises they have underpaid tax or failed to disclose rental income, the first step is to notify HMRC directly, after which there is a 90 day window to calculate and pay what is owed. Penalties for voluntary disclosure typically range from 0 to 35 percent of the tax due, while penalties where HMRC uncovers the issue first can reach up to 100 percent, with criminal prosecution possible in serious cases. HMRC’s Let Property Campaign offers a structured route for landlords to bring their tax affairs up to date voluntarily.
How UK Property Tax Accountants Can Help
Property tax rules change frequently and the penalties for getting them wrong can be severe. Our team at UK Property Tax Accountants specialises in landlord tax compliance and planning, helping you avoid these common mistakes and keep more of your rental profits.
- We calculate your mortgage interest tax credit correctly and ensure unused finance costs are carried forward where applicable
- We distinguish clearly between revenue and capital expenses so your Self Assessment return only includes eligible deductions
- We prepare accurate Self Assessment returns covering all your rental income sources, including jointly owned property
- We advise on income splitting between partners in line with legal ownership shares
- We handle Capital Gains Tax calculations and the 60 day reporting deadline when you sell or transfer a property
- We review whether incorporation or continuing as an individual landlord is more tax efficient for your portfolio
- We support voluntary disclosures through the Let Property Campaign if you have undeclared rental income
- We provide proactive tax planning before you purchase, so your investment is structured correctly from the outset
Frequently Asked Questions
Can I still deduct my full mortgage interest from my rental income?
No, individual landlords can only claim a 20 percent tax credit on mortgage interest, not a full deduction, following the Section 24 changes.
What is the difference between revenue and capital expenses for a rental property?
Revenue expenses cover day to day repairs and maintenance and can be deducted against rental income, while capital expenses such as improvements are only deductible against Capital Gains Tax when the property is sold.
Do I need to file a Self Assessment return if my rental income is small?
It depends on your gross income and profit, with different thresholds triggering either no action, a simple notification to HMRC, or a full Self Assessment return.
How long do I have to report and pay Capital Gains Tax after selling a rental property?
UK residential property sales must be reported and any Capital Gains Tax paid within 60 days of completion through HMRC’s property reporting service.
What happens if I forgot to declare rental income in previous years?
You should notify HMRC as soon as possible, ideally through the Let Property Campaign, since voluntary disclosure generally results in lower penalties than HMRC identifying the issue independently.
Is it better to hold rental property personally or through a limited company?
This depends on your income level, portfolio size and plans, since limited companies retain full mortgage interest deductibility while individuals are restricted to the 20 percent credit.
