How Does Buy to Let Property Income Affect a Dental Associate’s Tax Position?

Why Buy to Let Matters for Associate Dentists

Many associate dentists build a property portfolio alongside their clinical work. Buy to let (BTL) offers an alternative to pension saving, a way to use retained profits from a limited company, or simply a long-term investment strategy. But the tax treatment of BTL income has changed significantly since 2017, and the interaction with your dental earnings can create unexpected tax bills.

This guide covers the key tax rules for an associate dentist with BTL property income in 2025/26. It explains how mortgage interest relief works now, how rental profits affect your personal allowance and higher rate threshold, and why the National Insurance treatment differs from your associate income. Worked examples use the 2025/26 rates from the system prompt.

Section 24: The Mortgage Interest Restriction

Before April 2017, landlords could deduct all mortgage interest from rental income before calculating tax. Section 24 of the Finance (No.2) Act 2015 changed that for individuals. Since 2020/21, you cannot deduct finance costs (mortgage interest, arrangement fees, etc.) from your rental income to reduce your tax bill. Instead, you receive a tax credit equal to 20% of your finance costs.

This matters for a higher-rate associate dentist. If you earn £80,000 from associate work and have £15,000 of rental profit before finance costs, with £10,000 of mortgage interest, your old calculation would have given you £5,000 of taxable rental income. Under Section 24, you pay tax on the full £15,000, then get a £2,000 tax credit (20% of £10,000). The net effect is that you pay 40% on the £10,000 of interest, not 20%.

Worked Example: Higher-Rate Associate with BTL

Consider a self-employed associate dentist with the following 2025/26 figures:

• Associate fee income: £90,000
• Practice expenses (lab, materials, CPD, indemnity, travel): £15,000
• Net associate profit: £75,000
• Rental income: £24,000
• Rental expenses (agent fees, repairs, insurance): £4,000
• Mortgage interest: £12,000

Tax calculation without property:

• Total income: £75,000 (associate) + £24,000 (rent) = £99,000
• Rental profit before finance costs: £24,000 - £4,000 = £20,000
• Total taxable income: £75,000 + £20,000 = £95,000
• Personal allowance: £12,570 (not tapered, as income is below £100,000)
• Taxable after PA: £95,000 - £12,570 = £82,430
• Basic rate band: £50,270 - £12,570 = £37,700 at 20% = £7,540
• Higher rate: £82,430 - £37,700 = £44,730 at 40% = £17,892
• Income tax before credit: £25,432
• Section 24 credit: 20% of £12,000 = £2,400
• Net income tax: £25,432 - £2,400 = £23,032

Class 4 NI on associate profit: 6% on (£50,270 - £12,570 = £37,700) = £2,262, plus 2% on (£75,000 - £50,270 = £24,730) = £494.60. Total NI: £2,756.60. Note: rental income does not attract Class 4 NI.

Total tax and NI: £23,032 + £2,756.60 = £25,788.60. The effective tax rate on the rental income is 40% on the profit plus the Section 24 restriction, which pushes the overall rate on the interest element to 40% instead of 20%.

Personal Allowance Tapering and BTL Income

Your personal allowance of £12,570 is reduced by £1 for every £2 of adjusted net income above £100,000. It is fully gone at £125,140. For an associate dentist, BTL income can push you over these thresholds.

If the associate in the example above had additional BTL income or a higher associate profit, the taper could apply. Suppose the associate profit was £85,000 and rental profit before finance costs was £25,000, total £110,000. The personal allowance reduces by (£110,000 - £100,000) / 2 = £5,000, leaving £7,570. This adds an effective 60% tax rate on the income between £100,000 and £125,140.

This is a common trap for associates who build a large BTL portfolio while still working full-time. The rental income stacks on top of associate earnings, and the personal allowance taper creates a marginal rate of 60% (40% income tax plus the loss of 20% of the allowance).

National Insurance: The Key Difference

Rental income is not subject to Class 4 National Insurance contributions. This is a significant difference from your associate income, which attracts Class 4 at 6% on profits between £12,570 and £50,270, and 2% above that.

For a higher-rate associate, the NI saving on rental income compared to additional associate work is 2% on profits above £50,270. But the Section 24 restriction means you still pay 40% income tax on the rental profit, whereas additional associate income would attract 40% income tax plus 2% Class 4 NI. The rental income is slightly cheaper from an NI perspective, but the Section 24 restriction can offset that advantage if you have significant mortgage debt.

Worked Example: BTL vs Additional Associate Work

Compare £10,000 of additional rental profit (after all expenses except mortgage interest) with £10,000 of additional associate profit. Assume the associate is already a higher-rate taxpayer.

Additional associate work:

• Income tax: £10,000 at 40% = £4,000
• Class 4 NI: £10,000 at 2% = £200
• Total: £4,200
• Net: £5,800

Additional rental profit (no mortgage interest):

• Income tax: £10,000 at 40% = £4,000
• NI: £0
• Total: £4,000
• Net: £6,000

The rental income saves £200 in NI. But if the rental property has mortgage interest, the Section 24 credit at 20% does not fully compensate for the 40% tax rate on the interest portion. The overall advantage depends on your loan-to-value ratio.

Holding BTL in a Limited Company

Some associates hold BTL properties through a limited company rather than personally. This avoids Section 24 because companies can still deduct mortgage interest as a trading expense. But the trade-off is that you pay corporation tax on the rental profits (19% or 25% depending on profit level) and then dividend tax when you extract the money.

For a higher-rate associate, the effective tax rate on company rental profits extracted as dividends is roughly: 19% corporation tax + (33.75% dividend tax on the remaining 81%) = approximately 46.3%. This is higher than the personal rate of 40% on rental profits (plus the Section 24 restriction on interest). The company structure works better if you reinvest profits rather than extract them, or if you have very high mortgage interest relative to income.

For a detailed comparison of extraction strategies, see our guide on profit extraction for dental practices. The principles for BTL companies are similar, though the tax rates differ.

NHS Pension and BTL Income

Your NHS Pension contributions are based on your NHS pensionable earnings, not your total income. BTL income does not affect your pensionable earnings. However, if your total income (including BTL) exceeds the annual allowance of £60,000 (2025/26), you may trigger the annual allowance taper. This is more likely for associates who have high NHS pension growth plus significant BTL income.

The annual allowance taper reduces your allowance by £1 for every £2 of adjusted income above £260,000 (threshold income must exceed £200,000). Most associates will not hit these levels, but those with large property portfolios and high associate earnings should check. See our NHS Pension guide for more detail.

Practical Steps for Associate Dentists with BTL

Here are the key actions to take:

• Keep separate bank accounts for rental income and expenses. HMRC can request evidence, and mixing funds creates confusion.
• Track mortgage interest separately from other rental expenses. You need the finance cost figure for the Section 24 credit calculation.
• Consider whether your BTL portfolio is structured optimally. If you have multiple properties and significant mortgage debt, a limited company might save tax in the long run, especially if you plan to reinvest profits.
• Check your total income each year to see if you are near the personal allowance taper threshold. If you are, you might consider reducing associate work, deferring rental income, or making pension contributions to bring adjusted net income down.
• Remember that rental losses cannot be set against your associate income for NI purposes, but they can be carried forward against future rental profits.

If you are considering buying your first BTL property or restructuring an existing portfolio, speak to a dental-specialist accountant. The interaction between associate income, Section 24, and your NHS Pension position needs tailored advice. Contact us for a free practice health check or explore our associate tax services for more information.