If you own a rental property, there’s a good chance you’re claiming interest expenses as part of your tax deductions. It’s one of the most common (and often one of the largest) claims for landlords. 

But there’s a catch – you can only claim interest to the extent that it relates to earning assessable rental income.

That means in many situations, you’ll need to apportion (split) your interest expenses between deductible and non-deductible portions. 

Getting this wrong can lead to over-claiming, which may result in amended returns, penalties, and unwanted ATO attention.

Let’s walk through when and how interest needs to be apportioned – and some common pitfalls to avoid.

When Do You Need to Apportion Interest?

The ATO outlines several situations where apportioning is necessary:

1. Co-ownership of the Property

If you own a rental property with another person, you generally split interest expenses according to your legal ownership share.

• Joint tenants each own an equal share, so deductions are split 50/50.
• Tenants in common may own unequal shares (e.g. 70% / 30%), and interest must be split accordingly. 

Even if one person pays all the loan repayments, the deduction is still based on ownership unless there’s a legally enforceable agreement stating otherwise – and the payments match that agreement.

2. Mixed-Purpose Loans

If your loan was used partly for the rental property and partly for something private – such as buying a car or funding a holiday – you’ll need to work out what proportion relates to the property.

For example:

• Loan amount: $400,000
• $380,000 used for rental purchase, $20,000 for personal expenses
• Total interest for the year: $35,000
  Deductible interest = $35,000 × (380,000 ÷ 400,000) = $33,250
  

The non-deductible portion ($1,750 in this example) can’t be claimed. And if the loan is refinanced or repayments alter the mix, you’ll need to adjust the calculation each year.

3. Private Use of the Property

If you (or friends/family) use the property for any part of the year, you can’t claim interest for that period.

Say you rent the property for 9 months and use it privately for 3 months – only 75% of the interest is deductible. If only part of the property is rented (e.g. you rent out one room via Airbnb), you’ll also need to apportion based on both time and space.

4. Part-Year Rentals

If the property is only genuinely available for rent for part of the year – for example, due to renovations or because you didn’t list it on the market – interest must be apportioned to cover only the rental period.

How to Stay on the Right Side of the ATO

Here are some tips to make sure your claims are correct and easy to substantiate:

• Keep clear records of loan purpose, rental periods, and any private use.
• Separate loans for private and investment purposes wherever possible – it makes apportionment simpler.
• Document co-ownership agreements if your ownership or repayment arrangements differ from the norm.
• Be consistent in how you calculate apportionment from year to year. 

Why Accuracy Matters

Interest deductions can be substantial, and the ATO keeps a close eye on property-related claims. Overstating deductions – even by mistake – can lead to costly adjustments. On the flip side, under-claiming means you could be missing out on legitimate tax savings.

Getting apportionment right ensures:

• You claim the maximum allowed deduction without crossing compliance lines.
• You have the documentation to support your claim in case of a review.
• You avoid unexpected tax bills down the track.

Apportioning rental interest expenses might not be the most exciting part of property investing, but it’s an essential one. If your property isn’t purely rented 100% of the time or your loan isn’t solely for the rental, there’s a strong chance apportionment applies.

If you’re unsure how to calculate your deduction – especially with mixed-purpose loans or complex ownership structures – it’s worth getting tailored advice. 

We can help you work through the numbers so you can claim confidently, compliantly, and in full.