Interest-only vs principal & interest: which loan structure suits investors?

One of the most common questions property investors ask is whether to take out an interest-only (IO) loan or a principal and interest (P&I) loan. The honest answer: it depends — on your cash flow, your tax situation, your investment strategy, and how long you intend to hold the property. Here's a clear-eyed breakdown of both.

What's the actual difference?

Principal and interest (P&I): You repay both the interest charged and a portion of the loan balance with each repayment. Over time, your debt decreases and your equity grows.

Interest-only (IO): You pay only the interest each month, with no reduction in the loan balance. At the end of the IO period (typically 1–5 years), the loan reverts to P&I — over a shorter remaining term, which means higher repayments.

Why investors have traditionally preferred interest-only

For investment properties, interest on your loan is generally tax-deductible. If you maximise that deductible interest while directing your cash flow to non-deductible purposes (like paying off your own home), you can create a more tax-efficient structure overall.

The key logic: your owner-occupied home loan interest is not deductible. Your investment loan interest is. So by minimising repayments on the investment loan (IO) and directing surplus cash toward your home loan instead, you reduce your non-deductible debt faster while keeping your deductible debt higher. This is sometimes called a "debt recycling" approach.

IO loans also improve cash flow on negatively geared properties — keeping monthly outlays lower while the investor holds for capital growth.

The risks and downsides of interest-only

It's not a straightforward win. IO loans come with genuine trade-offs:

• Higher rates — lenders typically charge a premium of 0.1–0.4% for IO loans versus P&I
• No equity build-up — if property prices flatten or fall, you don't build a buffer
• Reversion risk — when the IO period ends, repayments can jump significantly as you repay the same principal over a shorter term
• Harder to get — APRA has tightened IO lending over recent years; lenders assess IO applications more carefully and some require a stronger serviceability buffer
• Discipline required — the strategy only works if you're actually directing the saved cash flow productively (e.g. into an offset account on your home loan)

When P&I makes more sense for investors

Interest-only isn't always the optimal choice. P&I may be better for investors when:

• The property is positively geared — you're not relying on deductible interest to manage cash flow
• You don't have an owner-occupied mortgage to pay down — the debt recycling argument disappears
• You're close to retirement and want to reduce debt levels
• You plan to eventually move into the property (converting it to your PPOR, where you'd want lower debt)
• The IO premium is large enough to erode the tax benefit

The offset account question

Some investors on IO loans hold excess cash in an offset account against the investment loan. This reduces the interest charged without reducing the loan balance — keeping the deductible debt "accessible" while reducing the interest cost. It's a strategy worth exploring, but the interplay with tax (particularly if you ever want to access that cash for personal use) needs to be carefully managed.

A worked example

Imagine two investors, both with a $700,000 investment loan and a $500,000 owner-occupied mortgage:

Investor A (IO on investment property): Pays $3,500/month in IO repayments on the investment loan. Directs all surplus cash to the home loan, which they clear 4 years earlier. Lower total non-deductible interest paid over the life of both loans.

Investor B (P&I on both): Builds equity faster in the investment property. More financially conservative. Less tax-optimised but simpler to manage.

Which is better? It depends on the tax bracket, the actual rate differential, how disciplined Investor A is with the cash flow, and the growth expectations for the property. A mortgage broker working with a good accountant can model both scenarios.

Get the right advice before you decide

IO vs P&I is ultimately a question that lives at the intersection of finance, tax, and strategy — which means the right answer looks different for every investor. Before you lock in a structure, it's worth having a conversation with both your broker and your accountant. The loan structure that's optimal for your situation might surprise you.