An interest only loan can seem like a great idea at the time – but there are plenty of risks involved in buying a property with a loan that fails to pay off any principal.

Interest only loans are popular with property investors, as they allow you to minimise your mortgage repayments in the short-term, while your property asset hopefully grows in value in the long term.

Interest only mortgages are exactly what they sound like: they require the borrower to repay only the interest on the loan, rather than a standard principal and interest loan.

Let’s say you own a property worth $360,000, with a $300,000 interest only loan at 7%. Your monthly repayments would be just $404 per week. On the other hand, with a standard principal and interest (P & I) loan, the weekly repayment would shoot up to $534.

Clearly, the immediate cost savings are significant, and they can help to ease the financial pressure in the short term - but it comes at a price.

The main disadvantage is that you’re making no headway on your overall mortgage. In the above scenario after five years have passed, you’ll still owe $300,000 on the property if you take out an interest only loan. With a standard P & I mortgage, after five years you would have shaved a good $50,000 off the balance of your home loan.

Essentially, this means that you will not have the benefit of gaining equity in your home, even though you are making mortgage payments every month.

Investors work around this by predicting that in five years time, the value of the property will have increased anyway. They hope that their $360,000 property will be worth at least $400,000, so they will have equity in the property without having paid a single cent off the principal.

But what if the property doesn’t increase in value? Just ask Darren Vistas, a sound engineer from Queensland who bet on an interest only loan – and lost.

“I bought an apartment in a regional coastal city in 2006, for $155,000,” he says.

“I thought it was a total bargain! I paid a 20% and got an interest only loan for the rest ($124,000). My goal was the hold the property for five years and then sell, so I could use the profits to buy my own home.”

Unfortunately, property prices haven’t increased in the five years since Darren invested in his two-bedroom apartment. In fact, they’ve gone backwards, and Darren’s property is currently worth around $120,000.

“I was going to hold onto it in the hopes that values will pick up the area eventually, but I can’t see things changing any time soon, and the bank won’t lend me any money for my own home with this investment debt hanging over my head,” he says.

The property is currently on the market for $125,000, and Darren says he’s prepared to wear the loss in order to move on with his home ownership goals.

“It’s a learning curve – a very expensive learning curve!” he says. “I’ve saved up enough money to use as a 20% deposit on my own apartment where I live, in Redcliffe, so I’m ready to sell this place and put it all behind me. I definitely won’t be getting an interest only on my next purchase, that’s for sure.”