McMansions and old homes could be devalued in a new green scheme. Picture: Sunday Herald Sun Source: Sunday Herald Sun

The Federal Government aims to introduce mandatory energy star ratings for homes being sold or rented out as soon as next year, the Herald Sun reported.

Under the favoured system, vendors and landlords would have to pay about $200 to have their property assessed - a total cost of $1.1 billion over 10 years.

And housing experts say most older homes and McMansions would be likely to score very poorly.

Mick Fabar, director of private energy ratings firm Green Homes Australia, said many two-storey McMansions would be lucky to score zero.

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Renovated old workers' cottages and Californian bungalows also were expected to score at the bottom end of the scale.

Experts said there would be significant financial implications for owners of these homes - either spend up on going green or face the prospect of a lower sale price.

A federal government study into a similar ACT scheme operating since 1999, which rates properties out of 10 stars, found a one-star difference affected selling prices by 3 per cent.

If mirrored in Melbourne, a one-star variation would equal $17,700 - based on the REIV's median house price of $590,000, as reported in June. A three-star variation would equal $53,100.

Choice head of campaigns Matt Levey said homes that were energy inefficient would cost more to run, but star-rating models have been criticised for failing to factor in actual consumption, leading to questions about whether the changes will even cut power use.

The Herald Sun asked Climate Change Minister Greg Combet's office if the scheme would have a negative effect on the sale price of some homes.

"It will allow buyers and renters to better compare different properties, making it easier to identify a property that uses less energy or water and thereby save money," a spokeswoman said.

She said the Government had introduced a $100 million energy-efficiency program for low-income earners.

Opposition climate change spokesman Greg Hunt said the scheme would create "enormous uncertainty".

"It could push up the cost of rent for people just when they are feeling the cost of living pressures," he said.

"It's another cost imposed on people from the Government."

But Angus Raine, chief executive of major national real estate chain Raine & Horne, suggested people were so consumed with getting on the property ladder that star ratings would be discounted.

"People look at the physical property first and then (a mandatory disclosure system) is going to be one of their second or third considerations in their purchasing matrix," Mr Raine said.

A July 2011 "consultation regulation impact statement" prepared for the federal and state governments forecast the system would affect hundreds of thousands of homes in its first year of operation.