But what’s it all about?

Highlanders can learn more about this on Sustainable House Day next month.

The Greeny Flat in Mittagong will be one of three Highlands houses to open its doors on September 11.

Andy Lemann and his partner Cintia Yamane built the house and moved into the Greeny Flat in 2014.

This year will be the third year the Greeny Flat has been open on Sustainable House Day and Mr Lemann said they often had people requesting a tour throughout the year.

“This whole project was really an experiment to see if we could build a house that was energy positive and cost effective,” he said.

The Greeny Flat cost $130,000 to build and Mr Lemann said in their first full year of living in the house it cost $312 to run.

The house has a three kilowatt solar power system. It does not have a heating and cooling system but instead uses passive solar design.

Passive solar design uses the sun to create a cost-effective method for keeping buildings warm in the winter and cool in the summer with little or no additional heating or cooling.

Mr Lemann said he hoped people who visited the house would learn the value of passive solar design.

“The first rule of building sustainably is to keep it small and simple. After that, passive solar design and reducing the amount of energy that you need. The last thing is adding a solar power system.”

Mr Lemann said Australia was among the highest energy users per capita in the world and also built the biggest houses in the world.

“There’s no excuse for it. We live in this glorious climate where it’s very easy to have a house that makes more energy than it uses.”

Sustainable House Day will be held on September 11. 

Houses will be open from 10am.

Visit http://sustainablehouseday.com/about-shd/ for more information.

To learn more about the Greeny Flat visit greenyflat.com.au