Built Green certified homes are trending in King and Snohomish Counties. Last year, more than half of Seattle's new homes were certified with the Built Green title.

One of the Built Green homes finished in 2011 belongs to the Thomas family, who designed it to take care of itself, whether it's strategically placed windows or Beatrice the dog walking herself on a treadmill.

"It didn't cost any extra. It wasn't super high technology. It's something anyone building a house, anyone remodeling a house, can do," Eric Thomas said.

Windows are designed to capture the most sunlight. Heat comes from recycled water that runs under the concrete floors. The walls are special pre-made insulated panels, and the solar panels on the roof produce more electricity than the Thomas family needs, so they get money back from the city.

"It's by far the best decision we've made in Seattle. It's worked out great. We love the house. It's very bright. It's very comfortable to live in, and the energy efficiency has been phenomenal," Thomas said.

Photos: A look inside a Built Green home in Seattle

Their house is part of a growing trend in King and Snohomish counties, with more than 17,000 certified Built Green homes.

"Since the recession, our numbers have only been going up. So, that's really exciting," Leah Missik said.

Leah Missik manages the Built Green program for the Master Builders Association of King and Snohomish County. Last year, they certified more than a thousand homes in Seattle.

Recently, they finished a comprehensive study to see how all of the homes are performing.

"We looked at every single home built, every single single-family or townhome built, in 2014. So, that's over 700 homes. We looked at real life data from these homes to see in real life, how do Built Green homes compare to non-certified homes. We found Built Green homes performed even better than we expected with over 40 percent energy savings," Missik said.

For the Thomas family, the house cost as much to build as more traditional homes, but they save thousands in utility bills each year – even with Beatrice regularly on the treadmill, which is powered by the sun.

"It's not something we really have to worry about. We live our lives like normal, and the house takes care of itself," Thomas said.