Until recently, “passive housing” — residences built to achieve ultra-low energy use — has been kind of a boutique-y effort, with eco-minded homeowners making upfront investments to radically reduce their carbon footprint. But now, Maine is on the leading edge of a surge in its development, on a large scale.

Last fall, the nation’s biggest passive housing complex opened in Brewer. The 48-unit Village Center, as it’s called, meets strict energy efficiency standards set by a national institute. It’s one of just 36 such buildings in the country.

And now, the Bayside Anchor — a big, green, somewhat boxy looking four-story building near Portland’s Franklin Street artery — has joined the group.