Goodbye to long months of hammering and sawing! As factory-built U.S. housing options expand, new homes are being assembled onsite within days.

These aren’t just modest abodes. Prefabricated building, once associated with Mobile double-wides, has evolved so much that even modular mansions are now arriving in upscale neighborhoods.

Within 32 hours last month, a 7,200-square-foot home (six bedrooms, six-and-half baths) was put together like a layer cake in Bethesda, Md., The Washington Post reports. To see how this worked, click here.

“We’re instant-gratification people,” owner Bob McCarrick, a 39-year-old investment banker, told the paper. While interior finish work remains, most of his insta-mansion was built in two weeks on an assembly line in State College, Pa., and trucked in 21 boxes to his lot.

For anyone who’s ever spent months – even years – building a custom home, the speed of these prefabs is amazing. I say this as someone who’s spent the last year designing a green home. My family submitted its plans in mid-January and is waiting for permits to break ground in Falls Church, Va.

Almost every week, new modular options cross my desk, many of them stylish and eco-friendly and some – by Clayton Homes, Nationwide and Excel – cost less than $100,000. In her latest book, Prefabulous & Sustainable, Sheri Koones showcases the increasing variety. I’ll soon be talking to Koones about her book.

Austin-based design-build firm KRDB recently launched Ma, energy-efficient modular homes that will be available nationwide and start at $125 per square foot. They offer green upgrades such as geothermal heating and solar panels. You can see pictures and read more about them on Jetson Green.

Last week at the Portland Home and Garden Show, Oregon-based prefab maker, Ideabox, unveiled its new Fortino model, a two-bedroom, two-bath, 1,250 square-foot home that sells for about $150,000.

The Fortino carries the government’s Energy Star label for efficiency. Its walls (R21) and roof (R38 to R42) are well-insulated, says Ideabox’ owner Jim Russell in an interview.

“It’s a clever way to show how you can live in a small house,” he says, and still live stylishly. “We decided to build houses we would want to live in.”

He says the model, which offers varying heating and other packages, can be ready for shipping within 8 weeks of ordering. It’s made in Oregon and can be transported to sites west of Colorado.

Russell expects installation, including permits and connection fees, to cost an additional $20,000 to $30,000.

Readers: What, if any, recent experience have you had with prefab homes?