Medical marijuana is legal in more than 20 U.S. states, and places like Colorado, Washington, Oregon, Alaska and Washington, D.C., have decriminalized recreational marijuana use for adults over the last few years. Marijuana advocates say this has led to an expanding industry in the home remodeling business: creating rooms in homes in which to grow pot.

But finding a contractor who would be willing to put a “grow room” into a home was nearly impossible until recently. Such rooms need, among other things, high-voltage metal halide electric lamps, high-capacity intake and exhaust fans to maintain carbon dioxide and oxygen levels, along with the proper heavy-duty electrical wiring and plumbing in a suitably-sized room.

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“No contractor would touch stuff like this a year ago,” said Eli Bilton, the chief executive of the Attis Group, an online marijuana supply company in Portland, Ore. “They didn’t want to be on a job site where cannabis plants were around,” he said.

So people who wanted to build a grow room had to learn how to do it themselves, or pay somebody to do work without the proper permits.

Fire investigators say grow rooms in homes, especially those built by amateurs without licensed contractors are a hazard. Some operations contain 800 to 1,000 pot plants in a 2,500 square foot home. Post legalization, many amateur pot growers continue to rig their homes with unsafe grow rooms while hoping to get rich quickly, said Bilton. “They don’t engineer and design correctly,” he said.

A single home of that size could produce enough pot in a year to net as much as $1.5 million once it’s harvested and sold according to Victor Massenkoff, an arson investigator with the Contra Costa County Fire Protection District in Pleasant Hill, Calif., about 20 miles east of San Francisco. California accounts for nearly half of the sales of pot in the U.S., according to ArcView, a San Francisco-based marijuana industry research firm.

Dave Perry, a property loss insurance lawyer from Littleton, Colo., said the increased use of electricity is the primary cause of most fires, as most home growing operations need as much as 600 amps of power, where a typical home only uses about 200 amps of power. And without a licensed electrician, a house could be rewired improperly, resulting in an overload. In Colorado, a year after marijuana was legalized in 2012, there were 20 marijuana-related fires, which jumped to 32 in 2014, and 50 in 2015, according to Perry.

Even more dangerous is the spread of something called hash oil or “honey butane oil,” which is a super concentrated oil containing 95% tetrahydrocannabinol, or THC, the drug that creates the high in marijuana. A one-inch vial of honey butane oil can sell on the streets for as much as $4,000 and cost as little as $100 to produce. But the drug needs large amounts of highly flammable butane to produce and has led to explosions and fires in homes when the butane finds an ignition source in a home like a pilot light, says Massenkoff.

Still, experienced contractors who work within the building code and permits process are beginning to build grow rooms in high-end developments, which can cost up to $2,000 to complete.

Bilton says that in Oregon many licensed contractors will build grow rooms now that marijuana is legal. “It’s becoming part of our culture here in Portland,” he said. “You’re more likely to know somebody who’s involved in the growing industry so it’s more acceptable now,” he said.

For example, in Washington, D.C., after voters in the District approved an initiative in November 2014 that legalized recreational marijuana use for adults beginning this February, Eric Hirshfield, a real estate developer, is adding grow closets to some of his condominium projects.

Hirshfeld said he used a licensed electrician to beef up the wiring for the lighting and the exhaust fans as well as plumbers to ensure the proper drainage. He also ensures that the closets are built small enough to hold just six plants, the maximum allowed by D.C. law.

“I like to include a unique amenity in each of my condo projects like a wine cooler or a dog washing station and this year the grow closet is the hot amenity for 2015,” Hirshfield said in an interview. “It’s a sign of the times,” he said.