From left, Lexi Goodman, 10, Lucas Goodman, 8, and Devin Goodman, 14, play at their tree house in their backyard. The tree house, which took around four months to build, cost more than $20,000.

SPARKS, Md. — Lexi, Lucas and Devin Goodman wander up the two-tier staircase and through a trapdoor, revealing the inside of a child's dream room. The walls are painted white, their names in red, yellow and blue letters with several of their handprints to match. And in the middle of the room is the trunk of a large oak tree.

It's their very own treehouse — and it's far more involved than a few pieces of wood nailed to some branches. With the intricate design of the 12-foot-high octagon that floats on wooden beams, it's hard to believe that it all started with their father's sketch on a paper napkin.

Barrett Goodman, 45, of Sparks, decided to build the treehouse after remembering his own as a child.

“I enjoyed it when I was growing up,” he said. “I decided we'd build it all together.”

But treehouses these days aren't what they were when Goodman was a kid.

With TV shows like Animal Planet's “Treehouse Masters” and DIY Network's “The Treehouse Guys,” there has been an increase in treehouse building within the past five years, said Tanya Breck, a designer and project manager at Tree Top Builders Inc. in Pennsylvania. And they're not just a DIY project anymore.

People are turning to companies like Tree Top Builders to help them create elaborate backyard structures, with costs ranging from $10,000 to close to $100,000 for large-scale treehouses with plumbing and electricity.

Though the newest trend is building treehouses for guest or rental use (think Airbnb), Breck said that many people are looking to build treehouses for their children and family.

That's what Goodman was hoping to achieve when he reached out to Tree Top Builders and its sister company, Treehouse Supplies, in 2010 after doing a rough sketch of his intended design on a napkin.

Over the course of four months and with the help of a couple of builders, the Goodman family built their dream treehouse, with stained wood, a recycled plastic shingle roof, five screened windows, two working doorbells, electrical outlets, a front door, a custom cedar handrail, and two hammocks on the inside. Outside, there's a tire swing and a “milk and cookie” zip line — a bucket on a set of wires used to transport treats from their home's back deck to the treetop dwelling.

Inside, three painted hard hats are nailed to the wall, a testament to the children's involvement in this labor of love.

“We just like to hang out,” said Lexi, 10, while sitting in a hammock inside the treehouse. “And sometimes we'll bring snacks.”

Sometimes there are bugs, 8-year-old Lucas, said, and Devin, 14, notes that they've seen squirrels. But Goodman said he does his best to clean out the critters and keep up with maintenance, which has been minimal. So far, only the rope railings on the stairs have needed to be replaced, he said.

Goodman said the treehouse, which cost between $20,000 and $25,000, was a worthwhile investment. (In Breck's experience with Tree Top Builders, this is around the average amount people spend when building a treehouse.)

The kids “use it every time a new friend comes over,” Goodman said. “That's when it's really the coolest thing in the world.”

But the process wasn't without hiccups. Just a week before work started, the tree Goodman was looking to use fell to the ground. Builders had to assess another tree to see if it could support the project.

Luckily, the tree that was used has remained sturdy for nearly six years.

“It was a lot of fun to build with the kids. It's something they certainly won't forget.”