Here are a few truisms about tiny homes: They are ecologically friendly. They are generally inexpensive. They can help avoid house debt. They reduce one's carbon footprint. They offer the possibility for a freer, simpler life.

Plus, they're cute.

And now, tiny homes have something else: a coolness factor. Architectural innovation has become part of the tiny home movement, with some of today's top designers testing the boundaries of imagination and possibility, transforming ultrasmall spaces into marvels of eco-sustainable, microminimalist design.

Just look at what a tiny home can be: A "free-spirit" tree sphere, hanging above the forest floor. A steel hut constructed of salvaged car parts. A plastic "loft cube" that can be airlifted by helicopter. A Space Age microhouse with round, rotating rooms. A diamond-shaped glass house suspended on a pole.

"There are a lot of architects today who are pushing the envelope," said California architect Cate Leger. "We're among them. It's fun."

Leger and husband Karl Wanaselja, founders of Leger Wanaselja Architecture, are "green builders" who recently designed a residence in Berkeley, Calif., the McGee House, that utilized car windows and 100 salvaged car roofs. Needing a storage shed during construction, the team quickly slapped together a metal hut-like structure using some of the leftover car parts, and the result was unexpectedly appealing. It looked futuristic yet primitive, like a prop for a science fiction movie. Leger and Wanaselja were intrigued.

"In the next five years or so, you'll see us turn that idea into tiny house architecture," Leger said. "I believe tiny homes are the future. Or they should be. It's hard to tell people you have to live small. But building ecologically responsible housing is essential to our survival as a species."

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Many architects agree, and are putting that idea to work. Tiny homes are being retrofitted with ecologically conscious features like solar panels, rainwater collection systems and compost waste management systems. Some even have state-of-the-art internal heating and cooling systems that can be digitally controlled by laptop computers or smartphones.

Some of the most cutting-edge design is happening outside the U.S., such as in Tokyo, the world's most populated city and one of the most expensive. There, living tiny is often not a choice or a philosophy but a necessity. Radical tiny homes abound there, such as the microcompact Paco House cube or the bullet-shaped Lucky Drop House, which is only 30 inches wide at its narrowest point. Even tinier housing possibilities include portable Tricycle Homes that can be joined together like Legos, or prefabricated Pop-Up Houses.

Most extreme of all may be Japan's so-called coffin apartments—mausoleum-like lockers barely big enough for a narrow mattress and a few belongings (that rent for $600-$1,000 a month.)

Denmark is also at the forefront of the tiny home movement. A futuristic structure called Primeval Symbiosis, an ultra-high-tech glass dwelling suspended on a pole above the forest floor, was designed by Konrad Wojcik, with an accompanying promotional video that asks, "Could this be a first step in the direction of resolving the ecological problems of the planet?"

"Wojcik's tree house is a good example of architecture students who have cast off the rule book," countered Michael Janzen, 46, a corporate Web designer in Fair Oaks, Calif., and the founder of tinyhousedesign.com. "It's extremely expensive to put in photovoltaics and all those high-demand appliances. I don't think going the high-tech route is sustainable. Sure, people like to share these ideas—I like to share them on the blog—but when the rubber hits the road, can you really make a glossy house like that happen?"

Architect Jordan Parnass concurs. He notes that there are simpler means to an eco-friendly lifestyle.

"You see a lot of tiny homes that consciously avoid making a great impact on the environment, that aim to be off the grid," noted Parnass, principal at Jordan Parnass Digital Architecture in Brooklyn, N.Y. "The passive house movement is something we're getting into now. It's a progressive way of leaving a compact footprint."

Because a tiny home can be anything you want it to be—a refurbished shipping container, a yurt, an "earthbag dome"—its cost is hard to quantify. Some businesses, like the Tumbleweed Tiny House Co., offer basic, ready-made starter homes starting at $57,000 ($433 a month) as well as DIY workshops.

Cheaper options are available, however. The "OTIS" house (optimal traveling independent space) for example, designed by students at Green Mountain College in Vermont, has features like a rainwater catchment system, composting toilet and a solar-powered electrical system, and still only costs $8,000-$10,000.

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Many owners of tiny homes say customization is the secret to living happily in tight quarters. Intricately designed interiors that utilize every square inch of space, with moveable walls, telescoping furniture and beds suspended from the ceiling, offers state-of-the-art, eco-conscious living with the maximum cool factor.

But for aficionados of tiny homes, a houseboat or a treehouse may be the ultimate fantasy. Both minimize the home's environmental impact (because the earth is undisturbed by the home's construction) and allow the inhabitants to live harmoniously with nature.

Not to mention, living on water or in the treetops satisfies many a childhood fantasy.

Among the world's most ingeniously designed, unique tree homes are the Chudleigh family's Free Spirit Spheres on Vancouver Island, British Columbia. Inspired by Jacques Cousteau's round, steel deep-sea diving submarine, "Bathysphere," Tom Chudleigh, 62, custom builds uni-room, round dwellings that hang from trees.

An artist with a background in biology, boat making and engineering, Chudleigh handcrafts the beds, chairs, tables and circular windows so they fit precisely into the curved space. One, the "wellness sphere," even contains a folding massage table concealed under the floor.

All the spheres are unique, measuring between 8 feet and 10 feet in diameter, and have names like Eryn, Eve, Melody and Gwynn. They are currently being rented out as vacation homes, though Chudleigh and wife, Rosey, envision a future "populated with spheres, connected by rope suspension bridges."

Chudleigh said the spherical shape of his creations, while difficult to construct, is an important part of his vision

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"Spheres have a healing effect on people. Walls separate people, while spheres embrace the concept of unity. Our spheres are at one with nature, moving with the trees."

It may be hard to imagine a future where "free spirit spheres" are a standard housing option, but Chudleigh said he's open to the possibility.

After all, the sky's the limit in the future of tiny home design.