Treehouses have come a long way from a few planks nailed to a tree with "no girls allowed" scrawled across one of the boards. In fact, the best ones take quite a bit of fresh engineering. Take this treehouse, for instance, on one of the San Juan Islands in Washington. At 1140 sq ft, with several desks, a lofted bed, plumbing, electricity and all the fixings, it's a house like any other. But it's also a house that needs to accommodate the Douglas fir trees it's built on, and trees sway and grow.

There's not a thing about this loch-side dwelling that isn't scenic.

Bill Allen of Forever Young Treehouses has made a name for himself building handicap-accessible treehouses

the Longwood Gardens abode in Pennsylvania looks more like a cathedral than anything else.