The folks at Inhabitat have found a prefab cabana that has a 10×10 footprint and comes pre-built and only requires common tools to fully assemble. They start at $12,500 and you’ll have to wire or plumb as necessary. You can change the siding, roofing, and windows to match your home.
A few commenters have mentioned a similar and more cost-effective solution from Readymade as well as making a visit to Home Depot in the true DIY spirit. But the redwood siding and cedar shingles that are available would be costly to replicate from a big box.
Modern Cabana [Inhabitat]