Dreaming of a small private studio tucked behind your home, or a guest room, or a child's play space? Flexa Studio, an architect-designed 10-by-12-foot prefab structure, is an easy way to add extra space without the expense, disruption, or building constraints of an addition. All that's required is a foundation on which to bolt the structure (which comes preassembled or as a panelized kit). Designed by San Francisco architect Casper Mork-Ulnes (see Architect Visit: Mork-Ulnes in San Francisco) in collaboration with Houseplans.com, the eco-friendly structure uses FCS-certified lumber, recycled denim insulation, and prefabricated elements for minimal waste. Another great selling point: no permits. According to Houseplans, "in most jurisdictions, permits are not required for structures that are 120 square feet or less." Flexa Studio starts at $8,950 plus shipping.