Rocio Romero, the architect behind the LV series of prefab homes, just announced the availability of stackable prefabs. Referred to as the LV2 — a 2-story stack placed on any LV series unit, the custom add-on costs the same as regular LV series units. Rocio Romero has sold over a hundred LVs and says the average cost to build, including the kit, shipping, foundation, and finish costs, is about $120 per square foot (not including land).
LV series prefabs come with a number of green elements and options, including well-insulated walls and roof, energy efficient lighting, ventilation, solar, and anything else that a homeowner wants in their green home. It's really up to the purchaser.