I came across the cutest and smartest passive home design recently at Inhabitat that I wanted to share. This house uses only 1/10 the energy that a conventional home uses, and relies on passive solar energy and heat recovery ventilation. The structure is small and simple (1200 sq ft) but allows for a central great room with tons of light and space, and a bedroom on each end. There is even a loft for a get-away space. I love how it incorporates outdoor living on both sides. The design is by Zero Energy Design and they have other great projects in their portfolio as well.
My parents are thinking about building a home for their retirement years that will be easy to live in as they get older. A house such as this one is on a slab, has no stairs, is efficient, and isn't too big. I think this would be perfect for a downsizing option. If you have other designs that might work, please let me know!