Now what we could do with the garden is bring bulldozers in and dig large terraces. Then build retaining walls to support the terraces. Then use the extra soil as the building blocks for a guest house, and the back walls of a passive solar greenhouse. What do you think? The costs would be lower, and you use the matrial that you have (extra soil from the hill).
HERE is a web site that takes you step by step on how to build an earth bag small dome house. This would be perfect for our guest house.