Buildings are generally priced per square metre of internal floor space. As a comparison, a modest 2 bed terraced house is usually about 55sqm, a large 4 bed detached house could be in the region of 90sqm.
From start to finish, a reasonably straightforward straw clay building will cost in the region of £900 per sq metre excluding connection to services such as drains and electricity. There are many factors that can increase this figure - a complicated design involving curves, non-standard doors + windows, a sloping site, expensive internal fittings such as bathroom and kitchen.
Factors that can decrease the guide price are: simple design, level site and modest internal fittings. The simple rule of thumb to keep costs down is " Keep it small, keep it simple "
Generally the best value for money is a one-and-a-half storey design, involving a pitched roof with the "loft space" used in some way, possibly for storage or sleeping.
Guide price on a simple, yet beautiful, 35sqm building is in the region of £32,000.