How much space is enough space?

A family invests every possible moment into a VW-hippie-van-style travels, feeling at home everywhere. On a small plot of land they envisioned a perfect base camp: a small house, a tool for high quality life with sustainability as a key baseline. Besides being sustainable by size and price, the house also proves to be sustainable to live in. Building materials mostly come from renewable sources, energy consumption is low, rain water is used cleverly and a romantic fireplace acts as a secondary heating source.