Based near Glastonbury House-Box was set up by Dean Crago in 2010 and converts old horse-boxes into tiny mobile homes (although they will also happily convert other structures including ice-cream vans and will also build from scratch). Here's what they say about their work:
"Our main areas of work include converting mobile and static 'off-grid' spaces into bespoke, warm, and technologically equipped handmade homes. As well as converting existing structures (busses, boats, dens etc.) we also build structures bespoke, whether mobile or static (tree-houses, stacked shipping containers, coachbuilding etc.)"
There is an excellently tempting gallery of their work with case studies here and a useful FAQ. House-Box also take on smaller commissions for items such as chests, staircases, and even a hidden bookcase door.