house in wales, modified to wofati

Paul's wofati article says the house in Wales, which I adore, can easily be modified into a wofati. How so? I'm assuming he meant the roof (shape and layers) and the back patio wall. Would that do the trick? Or, are we talking about the walls too?

Also, what's perspex? I'm curious about the back wall on that house. Instead of hay/straw and plaster I'm assuming you'd have to do hay and polyvinyl against the dirt. Would the hay/straw breathe well with one side plastered? I don't like to.assume....

Perspex is what they call Plexiglass in the UK