A proper log cabin is damn expensive and could be too hot in summer and too cold in the winter. I got a cheap one: a summerhouse with an attached side shed. I have wired in electrics to both the summerhouse and shed parts and in the summerhouse, I've put a multilayer foil insulation on the walls and ceiling, and a damp proof membrane on the floor. On top of that I'll put some moisture resistant MDF, and get some proper doors and a window, a cheap laminate floor and hopefully move my office in there. Finishing it will be a job for when it gets warmer though. Can't be arsed being out in the cold.