We've converted the old shed at our house into a games room/spare room over the the course of a year.
The shed is 6m x 5m inside and the covered area outside is 3m x 5m, so it's a nice size
We lifted the roof off and insulated underneath, then screwed it back down. It sounds like a big job but with the missus on the ladder we did it in about four hours.
We laid 4mm insulation across the rafters, and I then got a tradie to install the sheeting from underneath straight onto the roof battens. We did the ceiling in 6mm cement sheeting as we are in a really high-rainfall area and didn't want to worry about Gyprock getting wet as it is still essentially a shed.
All the interior walls are insulated with 120mm insulation and lined with 9mm plywood.
We had an asbestos removalist remove the end wall so we could put in the sliding door. We had a sparky run the power inside and outside, as well as the antenna connections.
The cabinets are from Bunnings and do the job perfectly