My detached garage is single skin brick. Can it still be converted?

Usually yes, and it is very common in older garages. Single skin walls will not meet the insulation requirements or thermal performance expected of a habitable space, so they are either lined internally with an insulated stud build-up and plasterboard, or rebuilt in blockwork where the brickwork is poor. Lining takes some floor width, which matters more in a single garage than in a double garage conversion. At the same time we usually replace the old garage door with a new wall, and you can glaze part of it with French doors or a window so the finished building suits the appearance of your home rather than still reading as the original garage. There are examples in our project gallery.