These are not necessarily just for the professional person who wishes to record their music. They are basically rooms which we build for customers who wish to have a 'live' area and a control, or 'chill out' area, all within the same room. We normally divide the area/room, with a thick soundproof wall, incorporating a double baffle door entry system, and ideally a couple of multi layered soundproof glass viewing panels.
Depending on the size of either the bespoke built garden room, or indeed the existing building, the control or reception area can be a fairly nice place to be, where you can see and hear the people (to a degree) inside playing their music, but still be able to enjoy not being deafened!
A lot of people install their recording gear in this area, and various control desks.
Obviously, although certain soundproof improvements and enhancements need to be made to this area, it is not necessary in most cases to fully soundproof the control room. The only time we do is if playback is going to be as loud as the live area, which as yet we have not come across.