100% right, Tweaky. One of my friends has been building, installing and updating studios - from project to pro - for the past 20 years (plus general acoustics and soundproofing advice). He's had to give up recently, partly because of his physical health and partly because people just weren't paying their bills. But making somewhere soundproof can be very expensive indeed. The biggest reason is that people buy a new property thinking that 'this room/outbuilding would make a nice project studio space' without calling in an acoustics consultant first. That's when they find their $8k/$12k/$20k/$40k etc. budget for studio modification building works suddenly needs to triple or quadruple because the space just isn't suitable for the purpose. A lot of people seem to chose rooms that aren't on the ground floor (a lot in attic spaces) and often directly adjoining neighbour's properties (especially in London, where there are a lot of older large terraced or semi-detached houses). They haven't really given any thought to the amount of soundproofing required (almost always requiring room-in-room construction as Tweaky said) and the associated weight of such a construction - often requiring major structural support works. Sometimes the buildings are so unsuitable that the resulting studio space would be so small once constructed as to make it impractical to work in - so no studio gets built and the clients end up moving again.

As Tweaky says, power is important, and you'd really want to run separate circuits with separate earths for equipment power and lighting/AC loads. Running in spare cables (of all sorts) whilst the build is going on is so much cheaper than having to retro-fit them later.