Audio Room...prefab shed?

I thought I would just ask, although I don't expect too many replies.

I was thinking about extending a portion of my house out about 20 feet but unfortunately one could only access this room by walking through a bedroom. Then I thought what about building a detached 20x14 room. And then someone who isn't an audiophile asked if I had ever thought about a prefab shed. I looked into this and there are some really nice sheds out there, e.g. cabinfever and modern-shed.

I'm curious, has anyone gone this route? I see several issues like slightly pitched roof, a lot of windows (I assume ventilation would be an issue too) and the wood panels would need treatment. I could lay a cork floor myself which would dampen some of the liveliness of a near empty space.

I called a couple of these companies and they said that they have built studios, but I'm sure they didn't consult the Master Handbook of Acoustics or take any measurements. Any opinions?

Thanks!