Hi Mike,
With regards to planning permission/building regulations:
1,The building must be 5 meters from the main dwelling.
2,The building must not take up more than half of your garden.
3,The building must no higher than 10 feet with a flat roof or 12ft with a pitched roof.
4,The building CAN be built right up to your boundary if it is built from brick OR within 1 meter if built from other materials.
5,The building is used for domestic use only & not used as a dwelling.
I Told the council it was a games room & they wanted a basic sketch of the proposed building.
These are the guidelines set out by Basildon council in Essex but as far as i'm aware they are standard for most councils,the exceptions i think are for conservation areas or green belt.
My building cost approx 6k with all the first fixing (footings, bricks, blocks plaster roof etc) all done by myself except the brick laying plaster work & the felt flat roof.
The advantage i have over people who have their cinema's in their homes,is that i can have as LOUD

as i want at ANY time it's the absolute dogs doo-dahs,if you get the chance go for it you won't regret it.
Contact me here or by email if you need advice blah blah.
Hope it helps
Tim