generally depends on a few things
requires permission if..
Above 70 cu mtr requires permission,
is less than 20mtrs from a public road footpath
you already have an extention
it is positioned nearer the road than house
usually if the garage is detatched and in the rear/side garden you wont need permission if its under 70 cu mtr
remember its alot cheaper to attatch the garage to your house than free standing
last garage i built approx 3m x 5m was around 7k + vat...not inc door for a freestanding brick garage
[Edited on 4-5-2006 by Real McCoy]