What about ''permitted development'' ??
I'm just looking into this for a garage that I'm going to build! My take on this goes thus: You are allowed to build an 'outbuilding' within 'permitted development'' rules IF -- 1:It does not cover more than 50% of your garden space , 2: Is more than 5 metres from your house , 3: Is not more than 3 metres high (for a flat roof),or not more than 4 metres high (if it's a pitched roof) i.e^. Note that there is NO limit on the actual plan size,other than the 50% rule ! You would of course need 'building regs' approval . This is purely to ensure that you build it to an acceptable standard.

My garage will be 36 ft long x 30 wide (1080 sq ft,if my maths is correct).

Hope this helps!