Building Regulations not required if floor area less than 30m sq and garage is materially incombustible i.e.brick/block walls,tiled/felted flat roof.
If it contains a controlled fitting i.e.drainage or fixed heating Building Regulations will be required.
Planning Permission:If the building doesn't face a public highway and is more than 5m from dwelling you can cover up to 50% garden area.Max height 3m flat roof,4m pitched roof.Check with Local Authority Planning Dept in case you're in conservation area,AONB,listed building,etc.
Any other queries mail me or phone 07957-274348 Andy Laight