depends where you are and size you want (hight mainly) may need planning permission, id google planning in your area or contact your planning office to ask them what you can and cant do which is all free, and re-ask question, ruf 5k bricky, concrete, blocks, mixer, digging, time, shutter, doors, chippie, roof, are a few of the factors

considering in your position you have a wooden garage in its place at the moment i cant see planning being a problem as mentioned above draw a rough sketch of your project contact your local planning office arrange an oppointment for a building inspector he will advice on footings depth,roof height,etc try to plan with greater detail that way your guaranteed the spec your after.regarding price you need my builder for reputable builders for quotes and inspections before works are carried out.due to foundations condition of the soil & wether the slab needs steel etc etc but you will have a better idea once your plans are shown to the building inspector regarding spec i hope this answers your question.
estimated quote from previous builds between 5k - 10k

hi paul you need to contact your local planing dept for regs how close to house hight off garage wether you need footing or slab reinforced about 5to 7000 hope this helps ian all build services 8 6 2011