Just to add on to clubman's reply - the following is from the planning faq's on Clare Co. Councils web site (just the first one I found when researching for myself)
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Based on your measurements you will have to get planning permission. 375 sq ft = 34.83 sq metre
Can I build a garage?
You can build a garage, carport, shed, greenhouse, kennel for domestic pets etc., as long as it does not extend out in front of the building line of the house and does not exceed 4 metres in height, (if it has a tiled or slated pitched roof), or 3 metres (if it has any other roof type). The floor area limitation for exempted development is 25 square metres. The structure may not be lived in, used for commercial purposes or for keeping pigs, poultry, pigeons, ponies or horses. Garages, sheds etc. to the side of the house must match the finish of the house. You cannot reduce the open private space, reserved exclusively for the use of the occupants of the house, at the side or rear of the house below 25 square metres.