In England and Wales you will not usually require planning permission, provided you do not exceed the permitted development limit for your property (which is 50-70 cubic metres, depending on where you live). Notable exceptions to the above are listed buildings and conservation areas. Please note that if your property has been extended in the past, you will have used up some or perhaps all of your “permitted development” area and may need permission. Its also worth noting that occasionally “permitted development” rights may be withdrawn from a property and any extension however small needs planning.
If in doubt check.