It depends on the size and type of extension. Small porches or flat roof extensions are very reasonably priced. More substantial two storey extensions can be expensive. In general, the price is roughly proportional to the amount of space you add to your home.
Brockenhurst
Brockenhurst is the largest village by population within the New Forest in Hampshire, England. The nearest city is Southampton some 13 miles (21 kilometres) to the North East, while Bournemouth is also nearby, 15 miles (24 kilometres) South West. Surrounding towns as well as villages include Beaulieu, Lymington, Lyndhurst, as well as Guide. The western part of the village substantially increased in the 1970s and, in the very early 1990s, Berkeley Homes constructed Ober Park, which is now known as The Coppice, this in spite of having actually been referred to as Staffs (or variations thereof) from the 13th to 19th centuries. A lot more building of the town still proceeds today by Pennyfarthing Homes. Brockenhurst is twinned with Pont St Martin, in Loire-Atlantique, France and has a population of 3,552 people (at 2011).