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.
Fakenham
Fakenham is a market town and civil parish in Norfolk, England. It is positioned on the River Wensum, about 19 miles (31 kilometres) north eastern of King's Lynn, 19 miles (31 kilometres) south west of Cromer, and 25 miles (40 kilometres) north west of Norwich. The civil parish has a location of 3.49 sq mi (9.0 km2) as well as in the 2001 census had a population of 7,357 in 3,292 households, the population boosting to 7,617 at the 2011 census. For the functions of local government, the parish drops within the district of North Norfolk. The name Fakenham is Saxon, potentially implying Fair Place/Place on a Fair River, or Community (Ham) on the river (Ken) Fa/Fair (Fa).