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.
Dulverton
Dulverton is a town as well as civil parish in the heart of West Somerset, England, near the border with Devon. The town has a population of 1,408. The church consists of the districts of Battleton and Ashwick which lies about 3.5 miles (5.6 kilometres) north west of Dulverton. To the west of the hamlet exists Ashwick House, built in the Edwardian style in 1901. Likewise close-by is the estate of Northmoor, previously a seat of Sir Frederick Wills,1 st Baronet of Northmoor, one of the four Wills Baronetcys, as well as the owners of the Imperial Tobacco Company. In 1929 Sir Frederick's son & beneficiary, Sir Gilbert Wills, 2nd Baronet, was increased to the peerage as Baron Dulverton, whose principal seat was at Batsford Park, near Batsford, Gloucestershire. Dulverton is a popular vacationer location for discovering Exmoor, and is house to the Exmoor National Park Authority head office. The town pushes the path of the Exe Valley Way and also Land's End Trail.