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.
Perranporth
Perranporth is a seaside hotel town on the north shore of Cornwall, England, United Kingdom. It is 1 mile east of the St Agnes Heritage Coastline, as well as around 8 miles south-west of Newquay. Perranporth as well as its 3 miles (5 kilometres) long coastline deal with the Atlantic Ocean. It has a population of 3,066, and also is the biggest settlement in the civil parish of Perranzabuloe. It has an electoral ward in its own name, whose population was 4,270 in the 2011 census. The community's modern name originates from Porth Peran, the Cornish for the cove of Saint Piran, the patron saint of Cornwall. He started St Piran's Oratory on Penhale Sands near Perranporth in the 7th century. Buried under sand for many centuries, it was discovered in the 19th century.