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.
Dunbeath
Dunbeath is a town in south-east Caithness, Scotland on the A9 road. It was the birthplace of Neil M. Gunn (1891-1973), author of The Silver Darlings, Highland River etc., much of whose novels are embeded in Dunbeath as well as its Strath. Dunbeath has an extremely rich archaeological landscape, the site of countless Iron Age brochs and a very early middle ages reclusive site (see Alex Morrison's historical survey, "Dunbeath: A Cultural Landscape".) Of Dunbeath's landscape, Gunn composed: "These little straths, like the Strath of Dunbeath, have this intimate elegance. In boyhood we learn more about every square backyard of it. We incorporate it physically as well as our memories hold it. Birches, hazel trees for nutting, swimming pools with trout as well as an occasionally noticeable salmon, river-flats with the wind on the bracken and also disappearing rabbit scuts, a wealth of wild flower and small bird life, the rising hawk, the unexpected roe, the old graveyard, ideas of the individual that once lived much inland in straths and hollows, the past and also the here and now kept in a moment of day-dream." ('My Bit of Britain', 1941.). There is an area museum/landscape interpretation centre at the old town institution.