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.
Dolwyddelan
Dolwyddelan is a village as well as community in Conwy county district, north Wales, on the primary A470 road between Blaenau Ffestiniog and Betws-y-Coed. As a community, the population of Dolwyddelan was recorded in the 2001 Census as 427, and also 55.8% of those homeowners might talk Welsh. A mild population increase to 474 was recorded in the 2011 census with the percentage of Welsh audio speakers falling to 50.8%. Close-by hills include Moel Siabod to the north, Moel Penamnen and Y Ro Wen to the south and, additional afield, Snowdon (Yr Wyddfa) lies about 10 miles to the west. Carreg Alltrem, a crag used by many rock climbers, can be found about a mile south of the town. Dolwyddelan is included in the Betws-y-Coed electoral ward which is represented by a county councillor on Conwy County Borough Council.