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.
Moreton-in-marsh
Moreton-in-Marsh (population 3,493) is a small market town in the Evenlode Valley, within the Cotswolds Area of Outstanding Natural Beauty in Gloucestershire, England. The town stands at the crossroads of the Fosse Way Roman road (currently the A429) and also the A44. It is offered by Moreton-in-Marsh railway station on the Cotswold Line. It is reasonably flat and also low-lying compared to the surrounding Cotswold Hills. The River Evenlode rises near Batsford, runs around the side of Moreton and meanders in the direction of Oxford, where it streams right into the Thames simply east of Eynsham. Just over 1.5 miles (2.4 kilometres) east of Moreton, the 4 shire rock marked the limit of the historic counties of Gloucestershire, Warwickshire, Worcestershire and also Oxfordshire, up until the re-organisation of the county borders in 1931. Since then it marks the meeting point of Gloucestershire, Warwickshire and Oxfordshire. Moreton is originated from Old English which indicates "Farmstead on the Moor" and "in Marsh" is from henne and mersh suggesting a marsh made use of by birds such as moorhens. An alternative tip is that 'Marsh' is a corruption of 'March', early English for border.