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Extension

We are looking at purchasing a property with a an existing dwelling- a small bedsit, we are interested in building on two possibly three bedrooms, what kind of cost are we looking at for the house extension, there is flat area next to the house suitable for building, to the front of the house it is sloping contour. We would be wanting Two Rooms size of approx 3mx3m? Only wanting the basics, There is walk on access only.