You would only know by getting a couple of builders round to give you estimates. You could do with someone who draws up plans first though to come round - have a look at what you need and say whether its viable or not. These people usually draw the proposed design in great detail and in line with all the local building regs. Once you have a plan - if it's all OK - a builder can then give a quote as opposed to an estimate and a planning application can be submitted.
I'd estimate at below �7500 - depending on what you need doing inside (plastering electrics, plumbing etc) - Only an estimate.