We have an external triple garage and are interested in converting around half to 2 thirds of it into a gym/office with possibly sink/toilet facilities. Has anyone done similar and can give me an approx cost.? I was also wondering about heating it. What is the cheapest option given electric is hugely expensive to run? The rest of the house is gas but it would take quite an upheaval to run gas pipes to it. Would a seperate system e.g air source heat pump work better? Solar is not an option due to shading.