A rough method of assessing the cost of refurbishment work is to apply a percentage of the work to be done to the cost of a new build. For instance, assume in your case that the foundations, external walls and internal partitions are okay and these elements are approximately 40% of the value of the new work. Taking a figure of £800 per m² for a new build, you can see that the cost of upgrading is 60% of £800, or £480 per m². If the floor area of the house is 180m², the refurbishment work should cost 180 x £480, ie. £86,400 – plus the cost of new fencing, paths etc. if required. This is a rough and ready method of calculating refurbishment costs, but at least it gives you a budget figure to work on. The best way, of course, is to obtain three firm estimates from different builders.