The idea of lowering the floor level for your new extension by 800mm is an attractive one given the slope on site. However, this will mean that your excavation, if immediately adjacent to the existing house, will undermine your existing foundation in two ways. Firstly from a lateral perspective where the foundation itself will in effect become a retaining wall and, secondly, from an adjacent excavation point of view where it will need to be underpinned.

Generally the premium cost of underpinning a foundation is circa £1,000 to £1,200 per linear metre, so that will be quite straightforward for you to calculate with a bit of a premium being added if it is only a short run. But you should also budget for some form of waterproofing to the exposed side of the foundation where it has now become a retaining wall. The treatment for this will be highly dependent upon your water table, the soil type and other forms of land drainage around your building and, of course, the final elements of your design. From a budgetary perspective you can then apply your normal rates to the size of the extension which, of course, must include VAT for the whole of the works.

Tim Doherty, Build It magazine contributor