The Royal Institute of Chartered Surveyors (RICS) gives a guide cost of between £1,260 and £1,680 per square metre for an extension with plastered finish. So they estimate that the shell of a 20-square metre, single-storey extension might range from £25,200 to £33,600 – that’s for base, walls to plaster finish, tiled roof, one radiator, one ceiling light and 2 double sockets. You then need to put down flooring and add extra costs for, say, roof lights, bi-fold doors or installed kitchen. Remember also to allow a further 10-15% for professional fees (architect, planning application, building regs, structural engineer) + VAT.
How about a double storey extension with the same footprint?
As a rule of thumb you can add 50% extra to the build cost of a single storey extension. So, £25,200 + 50% = £37,900. Add on 10-15% for professional fees (architect, planning application, building regs and structural engineer) + VAT suggests a Double Storey Extension = £52,302.
At Xtenda, however, we cover the cost of our Architectural fees and Structural Specifications. We also provide other extras, from time-to-time, such as additional ceiling lights and wall sockets within our fixed price quote. A typical loft or garage conversion should cost from £900 to £1,400 per square metre, but this can easily rise to £400 a cubic metre depending on the design.