Will this add value to my house?

Provided it has been completed by a professional company and has the correct local authority permissions in terms of planning and building regulations then a conversion will almost certainly add value to your existing home. The added space the conversion brings, together with a contemporary feel will prove more attractive to buyers than if you had decided not to go ahead with the work. A typical dormer extension for an additional double bedroom and en-suite costs from £35,000 to £45,000 but it should add 21 per cent to the value of your property according to one London estate agent at Strutt & Parker.