One of a series giving practical advice on methods of reducing radon levels in existing dwellings, based on a large body of remedial work carried out to advice given by BRE, and on discussions with others working in the field. It is intended to help surveyors, builders and householders who are trying to reduce indoor radon levels in houses with cellars or basements. It describes measures applicable, in principle, to all dwellings and other buildings of a similar scale and construction which have cellars or basements. Contact details of other organisations are listed. It assumes that radon measurements have been made in the building and that the annual average indoor radon level was shown to exceed the action level.