There is a growing interest worldwide in utilising water more efficiently. This paper presents a study undertaken to explore the financial aspects of an in-building water re-use system. The financial issues are defined in terms of: capital costs, running costs, maintenance costs and water saving benefits. A model was developed to allow the user to specify factors pertinent to a net present value calculation and to map the viability space (i.e. the set of conditions required for the re-use system to be financially viable). The most important variable was found to be the annual maintenance cost. A scoring system was developed to indicate the degree of financial viability. Use of the model is explained by the application of three example cases.