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Making an accurate estimation of peak water demand in buildings is essential for engineers and designers in order to ensure proper sizing of water supply systems, storage tanks, boilers, and booster pumps. Over recent years, the amount of potable water used in buildings has reduced considerably as a result of the prevalence of water-efficient appliances and a heightened awareness of the need to conserve water. This has, in part, led to oversizing of water supply networks; a phenomenon that has given cause for concern to those responsible for the design of building plumbing systems. This oversizing problem does not only result in a material and financial cost, it also has negative health consequences. In the UK, despite a clear reduction in consumption at end-use points, traditional design approaches are still used for determining design flow rate. Although different design methods have been presented in various British standards and guidance documents, all use the Loading Unit (LU) approach, which is based on the application of probabilistic techniques, to estimate the design flow for both residential and non-residential buildings. In recent studies, the focus has generally been on residential buildings and there has been little, if any, research to assess the validity of current design methods for non-residential buildings. This study, therefore, focuses on developing a new design approach to estimate demand flow in non-residential buildings. This research starts by providing background information on the water situation in the UK and discusses the reasons for oversizing and its consequences. Water demand is also discussed, as is water conservation, per capita water consumption and the demand from micro-components. In addition, the history of system design and the most commonly used UK design approaches are discussed. After undertaking a critical review and comprehensive investigation of statistical methods and recent studies used to estimate demand flow, a new design methodology for estimating water demand, specifically for non-residential buildings, is introduced. This has also allowed for the presentation of a new stochastic model, namely the Water Demand Estimation Model (WDEM). The model is underpinned by the interaction between users and the provision of sanitary appliances in conjunction with the generation of a comprehensive range of probabilities to capture all possible simultaneous uses of appliances. The Monte Carlo technique has been applied to calculate flow rate values based on a given number of users. A specific type of non-residential building i.e. the 'workplace' has been selected for application of the model and for which new design equations have been derived. Taking into account the water saving appliances used in modern plumbing systems, five design equations have been derived based on efficiency levels of corresponding appliances. In order to validate the model and to assess its accuracy, high quality flow rate data was gathered from three case study buildings. The effectiveness of the WDEM and its impact on the oversizing of water systems has been confirmed by comparing simulated, measured and design flow rates. The results show that the simulated demand is very close to the measured flow rate, and that its use results in a significant reduction of design flow rate compared to those determined by using current design codes. The main outcome of this study is hence a novel approach for the estimation of demand flow for non-residential buildings and a set of design equations to estimate the simultaneous demand flow rate for workplaces. This new approach will be of value to all engineers and designers who seek to establish a more accurate estimation of water demand in buildings.