The purpose of this publication is to provide advice to the regions, and others working in this area, on the quality and availability of data relating to housing affordability. It is intended to be a resource, particularly for those with less experience in understanding the housing market as well as those making decisions about planning for housing. Public Service Agreement 20 (PSA20) commits the Government to ‘increase long term housing supply and affordability’1 and recognises the lead role of regional and local planning authorities in delivering 240,000 net additional homes per year, of which 70,000 should be affordable homes In addition, Planning Policy Statement 3 (PPS3) on housing sets a new obligation on authorities to establish the impact of housing supply plans on affordability prospects for their communities over the medium and long term This publication brings together in one place information on the sources of standard data that planning authorities can use to understand their housing markets and specifically indicators of affordability. Where possible, it identifies which data sources are the most useful and / or robust at the different spatial scales. This publication includes an evaluation of the strengths and weaknesses of the relevant data; sets out recommended standard national data sets; and relates the use of the data to the main modelling approaches currently being used. The next three sections of this publication cover measuring, understanding and modelling market housing affordability. This is followed by a section on gaps and shortcomings in the available data and how to address them. Finally, conclusions are drawn about the use of data on affordability and housing markets more broadly