Projections of the ethnic composition of the UK population presage radical change in the future at both national and local levels. The first question that users of such population projections ask is ‘what is driving the results?’ The answer can be found in the assumptions made for future mortality rates, future fertility rates, future migration rates and flows, and the age–sex structure of the starting populations. But it is difficult to disentangle the component effects and demographic momentum. We adapt a methodology proposed and used by Bongaarts and Bulatao in 1999 (Population and Development Review 25 515–29) by extending to national, subnational and ethnic group projections, applying it to projections of the ethnic group populations in English local authorities. We assess the roles played by immigration assumptions (the subject of continuous public debate), fertility assumptions, mortality assumptions and internal migration assumptions together with the role of the existing population age structure of each group in each area. Our findings are that positive immigration assumptions contribute to population growth in all local authorities, below replacement fertility lowers the population everywhere, declining mortality compensates for missing babies but substitutes elders and internal migration has very different effects depending on the local authority. Ethnic groups vary enormously in terms of the contribution of the current age structure. The demographic momentum of the White groups produces population declines while it is the most important demographic driver for some but not all of minority ethnic groups. The paper reports on the methods, assumptions and results associated with a systematic set of ‘what if’ projections. These enable us to determine the demographic drivers of future ethnic population change for England's local areas over the time horizon 2001–2051.