Demographic characteristics vary between ethnic groups, as a function of cultural preferences and norms, consequences of immigration, compositional differences in age structure, and discriminatory practices. The differences are of policy relevance, but one should not expect a single pattern of convergence, persistence, or divergence. This paper's key distinction between child and adult contributions to household size allows an explanatory account that recognises both fertility and traditional preferences for extended family households. Aggregating small area populations allows the statistical analysis of a demographic indicator across ethnic groups in relation to both area composition and time. Convergence over time of the number of children per household is interpreted as a fall from high fertility immediately after immigration. Stable high numbers of adults per household among Indian, Pakistani and Bangladeshi groups suggest continued demand for larger houses. These two patterns for England and Wales from 1991 to 2001 are confirmed with survey data for 2010. There is relatively little relationship between household size and local co‐ethnic concentrations. Copyright © 2013 John Wiley & Sons, Ltd.