Planning and housing policies in the UK have recently adopted the principle of developing mixed schemes, whereby a mixing of tenures stands in for a mixing of income groups. A series of recent research studies has informed the future production of mixed income new communities. This article draws on these wider studies to consider the issue of social mixing and design within developments in detail. In particular the concept of "tenure blind" development is critically investigated with regard to three mature case studies, combining design analysis with social research. Conclusions are drawn about the dangers of over-specific prescriptions towards design and the continuing relevance of urban design theory.