This study investigates how three secondary subject teachers think about and take account of learner diversity in their lesson planning, enactment and reviewing, with a specific focus on pupils designated as having special educational needs (SEN). The study is significant in the context of international moves in contemporary school teaching towards greater personalisation and inclusion. Focussing on three different subject teachers whose lesson planning was nominated as high quality, the study uses a sequence of interviews and lesson observations alongside the analysis of lesson planning artefacts, to present three in-depth cases and, through cross-case analysis, to develop a provisional situated model of lesson planning for diversity. Different forms of planning are identified -- formal, personal, expanded and in-flight -- which differ with regard to dimensions of purpose, formality, whether recorded, and timing. The teachers understandings of diversity are broader than differences related to SEN, and this relates to different kinds of differentiation, connecting the concerns of special needs education with wider issues of lesson planning for diversity.