BACKGROUND: Urban environments impact negatively on the risks of non-communicable diseases and perpetuate health inequalities. Against this, law could play a critical role, notably through implementing and securing visions of health and well-being, and evidence-based interventions. METHODS: Seven teams conducted 123 interviews with 132 actors in urban planning in England. Teams had expertise in urban planning, transport, real estate, public health, public policy, administration, and management. An additional team with expertise in law analysed data from all interviews to explore how the law is perceived and used to promote health in urban planning. RESULTS: Six issues were identified as preventing actors from using the law to improve health in urban planning: (i) density and complexity of the law; (ii) weak and outdated regulatory standards; (iii) absence of health from legal requirements in the decision-making process; (iv) inconsistent interpretations by actors with competing interests; (v) lack of strong health evidence-based local planning policies; and (vi) inertia of the law. CONCLUSIONS: The legal determinants of health listed in the Lancet-O'Neill Commission's report need to be strengthened at the local level to effectively deploy law in English urban development. The findings call for strong, evidence-based local planning policies and decision-making frameworks, placing health as (one of the) core value(s) of urban planning and showing what types of development benefit health, i.e., prevent NCDs risks and reduce health inequalities on the long term. The legal capacity of local government should be strengthened to empower decision-makers in shaping urban development that promotes health for everyone.