In India, 59% of urban slums are “non-notified” and lack legal recognition by the government. Data on 2,901 slums from four rounds of the National Sample Survey spanning nearly 20 years is used to assess the relationship between a slum’s legal status and the severity of deprivation in access to basic services, including piped water, latrines, and electricity. There is a progressive reduction in deprivation the longer a slum has been notified. These findings suggest that legally recognising non-notified slums, and targeting government aid to these settlements, may be crucial for improving health outcomes and reducing urban disparities.