This study assessed whether an upgrade of playgrounds in a community was associated with changes in the physical activity of local children. The study used a natural experiment design with a local authority project to upgrade two community playgrounds as the intervention and a matched control community. Children's physical activity was measured by an Actigraph GT1M accelerometer worn for 8 days, enabling up to 6 days of data to be analyzed. A self-administered parent/guardian questionnaire was used to collect additional data, including perceptions of the neighborhood, school-travel modes, days involved in extracurricular activities, ethnicity, caregiver age, caregiver sex, household vehicle access, and household income. At baseline, 184 children (5-10 years old) participated. Of these, 156 completed the 1-year follow-up assessment (20% lost to follow-up). There was statistically significant evidence that change in mean total daily physical activity was associated with on an interaction between participant's body mass index (BMI) z-score and her or his community of residence (p = 0.006), with the intervention being associated with higher levels of activity for children with lower BMIs but lower levels for children with higher BMIs. Physical activity is not the only focus of local authority playground provision as playgrounds also have benefits for social development and fundamental movement skills. However, making sure that physical activity is always included in the design rationale and that playgrounds are designed to encourage and sustain physical activity could be a useful population health intervention. The effects of such interventions on different subgroups are of importance, especially if the effects differ over levels of BMI.