Background The Nutrition And Physical Activity Self Assessment for Child Care (NAP SACC) intervention has shown evidence of effectiveness in the USA but not been adapted or assessed for effectiveness in the UK. Objectives To evaluate the feasibility and acceptability of implementing NAP SACC in the UK. Design Adaptation and development of NAP SACC and feasibility cluster randomised controlled trial (RCT) including process and economic evaluations. Substudies assessed mediator questionnaire test–retest reliability and feasibility of food photography methods. Setting Nurseries, staff and parents in North Somerset, Cardiff, Gloucestershire and Bristol. Participants Development – 15 early years/public health staff and health visitors, 12 nursery managers and 31 parents. RCT – 12 nurseries and 31 staff, four partners and 168 children/parents. Mediator substudy – 82 parents and 69 nursery staff. Food photography substudy – four nurseries, 18 staff and 51 children. Intervention NAP SACC UK partners supported nurseries to review policies and practices and set goals to improve nutrition, oral health and physical activity (PA) over 5 months. Two workshops were delivered to nursery staff by local experts. A home component [website, short message service (SMS) and e-mails] supported parents. The control arm continued with usual practice. Main outcome measures Feasibility and acceptability of the intervention and methods according to prespecified criteria. Data sources Qualitative data to adapt the intervention. Measurements with children, parents and staff at baseline and post intervention (8–10 months after baseline). Interviews with nursery managers, staff, parents and NAP SACC UK partners; observations of training, workshops and meetings. Nursery environment observation, nursery Review and Reflect score, and resource log. Child height and weight, accelerometer-determined PA and sedentary time, screen time and dietary outcomes using the Child and Diet Evaluation Tool. Staff and parent questionnaires of knowledge, motivation and self-efficacy. Child quality of life and nursery, family and health-care costs. Food photography of everything consumed by individual children and staff questionnaire to assess acceptability. Results Thirty-two per cent (12/38) of nurseries and 35.3% (168/476) of children were recruited; no nurseries withdrew. The intervention was delivered in five out of six nurseries, with high levels of fidelity and acceptability. Partners found it feasible but had concerns about workload. The child loss to follow-up rate was 14.2%. There was suggestion of promise in intervention compared with control nurseries post intervention for snacks, screen time, proportion overweight or obese and accelerometer-measured total PA and moderate to vigorous PA. Many parental and nursery knowledge and motivation mediators improved. The average cost of delivering the intervention was £1184 per nursery excluding partner training, and the average cost per child was £27. Fourteen per cent of parents used the home component and the mediator questionnaire had good internal consistency and test–retest reliability. Photography of food was acceptable and feasible. Limitations Following nursery leavers was difficult. Accelerometer data, diet data and environmental assessment would have been more reliable with 2 days of data. Conclusions The NAP SACC UK intervention and methods were found to be feasible and acceptable to participants, except for the home component. There was sufficient suggestion of promise to justify a definitive trial. Future work A multicentre cluster randomised controlled trial to evaluate the effectiveness and cost-effectiveness of NAP SACC UK has been funded by NIHR and will start in July 2019 (PHR NIHR 127551). Trial registration Current Controlled Trials ISRCTN16287377. Funding This project was funded by the National Institute for Health Research (NIHR) Public Health Research programme and will be published in full in Public Health Research; Vol. 7, No. 13. See the NIHR Journals Library website for further project information. Funding was also provided by the North Somerset and Gloucestershire Councils, Development and Evaluation of Complex Interventions for Public Health Improvement (DECIPHer) (MR/KO232331/1), and the Elizabeth Blackwell Institute.