PurposeThere is some evidence of the benefits of physical activity (PA) in patients with lung cancer; however, there is a lack of understanding of acceptable PA for patients with established cachexia and how to facilitate sustainable behaviour change to promote PA. Therefore, this study explored the views of healthcare professionals (HP), patients with lung cancer and cachexia, and their carers on preferences for, barriers and facilitators of homebased PA.MethodsThis qualitative study involved ten telephone interviews with HPs and face-to-face interviews with seven patients with lung cancer and cachexia and their carers. Interviews were transcribed and analysed thematically. The Capability, Opportunity, Motivation and Behaviour (COM-B) model was used as a framework for the thematic cross-group analysis.ResultsThe types of homebased PA suggested by patients with lung cancer and cachexia (n= 7), their carers (n= 7) and HPs (n= 10) were functional, flexible, individualised and initially of short duration and low intensity. PA was influenced by themes within physical and psychological Capability, physical and social Opportunities as well as automatic and reflective Motivation.ConclusionBased on a behaviour change theory, principles to promote homebased PA were developed. These principles need to be integrated into tools to promote PA in people with lung cancer and weight loss.Implications for Cancer SurvivorsThe application of the proposed principles by clinicians will promote physical activity, enhancing the function and wellbeing of patients with lung cancer and reducing burden on carers.Supplementary InformationThe online version contains supplementary material available at 10.1007/s11764-023-01376-3.