Background Co-designing healthcare interventions is increasingly recognized as an effective and collaborative approach to ensure contextual relevance and user engagement. This participatory method actively involves end-users from the outset, integrating their experiential knowledge into the design and implementation of health innovations. Despite its potential, co-design remains uncommon in primary care interventions supporting family caregivers of people with advanced chronic illness—a group facing substantial emotional, physical, and social challenges. Objective To describe the application of the Double Diamond model to co-design the Home-based pErson-centered care to Listen and suPport Family caregiving in advanced disease (HELP-F) intervention; a nurse-led, person-centered programme aimed at empowering and supporting family caregivers providing home care to relatives with advanced chronic illness. Methods The co-design process followed the four stages of the Double Diamond model. The Discover phase involved narrative interviews with 24 participants caring for relatives with advanced chronic illness. The Define phase integrated these insights with a systematic review and meetings with experts from the University of Gothenburg Centre for Person-Centered Care to create the intervention’s logic model. The Develop phase included a focus group with primary care nurses, physicians, and a caregiver representative to refine the intervention’s structure and delivery. The Deliver phase will pilot-test its feasibility, acceptability, and preliminary effects in eight primary care centers in Navarre, Spain. Findings Participants emphasized the need for a person-centered intervention acknowledging each caregiver’s perspective and lived experience. The co-design process fostered mutual learning, shared decision-making, and understanding between caregivers and professionals. Consequently, HELP-F emerged as a comprehensive, person-centered intervention addressing emotional, physical, social, and existential aspects of caregiving. Conclusions Applying the Double Diamond model enabled the development of a feasible, person-centered, and contextually grounded intervention. The co-design process enhanced caregiver empowerment, professional collaboration, and ownership of the HELP-F intervention. Registration: NCT07184216; ClinicalTrials.gov