Background: While citizen science approaches are emerging within both social and health sciences, projects aimed at improving the living conditions of older adults remain rare. To enable forward-looking housing provision for the ageing population, valid and detailed information is needed on environmental barriers in the housing stock. Moreover, to promote active ageing and avoid involuntary moves to residential care facilities, there is a need for both increased public knowledge and raised awareness about accessible housing among older adults. Thus, Swedish senior citizens were engaged in a citizen science project—the Housing Experiment 2021—using a smartphone application to report environmental barriers in dwellings. Aim: This paper describes in detail the process by which varied members of the public participated to develop an application that assured reliable data collection of environmental barriers by older adults. Methods and Results: The scientific foundation for the app was the Housing Enabler. The development process comprised six iterative phases including participatory activities, namely, developing a citizen science version of the Housing Enabler; developing a print mock-up; developing an app prototype; testing and improving usability; beta version testing for reliability; final tests; and finishing touches. Discussion: Through an iterative development process involving researchers, professionals, and members of the public, a reliable app suitable for senior citizens was created. The results can serve as an inspiration for development protocols increasing the involvement of older adults in app development as well as for citizen science projects targeting older adults.