Background & aim: Children and young people (CYP) with severe, sub-optimally controlled asthma and co-existing allergic sensitisation to indoor aeroallergens, such as pet dander and house dust mite (HDM), would likely benefit from reduced allergen exposure. Multiple allergen remediation interventions exist and are often suggested to families in secondary care asthma clinics in the United Kingdom. Evidence suggests remediation uptake is low or partial but there is sparse evidence to explain why. This study aims to explain how families in this situation make decisions about home-based allergen remediations. Methods: In-depth qualitative interviews with CYP and mothers were analysed, and a grounded theory approach was used to develop a theory to explain decision-making processes and behaviours. Results: Ten CYP aged 11-15 years and 11 mothers were interviewed. The core finding was that families iteratively respond to changes in how certain they are in their asthma management decisions and actions. For allergen remediation uptake, this certainty varied depending on seeing an outcome-exposure relationship, understanding asthma severity, variability, and asthma control at the time of remediation decision-making. Understanding the mechanistic role of allergen exposures in asthma was challenging for families, and ongoing bi-directional communication with clinicians was essential in supporting long-term decision-making. Conclusion: The theory explains the often elongated, reactive process of allergen remediation decision making and implementation. It also explains other elements of family management of asthma, and their interconnections. Families’ iterative responsiveness suggests opportunities to intervene and promote earlier, preventative behaviour change.