Objective To assess whether the START (STrAtegies for RelatTives) intervention added to treatment as usual is cost effective compared with usual treatment alone. Design Cost effectiveness analysis nested within a pragmatic randomised controlled trial. Setting Three mental health and one neurological outpatient dementia service in London and Essex, UK. Participants Family carers of people with dementia. Intervention Eight session, manual based, coping intervention delivered by supervised psychology graduates to family carers of people with dementia added to usual treatment, compared with usual treatment alone. Primary outcome measures Costs measured from a health and social care perspective were analysed alongside the Hospital Anxiety and Depression Scale total score (HADS-T) of affective symptoms and quality adjusted life years (QALYs) in cost effectiveness analyses over eight months from baseline. Results Of the 260 participants recruited to the study, 173 were randomised to the START intervention, and 87 to usual treatment alone. Mean HADS-T scores were lower in the intervention group than the usual treatment group over the 8 month evaluation period (mean difference −1.79 (95% CI −3.32 to −0.33)), indicating better outcomes associated with the START intervention. There was a small improvement in health related quality of life as measured by QALYs (0.03 (−0.01 to 0.08)). Costs were no different between the intervention and usual treatment groups (£252 (−28 to 565) higher for START group). The cost effectiveness calculations suggested that START had a greater than 99% chance of being cost effective compared with usual treatment alone at a willingness to pay threshold of £30 000 per QALY gained, and a high probability of cost effectiveness on the HADS-T measure. Conclusions The manual based coping intervention START, when added to treatment as usual, was cost effective compared with treatment as usual alone by reference to both outcome measures (affective symptoms for family carers, and carer based QALYs). Trial Registration ISCTRN 70017938