OBJECTIVES The Dose Adjustment For Normal Eating (DAFNE) structured education programme is shown to be effective both in terms of clinical outcomes and cost-effectiveness outcomes in the treatment of T1DM. DAFNEPlus aims to revise the DAFNE 5-day curriculum based on psychological and sociological findings in DAFNE, input from DAFNE graduates and emerging knowledge around behavioural science and technological developments. The current suggested primary endpoint is for the DAFNEplus programme to have an additional 20% DAFNE participants(70% in total) achieve either, (a) a reduction of at least 0.5% in HbA1c, or (b) to have an HbA1c below 7.5%(58.5 mmol/mol), at 12 months. This paper undertakes pre-trial what-if cost-effectiveness analyses concerning the DAFNEPlus programme, which aim to be useful both in the design of the intervention itself and of the proposed trial. METHODS The Sheffield Type 1 Diabetes Policy Model is an individual patient-level simulation model of T1DM. It includes long-term microvascular (retinopathy, neuropathy and nephropathy) and macrovascular (myocardial infarction, stroke, revascularization and angina) diabetes-related complications and acute adverse events (severe hypoglycaemia and diabetic ketoacidosis).  Econometric methods were used to obtain the target level of HbA1c responders in the DAFNEPlus arm. RESULTS DAFNEplus would be considered as cost-effective if the additional spending on the intervention would be limited to £455-£751 per patient per year, depending on the assumptions on the length of maintenance period for the HbA1c benefit and the target HbA1c responder endpoint (70% in total) being achieved in the future trial. To achieve a more favourable cost-effectiveness probability of 80%, for example, the additional per patient per year cost should be restricted to £393-£574 range. CONCLUSIONS Pre-trial modelling has enabled a clear understanding of the threshold range for the annual cost of DAFNEplus, which is still being designed, in order to be considered as cost-effective at the £20,000/QALY threshold.