Radiotherapy has proven efficacy in a wide range of cancers. There is growing interest in evaluating radiotherapy-novel agent combinations, and a drive to initiate this earlier in clinical development of the novel agent, where scientific rationale and pre-clinical evidence for a radiotherapy combination approach is high. Optimal design, delivery and interpretation of studies is essential. In particular, design of phase I studies to determine safety and dosing are critical to an efficient development strategy. There is significant interest in early phase research amongst scientific and clinical communities over recent years, at a time when scrutiny of trials methodology has significantly increased. To enhance trial design, optimize safety, and promote efficient trial conduct, this position paper reviews the current phase I trial design landscape. Key design characteristics extracted from 37 methodology papers were used to define a road map and design selection process for phase I radiotherapy-novel agent trials. Design selection is on the basis of single or dual therapy dose escalation, dose limiting toxicity categorisation, maximum tolerated dose determination, subgroup evaluation, software availability and design performance. Fifteen of 37 designs were identified as being immediately accessible and relevant to radiotherapy-novel agent phase I trials. Applied examples of using the road map are presented. Developing these studies is intensive, highlighting the need for funding and statistical input early in trial development to ensure appropriate design and implementation from the outset. Application of this road map will improve design of phase I radiotherapy-novel agent combination trials, enabling a more efficient development pathway.