INTRODUCTION: Approximately 40% of the 55 000 women diagnosed with breast cancer each year in the UK undergo mastectomy because they are considered unsuitable for standard breast-conserving surgery (BCS) due to tumour size or multiple tumour foci. Mastectomy can significantly impact women's quality of life, and only one in four women currently undergo immediate breast reconstruction (IBR).Level 2 oncoplastic breast-conserving surgery (OPBCS) combines removing the cancer with a range of plastic surgical volume replacement (eg, local perforator flaps) and volume displacement techniques (eg, therapeutic mammaplasty) that can extend the role of BCS and may allow some women not suitable for standard BCS to avoid mastectomy. High-quality research to determine whether OPBCS offers a safe and effective alternative to mastectomy±IBR is currently lacking. Preliminary work is needed to ensure a future large-scale study is feasible and well designed and addresses questions important to patients and the National Health Service. METHODS AND ANALYSIS: Mixed methods will be used to inform feasibility and design of a future large-scale study comparing the clinical effectiveness and cost-effectiveness of OPBCS and mastectomy±IBR. It will have four parts: (1) a National Practice Questionnaire to determine current practice and provision of oncoplastic breast and reconstructive surgery in the UK; (2) a pilot multicentre prospective cohort study to explore the proportion of patients choosing OPBCS versus mastectomy, the proportion in OPBCS is successful and clinical and patient-reported outcomes of different techniques at 3 and 12 months postsurgery; (3) a qualitative interview study to explore patients' attitudes to different procedures, rationale for decision-making and perceptions of outcomes; and (4) design of the future study.All centres offering OPBCS and mastectomy in the UK will be invited to participate. Recruitment is planned to commence winter 2020 and continue for 12 months. ETHICS AND DISSEMINATION: The study has ethical approval from the Wales Research Ethics Committee 6 National Research Ethics Service (REC Ref 20/WA/0225). Results will be presented at national and international meetings and published in peer-reviewed journals. We will work with patients to develop lay summaries and share these through patient groups and breast cancer charities. TRIAL REGISTRATION NUMBER: ISRCTN18238549.