Adaptation of existing building stock is an urgent issue due to aging infrastructure, growth in urban areas and the importance of demolition mitigation for cost and carbon savings. To accommodate the scale of implementation and address the complexity of building adaptation projects, the design decision-making process needs to improve. An early-stage design computational methodology is presented for integrating design optimization metrics, including energy use, daylighting, life cycle impact, life cycle costing and structural efficiency, to improve the quality of design options and speed of evaluation in building adaptation processes. Various design options that conform to determining design constraints are evaluated, and optimal early-stage design options are identified. The case of a modular extension to and re-cladding of a residential tower is presented for demonstration.