Sustainable construction (SC) practices are crucial for minimising environmental impacts and enabling the long-term viability of construction projects. However, sustainability considerations are often lacking during early project stages in developing countries like Sri Lanka. This research aims to mandate sustainability assessment at the critical project feasibility stage by proposing sustainable feasibility criteria. A qualitative Delphi study gathered experts' insights on incorporating sustainability factors into feasibility evaluations. The findings reveal 68 minimum sustainability requirements spanning economic, social, and environmental dimensions which should be assessed during feasibility studies. Further analysis identified 35 non-legalised sustainability criteria along with measurable factors and relevant Sri Lankan authorities to mandate their evaluation. By requiring consideration of these criteria, project owners and developers can be compelled to integrate sustainability principles during initial decision-making. This research provides a vital first step towards mainstreaming sustainability practices through mandated feasibility assessments. The proposed sustainable feasibility evaluation framework with specific criteria ensures that all projects account for sustainability impacts before approval, driving improved triple-bottom-line performance.