Singapore’s Long Island project aims to protect the East Coast, meet freshwater demands, and support urban development. It involves constructing a freshwater reservoir by closing off part of the sea using three islands and two barrages. The islands, totaling 850 ha, will be used for urban development. The project is currently in its conceptual design phase. Long Island presents several challenges. Singapore’s flood risk policy focuses on raising the platform level, increasing demand for scarce construction materials. The multifunctional nature of Long Island, providing flood protection, freshwater supply, and urban space, complicates design. Uncertainty in future sea level rise (SLR) further challenges sea defense planning and adaptation. This thesis develops a resilient conceptual design for Long Island’s land reclamation, focusing on platform level optimization and sea defense adaptability. Six reclamation variants are proposed, ranging from polder systems to conventional landfills, combined with a caisson or a dike as sea defenses. Sea defenses are designed to accommodate up to 5 m SLR and are integrated into adaptation pathways. Each variant considers reservoir dike design, effective land area, settlements, and polder pumping requirements. Designs are evaluated through capital cost analysis, lifetime cost assessments using Present Value, Multi-Criteria Analysis (MCA), and sensitivity analyses on design parameters, Social Discount Rates (SDRs) and SLR projections. The most cost-effective design combines a platform level of -4 m SHD with either a dike or caisson. This polder approach is technically feasible and reduces reclamation volumes by 80 million m3 and saves 3 billion SGD compared to a 5.1 m SHD design. Sensitivity analyses confirm its robustness under varying assumptions. Both sea defense types are adaptable and have comparable costs, though further research is needed to determine the optimal choice, including geotechnical design and naturebased integration. The MCA did not yield a clear preference due to close value-cost ratios and a lack of stakeholder validation. While technically and economically promising, the polder system’s societal acceptance and integration into Singapore’s urban context require further assessment. Future design phases should address public perception of flood risk, desirability of polder developments, and nature-inclusive coastal environments, supported by stakeholder engagement. Additional research into flood risk, the polder pumping system, and SDRs is recommended to improve the design and inform decision-makers on platform level selection. This thesis provides a technical foundation for Long Island’s next design stages and supports platform level decision-making. It also offers insights for other regions pursuing land reclamation developments, especially where unit rates are high and/or materials are scarce, demonstrating an integral optimization approach focused on multifunctionality and climate resilience.