Cost overruns (COs) in public sector social housing programs (PSSHPs) will continue unabated without immediate intervention to resolve the ad hoc amalgamation of root cause ontologies. This study applied a fuzzy synthetic evaluation (FSE) multicriteria decision support model to evaluate and prioritize the overall cost overrun risk in PSSHPs. In a Caribbean Small Island Developing State (SIDS) context, a questionnaire survey measuring 41 perceived critical risk factors (CRFs) was used to derive the risk impact and normalized impact values from 150 professional participants. Twenty-two normalized CRFs and four principal risk groups’ (PRGs) membership functions were determined using fuzzy synthetic knowledge-based inference procedures. Politically linked contractors and intentionally inferior building contracts caused most PSSHP cost overruns. The FSE risk model reveals that political PRG is the major root cause of COs, followed by socioeconomic, technical, and psychological PRGs. The study found that the overall risk level (ORL) of PSSHPs is high, meaning that government investment in public housing is a high-risk endeavor and ill-informed choices could lead to “wicked” social problems. Defuzzifying cost risks provided deeper insights into the nature and scope of the risk and related management implications that lead to unsustainable decision-making practices and provide opportunities for adequate control strategies to maintain economic viability. This research evidenced the fusion of different CO ontologies and raises awareness of the hidden political risk factors that should be prioritized to achieve sustainable PSSHP decision-making practices.