Background Septic systems are recognized as a contributor to water quality degradation. Septic to sewer conversion programs are being implemented to mitigate nutrient pollution but success of voluntary programs is dependent on homeowner participation. The purpose of this study was to identify elements of potential social marketing strategies that could advance the voluntary adoption of septic to sewer conversion programs. Focus of the Article This study used formative survey research to assess residents’ knowledge, attitudes, and perceived benefits of and barriers to septic systems conversion in Florida, USA to inform future interventions promoting this major, costly home infrastructure change that reduces wastewater nutrient contamination. Research Question This study attempts to answer the following questions: (1) Does septic system conversion status (mandatory vs. voluntary) influence homeowners’ knowledge, attitudes, and perceived benefits and barriers to septic to sewer conversion? (2) What social marketing variables are the best predictors of attitude about septic to sewer conversion? Importance to the Social Marketing Field In contrast to inexpensive, repeated habits which are the target of many social marketing campaigns, costly one-time behaviors have been minimally explored by social scientists and social marketers. This study demonstrates how social marketing research can be applied to advance the adoption of costly, one-time infrastructure improvements, with septic to sewer conversions serving as the study context. There are substantial opportunities for social marketers to support local government and other organizations in their promotion of major technologies, including, but not limited to septic system conversions. Methods 518 adult Florida residents who were also septic system owners completed an online survey instrument that assessed subjective and objective knowledge, attitudes about septic to sewer conversion, and motivators and barriers to conversion. Results Septic system homeowners are largely supportive of septic conversion programs; however, the greatest perceived benefits are personal and not associated with environmental improvements. Despite many septic to sewer conversion projects being initiated for environmental purposes, behavior change efforts should promote the benefits of conversion as perceived by the homeowner. The primary barrier is the upfront cost, and alternative funding structures should be investigated. Recommendations for Research or Practice This study indicates that target audience outreach is an essential component of the environmental planning process. Considering the costs and complexity of water quality infrastructure projects, environmental management professionals should consider a robust audience-specific social marketing program as part of the implementation and planning process. The findings presented here suggest opportunities for social marketers to partner with environmental management professionals tasked with encouraging septic to sewer conversion or other costly and significant one-time behaviors.