Scholars have recommended various strategies to combat land vacancy in shrinking cities. Side yard programs, in which adjacent homeowners purchase vacant lots, represent one such solution. We use the case study city of St. Louis, Missouri to evaluate this approach’s potential for reducing residential land vacancy. The analysis reveals that while demand-side issues (i.e., affordability) exist, the supply-side barriers (i.e., restrictive guidelines and inequitable or illogical pricing structures) are the larger constraints for the program’s success. In St. Louis, the program as currently structured could find buyers for approximately 10.8% of vacant residential parcels if all eligible buyers were interested. Through comparison of policy scenarios, we conclude that program policy significantly influences a program’s potential success, through a range of mechanisms including: restrictions regarding buyers’ owner-occupancy status, side yard lot width maximums, and pricing structure. State legislation regarding tax foreclosure auctions and elements of urban design also influence program effectiveness.