Outdoor watering of lawns accounts for about half of single-family residential potable water demand in the arid southwest United States. Consequently, many water utilities in the region offer customers cash rebates to replace lawns with drought tolerant landscaping. Here we present a parcel-scale analysis of water savings achieved by a 'cash-for-grass' program offered to 60 000 homes in Southern California. The probability a resident will participate in the program, and the lawn area they replace with drought tolerant landscaping, both increase with a home's outdoor area. The participation probability is also higher if a home is occupied by its owner. From these results we derive and test a simple and generalizable probabilistic framework for upscaling water conservation behavior at the parcel-scale to overall water savings at the city- or water provider-scale, accounting for the probability distribution of parcel outdoor areas across a utility's service area, climate, cultural drivers of landscape choices, conservation behavior, equity concerns, and financial incentives.