If housing markets exhibit slow adjustment to system shocks, then hedonic estimates of the price impact from environmental amenity trends may be time variant. This paper suggests an alternative to the cross-sectional model for estimating hedonic prices using an error correction approach that allows for endogenous environmental quality. The model is applied to data concerning an open space purchase program in Boulder, Colorado, and shows that the economic impact of an open space purchase takes several years to be fully realized. This observation questions using cross-sectional, hedonic models for evaluating willingness to pay for time-trended environmental amenities.