There have been a number of recent advances in probabilistic assessment of seismic‐induced losses for individual buildings. However, the possible losses for a portfolio of buildings are of interest for insurance and reinsurance companies, developers, and policy makers. Probabilistic estimates of earthquake‐induced losses to portfolios of buildings require quantifying correlations between losses of the different buildings comprising the building stock. This article examines spatial correlations in building seismic responses, showing that correlations are significant for closely spaced buildings (i.e., with site‐separation distances of up to 25–65 km, depending on the earthquake) and for buildings with similar first‐mode periods. The results demonstrate that correlation patterns in building response parameters are closely linked to a linear ground‐motion intensity measure, spectral acceleration measured at the buildings’ first‐mode periods. Based on this finding, enhancements are proposed to state‐of‐the‐art methods for regional loss assessment to account for correlations in building response. These enhancements define a transformation between ground‐motion intensity and building response that preserves the expected correlation for building responses. These building responses then provide the basis for computation of earthquake‐induced losses in the regional building stock.