West Antarctica is underlain by a laterally heterogenous upper mantle, with localized regions of mantle viscosity reaching several orders of magnitude below the global average. Accounting for 3-D variability in upper mantle structure in glacial isostatic adjustment (GIA) simulations has been shown to significantly impact the predicted spatial rates and patterns of crustal deformation, geoid and sea-level changes. Uncertainty in constraining the viscoelastic structure of the solid Earth remains a major limitation in GIA modeling. To date, investigations of the impact of 3-D Earth structure on GIA have adopted solid Earth viscoelastic models based on global- and continental-scale seismic imaging with variability at spatial scales >150 km. However, regional body-wave tomography shows mantle structure variability at smaller spatial scales (~50-100 km) in central West Antarctica (Lucas et al., 2020). Here, we investigate the effects of incorporating this smaller-scale lateral variability in upper mantle viscosity into 3-D GIA simulations. Lateral variability in upper mantle structure at the glacial basin scale is found to have a significant impact on GIA model predictions, especially in coastal regions undergoing rapid ice mass loss. For example, incorporating a transition from lower viscosity at the mouth of Thwaites Glacier to higher viscosity further upstream impacts the predicted rate and pattern of solid Earth deformation and sea-level change in response to ongoing and projected ice mass loss, with possible implications for the evolution of the overlying ice and the interpretation of geophysical observables.