The importance of including lateral Earth structure in the analysis of Fennoscandian glacial isostatic adjustment (GIA) is investigated using a finite volume numerical formulation. Comparing output from radially-varying 1-D Earth models and models which account for the presence of plate boundaries, lateral variations in lithospheric thickness and viscosity heterogeneities in the upper and lower mantle, we find that perturbations to present-day rates of surface deformation due to the inclusion of 3-D Earth structure significantly exceed current observational uncertainties. Predicted residuals between 1-D and 3-D Earth models may be improved with the use of a 1-D model which approximates the local depth-dependent mean of the 3-D model. However, the remaining misfit is still large enough to significantly bias inferences of Earth structure and ice history. We conclude that lateral variations at both global and regional scales must be accounted for when interpreting GPS observations from Fennoscandia.