Recently published analyses of postglacial relative sea level (RSL) histories from coastal locations which are proximate to broad continental shelves that were significantly exposed land at Last Glacial Maximum, have demonstrated that the Holocene RSL variations at such sites are not adequately explained unless particular care is taken to treat the influence of shelf inundation. This is herein shown to require a local adjustment to the water load based upon a mass conservation constraint. Previously revealed misfits of theory to data from such regions therefore do not signify a requirement for lateral heterogeneity in model viscoelastic structure. The quality of the revision to the theory required to account for this broad shelf effect is illustrated through analysis of RSL histories from the east coast of the South American continent and from Northwestern Europe.