Ground-based gravimetry is increasingly used to study mass distributions and mass transport below the earth surface. The gravity effect of local water storage variations can be large and should be accounted for in the interpretation of these data. However, the effect of hydrologic mass changes in the immediate vicinity of the gravimeter is not considered in standard routines for separating unwanted signal components. This applies in particular to the effect of the buildings in which gravimeters are installed. The building shields the underlying soil from precipitation and evapotranspiration and thus directly affects the water storage dynamics in the near-field of the gravimeter. A combined approach of in situ soil moisture observations and hydrologic modeling was used to quantify the altered water storage variations below observatory buildings. Subsequently, the errors caused by different estimation approaches for this umbrella effect in hydrogravitational computations were assessed. Depending on the site characteristics, the errors range from 4.1 to for the intra-annual amplitude when natural soil moisture data are considered for modeling the umbrella effect, and they range from 4.1 to when assuming no gravity change within 5 m below the building. These results were condensed to general recommendations, leading to a new simple and broadly applicable method to reduce observed gravity data for building effects, given basic information about the gravimeter location, building dimensions, climatic regime, and soil type of the observation site. This new reduction approach indicates errors of the intra-annual amplitude from 1.9 to .