The risk assessment for extreme floods is a key point to anticipate and to dimension a set of measures to improve the resilience of the territories. Because extreme floods are not observed, the calculation of hazard in the flooded areas comes from an extrapolation of models calibrated on more frequent events. The simplified approach called "Additional Depths Method" guesses the knowledge of a rare flood for which the flooded areas are considered to be accurately estimated.The limits of the extreme flooded areas are drawn starting from the water elevation interpolated between the values obtained at a set of cross sections. This simplified method is encapsulated in a plug-in of the QGIS software.