Given the difficulties in identifying suitable sites for the establishment of new sanitary landfills, the extension of existing sites is considered more and more often as a solution. This increasing interest is also linked to the possibility of increasing the waste storage capacity and using the operating infrastructures over a longer period, thus allowing savings. Beyond the benefits associated to this rather new landfill concept, it is necessary to carefully consider the integrity of the liner system considering the important settlements (reactivated under the new cell load) that can develop. Indeed, the settlements can alter the sealing and the drainage functions of the liner system. An iterative method inspired mainly from the Incremental Settlement Prediction Model (Olivier, 2003) was developed to look deeper into this issue. Applicable to various geometric configurations, this method helps designing the barrier reinforcement using high-performance geogrids. In addition to this aspect, the proposed methodology also helps designing bottom liner system slopes to guarantee free drainage and avoid leachate accumulation.