According to the new Ethiopian Standard Code ES EN 1998-1:2015, the Lateral Force Method (LFM) of analysis is allowed for building that are regular in plan and elevation. Building not regular in elevation can also be analysed by LFM if the fundamental period is less than 2 seconds or 4𝑇𝑐. The preferred method of analysis for irregular buildings is the Modal Response Spectrum (RSM). In this thesis two sets of parametric investigation is conducted on a total of 20 models by varying the stiffness and mass of the deferent storys. The buildings are analysed by LFM and RSM method of analysis and the percentage difference in the response were calculated. The results obtained from the study revealed that for the same percentage variation of stiffness and mass (model 10 of stiffness case study and model 7 of mass case study) the story response of mass model is higher than the stiffness case study.