This paper presents a quick method for estimating the lateral stiffness of building structures, including regular and irregular moment frames, braced frames as well as frames with shear walls, which can be used for preliminary analysis and especially final check purposes. The method can be utilized for the calculation of the building displacement at different levels under lateral loads, the contribution of various lateral resisting systems to carrying the lateral loads, and finally the natural frequencies of the system. The basic idea of the method is based on some facts about the lateral deformation and stiffness of building structures, which make it possible to consider an equivalent single-bay single-story frame module for every story of the real multi-bay multi-story frame. This leads to a 3-diagonal or banded stiffness matrix in most cases. Even in the cases resulting in a full stiffness matrix the proposed method does not require solving a system of simultaneous equations for obtaining the lateral displacements. Several numerical examples show the higher efficiency and precision of the proposed method in comparison with the Kan method. The use of the main concepts of the proposed method for preliminary design purposes is also possible as a secondary application.