This paper discusses the basic principles of a geometric method to aid the design process of residential buildings. It makes part of the initial phases of a research whose aim is to develop a computer system to aid the sketching and evaluation of floor plant design of multi-storied residential buildings. The fundamental idea of the research is the existence of some basic patterns of floor plants that reflect the designer’s mental models in this category of building. The models are regarding the usage of the space such as forms and dimensions, the elements for the circulation and the external skins. During the design process, architects work on each one of these models to generate the sketches of the floor plant layout. Generally, the layout of an apartment in multi-storied buildings depends basically on the internal dynamics of the users without the complex relationship with the neighborhood environment as in the case of houses. In this way, it would be easier to identify, to organize and to associate the mental models of multi-storied buildings on geometric basis, which, in their turns, might be effectively used as inputs for the layout planning of new design. By applying the geometric basis, the architects may reduce the universe of feasible alternatives into a small group of heuristic solutions that can be described by using few simple guidelines. In addition to this, the geometric bases of the existing buildings might be used to build a knowledge-based system to aid the architectural design. The objective of this paper is to show some initial results of the research obtained from a survey and the case studies of form, dimensions and topology of existing buildings. To limit the scope of the discussion, only residential buildings with two to three bedrooms are considered. At first, a survey of plants of residential buildings with two and three bedrooms, in Brazil, is carried out. In the next step, the dimensions, shape, external skin perimeter, circulation system and accessibility are analyzed. Finally, typical topologies of the building are investigated.