The thesis explores the possible use of parametric object definitions during capacity analysis to support Open Building design processes.The study proposes that design criteria regarding possible size, position and relation of design elements can be formulated and modeled parametrically. Then developed parametric data can be used as library objects during the exploration of dwelling unit layout alternatives. Parametric models, holding explicit design information can be shared, modified and re-used in different design cases. The process and criteria used in the study are based on S.A.R. (Stichting Architecten Research) methods described in the study, "Variations - The Systematic Design of Supports" focused particularly on residential building types. Parallel to the S.A.R methods, the study focuses on the spatial capacity analysis between a floor plate and a number of alternative dwelling unit layout arrangements. Other capacity analyses such as structural, daylight or thermal performances can be formulated and studied in a similar way, but are not included in this study.GDL (Geometric Description Language), a programming medium for ArchiCAD software, is used for the production of parametric models. The Keyenburg housing project designed by Dutch architect Frans Van Der Werf is taken as a base-building model to demonstrate the development and the use of parametric models.Keywords: Open Building, capacity analysis, parametric objects, design constraints, GDL (Geometric Description Language) / Department of Architecture