||We introduce the foundations of a novel approach that deals with constraint-based design methods to supporting participatory planning processes of low-income dwellings. We examine the space allocation problem inside the architectural domain on the basis of graph theory and combinatorics, providing a concise mathematical background for an implementation strategy called FLS (Floor plan Layout Support), which is analyzed here for the first time regarding this particular context of application. The philosophy underlying a design method that is mainly driven by the formulation of distinct constraints suggests to avoid the traditional procedure of first to create a yet not necessarily valid instance of the eventual design solution by directly choosing specific parameter values of its shape, and later on to evaluate its validity by confronting the designed model to a set of applicable constraints. Instead, constraint-based design poses a search procedure that operates in a space of planning-relevant constraint sets. The FLS methodology integrates some few principles of constraint-based automated reasoning with high user interactivity, into a design environment where as much dwellers as planners can collaboratively work in solving spatial organization problems of housing projects. The FLS model of application makes use of a combination of dweller-specified constraints, planning and zoning regulations, and a small library of modular space units. Constraint-based design ! methods are particularly capable of supplying efficient support for the collaborative involvement of dwellers into the architectural programming process of her/his own home. Mainly, because dwellers themselves tend to describe their space need and design intentions as a set of constraints on room quantity, space utilization, circulation system, allocation of available furniture, available budget, construction time, and so forth. The goal is to achieve an integrated tool for finding and modelling topologically valid solutions for floor plan layout alternatives, by combining user-driven interactive procedures with automatic search and generative processes. Thus, several design alternatives can be explored in less time and with less effort than using mainstream procedures of architectural practice. A FLS implementation will constitute one system module of a larger integrated system model called Esther. A FLS tool shall interact with other functional modules, like e.g. the BDS (Building Bulk Design Support), which also uses constraint-based design methods. A preliminary procedural model for the FLS was tested on Chile’s official social housing standards (Chilean Building Code – OGUC. Art. 6.4.1) which are very similar to most Latin American housing programs currently in operation.