In order to reduce the increasing energy consumption for the domestic demands of existing single-family housing and take advantage of frequent building enlargements, this paper presents a methodology and supporting software tool for determining the optimal design configuration of an attic with integrated solar collectors. The analysis procedure is based on parametric modeling, energy simulation and the use of evolutionary algorithms for finding optimal designs. It has been implemented as a Web-platform for public use that provides users with a proposal of an attic shape with maximum solar energy collection, maximum living space and minimum construction envelope for each house according its size and orientation. The attic integrates PV, thermal and hybrid solar panels on one side of the roof. This paper describes the methodology and software design, assessment of the Web-platform usage and case-studies to verify its behavior. In a matter of minutes, the Web-platform enables users to select a specific attic design for each house that has integrated solar collectors that can produce energy to cover almost 100% of domestic energy consumption. The attics designed provide a nearly 30% increase in living space through the extension of one to four rooms, and the construction cost of the envelope is similar to that of a standard housing extension.