This paper presents a prototype tool for the space planning phase, which automatically generates alternative floor plans, according to the architect’s preferences and desires, and assesses their thermal performance by coupling it with dynamic simulation. A case study of a single family house was carried out, which comprehended two design sets. The first set correspond to a single-level house and the second set is a two level house served by one stair. Each set is made up of twelve alternative floor plans that were automatically generated, assessed, and ranked according to their thermal performance. The ranking function weights and factors variability is analyzed.