The general thematic of our work tackles the question of the generative design tool efficiency to stimulate a creative architectural conception in the context of sustainable development. We focus our point of view on the conceptual research phases. We would like to characterise the human creative mechanisms in a situation of generative assistance where digital tool reveals some degree of autonomy and incorporates environmental constraints. Thus, we implement an evolutionary design tool in which energetic performances of the analogon are used in order to orient the evolution. Our tool is based on an interactive genetic algorithm that ensures both a broad exploration of the solutions space and the subjective user preferences accounting. Users groups were confronted to the tool in a conception situation and creativity was evaluated and characterized.