The use of 3D software in architectural field is too often limited to the vizualisation of already designed projects. While Blender is very good at it, I will mainly present my use of Blender as an architect during the design phase of a project : How to explore form and spaces directly in 3D with the modeling tools of Blender. How to design details of a building directly in 3D with the precision tools. How the interactive photorealistic rendering of Cycles can really help the way one choose lightning and materials of a project. Those fields, (but the « simple » vizualisation too), will be illustrated by exemples of my professionnal projects, and personnal researches.