Beside Blender there is FreeCad .
FreeCad is a free software developed by two German engineers witch is not production ready yet, but could become a very solid basis for open source architecture software. It's based on the OpenCascade technologies,the inventor language, coin3D, pivy and Qt.
FreeCad is aimed to be a fully parametric CAD like SolidWorks, Catia or Revit.
FreeCad can be use as an external Library. Potentially, it will be possible from Blender and throw Pyhon, to have access to all FreeCad functionalities such as parametric and drawing functions.
Yorik did a great work and did already implemented some core tools :