I've been using QCAD for a few years to do house plans and structural design. It is a fantastic easy-to-use product. But I've come to the limit on some of it's features.

Until today I didn't even know there was an alternative opensource version named FreeCAD but FreeCAD has 3D Modelling and a quick perusal of the website shows that there is an architectural module aswell.

May be worth checking out.... for the next home project I need to council approved: A new Garage, Carport & Driveway.

Refs:

Demo Video
LibreGraphicsWorld Article
Sourceforge site to download FreeCAD