[Sileni]

Doesn't have all the functionality that AutoCAD does, and there are some usability issues if you're really comfortable in AutoCAD already, but I imagine FreeCAD [0] would work well for your use case.

[0]: https://www.freecadweb.org/

[acidburnNSA]

I'm particular the BIM workbench is looking pretty slick these days [1]

[1] https://www.freecadweb.org/wiki/BIM_Workbench