I have been using Broderbundís 3Dhome for several years now, and I think itís the best under-$50 software on the market. I have bought and/or used some other more "professional" residential CAD systems since, but I found none as easy to use as 3Dhome. Get the Deluxe version, as it allows for roof designs as well as other useful stuff.
Using 3Dhome, I have designed many projects and never had a problem with getting a permit. In fact the municipalities really welcomed it, as I was told by them that the average DIY homeowner usually shows up with little more than some hand-done sketches, and many remodeling contractors donít do much better.
Now, for bigger projects like building a house or adding a second floor, knowledge of all the engineering factors really falls on the professional architect. No CAD program will tell you when you have exceeded a limitation. Itís only a tool to allow the architect to put ideas together faster. Even when I use 3Dhome for applying for a permit, I have to also provide a description of the materials and construction makeup of the project, so knowledge of building techniques is still essential. Professional CADís would help automate that process, but itís still up to the designer to know what can and cannot be done.
One big advantage of 3Dhome is that it can output images in DXF format, which basically means that the drawings you do can be imported to other professional architectural systems, so that when you get something worked up you like, you can give a copy to your architect, and he can finalize the plans for you. It is a pretty economical way to get a feel for what your house is going to look like before you build, from both the inside and out.
One big disadvantage is that I spend too much time with it on the computer, and itís gonna get me in divorce court.!!!
About 2 years back I upgraded to the professional version of this software, called Chief Architect. Of course it has many, many more features, but it costs about $800.
As an FYI, this question has come up numerous times on this BB, and I always answer the same. And, everyone who has taken my advice has been equally pleased. Good luck - TomR