The more I use it, the more I think that Google is fabulous. You can do all sorts of stuff other than simply searching the interwebs.
Today I am shilling a lovely, free design program called Google Sketch-up. Oh, the places you will go with this 3-D program: design houses and entire cities to scale before you build and share them with the world. You can add objects and people for the total picture.
Of course, if you want to pay some cash and get some additional features, you can go with Sketch-up Pro, but for your average student wanting to design/build, Sketch-up should be fine.