I just buy a ton of light gray bricks (usually 1 x 6) and build them into walls 5 bricks high. This is high enough to act as a good wall and fully conceal a minifig, but if you want to add a roof or a door somewhere I would make it 6 high, as most newer door frames are this height. Also, try to add some additional texture to your walls to make them look nice; the majority of mine suffer from Big Gray Wall Syndrome and so they are useful and cheap but look ugly.
For an average house, I usually take a 16 x 16 baseplate, add a door (usually one of the Harry Potter/Castle door frames), put in a couple windows, build a 5 or 6 brick high wall all around, and seal it all together at the top with a layer of plates or bricks. Windows and doors can help by taking up space and saving bricks. I have made about 6 houses using this method. They are rather bland, but work well.
Infantry may be the Queen of battle, but Armor is the chastity belt that keeps that bitch from getting snuggled.