Might make a post with pictures soon, but right now I'm in the middle of running a food-stand business while Gavin meditates, conserving motives and aspiration. Very low-maintenance business, and nice to have when you want to play and blog at the same time. But anyway, here's some info about building houses.
I started out not knowing how to design houses in Sims 2. Everyone goes through that phase where they build square or rectangle houses that look horrible and may have problems with routing. With the routing, it is a simple thing, like not placing double-beds sideways against the wall, but designing a good house is quite hard. I had to read many different tutorials which I need to link to soon - they deserve lots of attention!
Square houses are boring, so it's often best to make them L-shaped or even add a smaller square to the corner of the L. A simple design like that will make it so when you add the autoroof, it will have a more pleasing appearance. The gabled roofs will be lower, and will have multiple points, instead of one big point.
Diagonal walls can also make for some pleasing designs, though with some roof types, the results can look ugly. The simplest way to make a square house look good is to add a square at the entrance, and use the short and hipped or short gabled autoroof. For the roof walls, use a stucco wall covering with enclosed timbering.
If you're going for a high-roof design, put some windows on the attic walls to keep them from looking empty. It doesn't matter if you have stairs leading to the attic or not - the windows make it look pretty. Balance the empty space with used space.
You might instead want to build a house with a flat roof, possibly condo-style. If so, the best design would be two pentagons stacked up on each other. The pentagon shape is quite nice, and the roof from the lower pentagon can be used as a balcony. Make sure you put a fence on the balcony as a safety feature, though sims seem to be incapable of falling off of unfenced areas.
If you want to make a foundation, do so, but make sure you build a usable and nice-looking stairway. Maybe build a patio. I may put in some basic drawings of how my condos are set up later on.
Anyway, the internet is full of many different house-designing tutorials, but make sure you have a budget in mind, so you don't blow all your money on a big house and don't have any left for necessities.