Unfortunately, haven't the time to type out anything approaching a complete tutorial (and haven't found one on just this topic extant elsewhere), but you'll need to perform some trial and error using these two cheats:
moveObjects on
boolprop constrainFloorElevation false
Would suggest experimenting with a test lot first, as follows:
Start with a level, empty lot.
Put in your house foundation (decking) and the connecting stairs to where the garage should be first.
Use moveobjects on to place the driveway and extension.
The use boolprop constrainFloorElevation false to place all the garage walls and garage door.
Then place the house walls. You may find that you will need to use boolprop constrainFloorElevation true to get the walls in place without any sloping. Indeed, you may find it necessary to temporarily remove discrete sections only of the foundation (decking) - leaving just parts of the walls 'floating' - and then re-input the foundation in order to get the walls to show without a slope.
Do take the time to play with the cheats, both enabled and disabled, to see what they do.
Sorry I can't provide a step-by-step guide just now, but experimenting on a lot you don't intend to keep, using those 2 cheats (sometimes with them enabled, sometimes with them disabled), will in the end get you what you want.
Should I happen to find an existing tutorial, will add that here in an edit.
Don't be leery of experimenting until you get things as you want them. So long as you don't Save the game, all of your experimenting will be erased when you Quit.
EDIT: Okay, here are some existing tutorials (each probably uses a slightly different order of methodology, but you should get the gist):
http://www.modthesims2.com/showthread.php?t=93917&c=1&ht=0&page=1&pp=25#startcommentshttp://modthesims2.com/showthread.php?t=91504http://www.thesimsresource.com/downloads/articles/sims2/tutorials/houses/465757/
