Some building tips...
The right order is housebuilding is to first put in the walls, and then fill them with floor tiles. Corners with diagonal walls, which should only be filled with half a tile, will then be properly carpeted using Shift-click. But I tend to put down the tiles first and then draw the walls around them. Result: the corners with diagonal walls stay unfilled if they were empty, or if the wall was put on a tile, the bit of tile outside the wall visually remains there, although it no longer exists and therefore can't be deleted. This is especially ugly on the top floor:
The only way to get rid of such a filled corner is to lay a new tile on it and delete that. Taking an inexpensive tile, preferably. Diagonals that are not placed on top of a tile and therefore leave an empty corner, cost no extra money; they have to be filled individually, and half the tile price will be deducted from the budget.
Speaking of using inexpensive tiles: when placing a stairway, the top tile of that stairway will be the floor tile last selected. And that floor tile's cost will be deducted from the budget. So if the top floor doesn't have a carpet yet, best pick the cheapest tile while playing around with the staircase.
Using Shift-click with wallpaper papers the whole inside of a room. Except diagonal walls. They tend to be skipped, and need wallpapering separately, one by one. Also skipped are walls which the game thinks are in a room inside a room. Say, I've placed a staircase against a wall, and put a little fencing around the edge. Shift-clicking with a chosen wallpaper will now leave the three wall segments above the staircase bare. The fence is the culprit.
Fences can act like walls in other ways. A wall on the ground floor supports floor tiles on the top floor. So does a fence! The floating "Beryl's walkway" in the Animehood was built on a fence and a few pillars.
A tip I've seen in several places: build a floating room by putting a floor on walls or pillars and then delete these walls/pillars. The floor magically stays afloat. However, no floor tiles can be added or deleted after the ground floor support is gone, nor can any walls be built there. As building materials always sell for less than they cost, it's best not to get rid of the supports too soon. Or, if the supports are to be used in another part of the building; make the balcony first, then move the posts to where they were intended to be. Walls can't be moved, so it's more cost-effective to use pillars as temporary ground floor support.
Floor tiles can have an unexpected use. Sims don't use quite the same logic that real-reality folks do when computing the shortest path to get from A to B. Firstly, they will see doors that close as obstacles, preferring to go out the back door and walk around the house rather than crossing the hall to go straight to the front door. Secondly, they prefer walkways, and will walk extra distances hugging the concrete path along the road before they venture onto the grass. Laying stone (or rubber, or carpet) paths from mailbox to front door and from one outdoor destination to another may save valuable seconds of time. And it's comical to see the Sims obediently following these paths.
Wallpaper again: when Shift-clicking on a wall on the outside of a building, every "outside" wall is covered with that paper. And an "outside" wall is one that isn't "inside". And an "inside" wall is one in an enclosed space. A three-walled room is not seen as having any inside space. When working on outer walls, Shift-click with care.
As every closed-off space is seen as "inside", it's not possible to bring light into a house by creating a little courtyard inside its walls. The only way to bring daylight into the courtyard is by creating a corridor from the courtyard to the outer wall, so that the house is shaped like an almost-closed circle. (Or rectangle, more likely - there are no curved walls in The Sims.)
Finally: though diagonals may increase the Room rating and (literally) cut floor tile costs, they are a mayor nuisance. Sims can move diagonally, but they can't move through the half-square left by a diagonal. Diagonals don't allow doors, windows or anything that's hung on a wall. Until I can hack objects to take advantage of diagonals, they are literally a waste of space.
(See next section for cheats that allow building outside the standard
lot.)