The Sims 3 Guide
Remember to use ALT to place objects without the grid.
Hold CTRL F to allow diagonal tiles to be placed.
Use the 'Create a Diagonal Room Tool' to craft more interesting house designs.
Hold SHIFT to place the floor over an entire room.
Create a floor and put objects in before putting in walls. This ensures you don't waste space if you're on a tight budget.
Use the Day/Night Toggle to allow you to see how your new items or house looks at night.
Various cheats are available to help with building houses and object placement. Check them out in the Cheats section for Sims 3.