Remodeling is rarely without unexpected (and expensive) complications. There's no way to avoid every pitfall, but a little early investigation can help you prevent them from busting your budget. For example, says This Old House general contractor Tom Silva, it is fairly common to find water damage behind under-sink base cabinets in a kitchen or bath. So looking inside the cabinet and under the counter for dark stains can alert you to the problem before work gets under way.

Identifying load-bearing walls is another essential advance tactic; if you're not certain what holds up what, have a contractor, engineer, or architect advise you. Also, before you decide to cut into any wall-anywhere-determine what's inside it, says Tom. "Go into the attic and basement to see if there are pipes or ductwork running into the wall, and look on the other side of it for registers or radiators." Hacking away at live electrical wires and inadvertently sledgehammering plumbing are costly (and potentially dangerous) missteps.

There are, of course, some issues you just can't anticipate before you get started, like unsound plaster behind cabinets or structural damage from earlier renovations by careless professionals. (What were they thinking when they cut the joists to run that pipe?) To plan for unpleasant surprises, Tom suggests setting aside 15 percent of your total budget at the start so you won't find yourself 15 percent over budget at the end.