When does it make sense just to tear down an old house?Say you're driving around, looking at open houses in an upscale neighborhood that you'd badly like to buy into. You find a quaint, old two-bedroom surrounded by modern 4,000 and 5,000 square-foot homes that's the right price and fits the old real estate axiom: "Always buy the least expensive home on the block." But when you walk inside, the agent greets you with a sheet showing the dimensions of the lot and the news that "it's a teardown." Maybe. But what if you're more interested in preserving the look of the old architecture than building out a McMansion to the property lines? What if you don't want to dump a few truckloads of wood and drywall into the local landfill, and would rather put off a major rebuilding job until a few years down the road? Bottom line: How do you figure out when you should remodel, and when you should tear down and start over?