Addition Planning: Bumping Out

Avoid costly foundation or roof work by making new space that hangs off the side of the house

Here's a trick for building a small addition without paying for any foundation work—or maybe roof work either: Hang the new space off the side of the building. It sounds farfetched, but a "bump-out addition" is a clever, secure and economical way to add on, says Curt Schultz, a Realtor-architect-builder in Pasadena, Calif.

If your house's finished floor is above the grade of the landscape outside, your contractor can build a bump out of up to about 3 feet deep, and as long as 10 to 12 feet. That may be enough to add an eating area to your kitchen or a separate tub and shower to your master bathroom.

After he has opened up the wall where the addition will connect, he'll pull up the adjacent floor and attach new, longer floor joists (horizontal framing members) alongside the existing ones (a process called "sistering"), so that they extend past the outer wall of the house to hold the weight of the bump out. Or he can support the new floor with kickers, which are angled supports that run from the outer edge of the bump out back to the house, like the brackets under a bookshelf.

Because you'll need no foundation work, you can save about 15 percent compared with a conventional addition, says Schultz. And you may be able to save another 10 to 15 percent by tucking it under existing roofing. "In many parts of the country, it's common to have roof overhangs of two or three feet, so you might be able to tuck the bump out under that," he says. "Now all you're building is the wall—no foundation and no roof." In other cases, you can just extend the existing roof over the bump out, which is far less costly than designing and building a new, separate roof.

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