House Bump Out: Micro-Additions Can Add Space for Less Cost

Small House Addition (Bump Out)
Small House Addition (Bump Out). John Keeble / Getty Images

House bump outs offer a tantalizing solution to that age-old conflict of space and money: you want more space in your house but it costs money than you can ever afford.  

When you want multi-room space—bedroom, bathroom, and living room or office, for instance—you build a full-blown addition. However, full-blown additions blow your finances to smithereens. Expect to pay six figures for even a modest addition.

If you need less space (or cannot afford a full addition), try a room addition—one room tacked onto the side of the house, typically one bedroom or one bathroom. If you do not need that much space and money is tight, consider building a type of micro-addition commonly called a house bump out.

Bump Out Defined

Full-size, multi-room additions get most of the attention. Builders favor them because they generate the most revenue, as well as making clients satisfied. Homeowners love them because they increase the home's property value and they transform home life. Post-addition families are happy families because they are no longer living on top of each other. Except for the high cost, full additions are win-win for everyone.

A bump out is an extra space that is far smaller than a full addition and often does not even rise to the size of a room addition.

There are two schools of thought about bump outs:

  • Full Rooms: Home renovation writer Michael Litchfield in  "In-Laws, Outlaws, and Granny Flats" (The Taunton Press, 2011) defines bump outs as essentially in-law suites (full rooms) that are attached to the house.
  • Less Than Rooms: Another definition runs along the lines of "Black & Decker The Complete Guide to Room Additions," which calls a bump-out an addition to an existing room, but not a room itself.

    While not rigid rules, the following provide a general profile for most house bump outs:

    • As Small as 2 Feet:  Bump outs may extend as far as 10 to 15 feet from the house. But they can also be tiny "pop-outs" as short as 2 feet long.
    • Cantilevers: On an upper story, a bump out can be cantilevered (unsupported by posts) generally as far as 2 feet. Permits to extend farther may be difficult to obtain. In order to extend farther, you need deeper joists on the main house. Plus, you need to extend the bump out's joists farther along the existing house's joists.
    • No Extra HVAC: One benefit of bump outs is that they do not require additional heating or cooling services.
    • Lean-To Roof: They often have a shed (lean-to) or flat roof, rather than extending the existing roof.
    • Minimal Exterior Changes: Bump outs, due to their small size, do not substantially change the exterior of the house. They often blend in seamlessly with the main house.

    How Can a Bump Out Add Significant Space?

    A bump out extending 3 feet out, 15 feet wide, would not be considered a major space-maker relative to the entire house. These 45 extra square feet would only add 3 percent to a 1,500 square foot home.

    However, bump outs are often installed in rooms that already very small—often unbearably tiny.

    A 150 square foot kitchen, when bumped out another 45 square feet receives a 30 percent boost in space.  

    Suggested Uses

    Because bump outs are not full rooms, they tend to enhance an existing room. They can:

    • Provide space for a window seat with book shelves. This is one of those rare instances when the bump out is more about fun than function.
    • Allow for installation of a bathtub in a bathroom that currently only has a shower.
    • Give you enough room in your kitchen to put a length of counter, stove, and fridge. With the increased floor space in the main area, you can install a kitchen island.
    • Provide a kitchen with enough extra space for a dining area or breakfast nook.

    Costs

    While the total cost of a bump out is less than that of a full-size addition, a bump out will cost more on a square foot basis.

    This is because a large part of the cost is in initiating the project, making drawings, pulling permits, opening up the side of the house, pouring a foundation footer, calling in an electrician to move wires, and so on. You may need to do the same things with a bump out as you would do with an addition, just on a smaller scale.

    Costs vary wildly because costs vary according to homeowners' desires, locality, and a host of other factors. Some anecdotal price reports:

    • $17,000:  2 feet by 10 feet bump out
    • $30,000:  Bump out twice that size (4 feet by 10 feet)
    • $30,000:  6.5 feet by 28 feet bump out for a bathroom, on ground, poured foundation
    • $5,000:  2 feet bump out in a kitchen (length not provided) on a second story, cantilevered out

    Cantilevering an Upper-Level Bump Out

    Grade-level bump outs have simple foundations. They rest on a concrete slab or are elevated over a crawlspace. But upper-level bump outs are often so small that they do not need a foundation. Instead, they can be cantilevered out—essentially hanging out in mid-air.

    When you choose to cantilever your bump outs, the dimensions of your house's joists dictate the size of the bump out. Generally, bump outs can be cantilevered a couple of feet. More than two feet is unusual.

    Stated Joist DepthTrue Joist DepthDistance of Bump Out Cantilever
    65.522
    87.2529
    109.2537
    1211.2545

    To calculate how far you can go:

    1. Go into the basement, crawlspace, or any place where you may have exposed joists.
    2. Measure the depth of a joist. Depth means the vertical measurement. For example, if a joist is  2 inches by 10 inches, it is the 10-inch part that is the depth. Stated and true measurements (also called "dressed dimension") of dimensional lumber are not the same. In this case, the true depth of a 10-inch joist is 9.25 inches.
    3. Multiply joist depth by 4.
    4. 9.25 x 4 = 37. The resulting number is a rule of thumb for how far you can cantilever your bump out.

    For every foot that you cantilever outward, you need to sister twice that length along an existing joist.

    For example, if your bump out cantilevers 2 feet, you have a minimum of 4 feet running alongside an existing joist.

    Kitchen Bump Out Example

    How complicated and costly is it to do a bump out from a kitchen for a space that extends between 2 feet to 3 feet outward and about 6 feet to 8 feet wide? Is there a certain size below which less structural supports and work is needed and does that vary from area to area? The good news is that, given the reduced dimensions of a bump out, it will still be exponentially less expensive to build a bump out vs. a full addition.

    Whenever you remove a load bearing wall—and all exterior walls are load bearing—you need to duplicate its function in another way. One typical alternative is to replace the wall with an LVL, or laminated veneer lumber, beam.

    The 6-feet to 8-feet width is especially encouraging. When you get into these very long spans (say, the entire width of a kitchen) that becomes a major and expensive undertaking. Because the new space is so small, you will probably not have to add extra heating or cooling capacity.

    Still, it's like a mini-house you're building—foundation footings, siding, electrical, shingles, perhaps a window—so that drives up the cost. Bumpouts are less common projects, so pricing is difficult to determine.  Homeowners who have done similarly sized projects report that they cost between $5,000 and $15,000.