Add Space for Less Cost with a House Bump Out

An example of a small house addition (a 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 more money than you can 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/or money is tight, a bump out might just the right improvement, especially if you need only a little more space in a specific room, like a bathroom or kitchen.

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Everything You Need to Know About Bump Outs

What Is a Bump Out?

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 his book "In-Laws, Outlaws, and Granny Flats" defines bump outs as essentially in-law suites (full rooms) that are attached to the house.
  • Less Than Rooms: Another definition is followed in the book "Black & Decker The Complete Guide to Room Additions," which considers a bump-out to be an addition to an existing room but not a room itself.

As these differing definitions show, there are no standard rules about what makes a bump out, in terms of size or project scope. Bump outs also can employ a variety of designs and construction methods, but there are some common features and advantages that apply to most 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" that are no deeper than 2 feet.
  • Cantilevers: Small bump outs often are cantilevered (unsupported by posts) from the house. This is typically limited to about 2 feet in depth. Extending farther from the house usually requires footings or foundation walls.
  • No extra HVAC: One benefit of bump outs is that they usually do not require additional heating or cooling services.
  • Lean-to roof: Bump outs often have a shed-style (lean-to) or flat roofs, 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 that extends 3 feet from the house and is 15 feet wide adds 45 square feet to the home. This 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 for Bump Outs

Because most bump outs are not full rooms, they tend to enhance an existing room. Homeowners commonly use them to provide just enough extra floor space for specific goals, such as:

  • Adding a window seat with bookshelves
  • Installing a bathtub in a bathroom that currently only has a shower
  • Extending a length of the counter, adding space for an appliance, or installing a kitchen island
  • Providing a kitchen with enough extra space for a dining area or breakfast nook

The Cost of Bumping Out

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 (as applicable), 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 they change according to homeowners' desires, locality, and a host of other factors. According to some anecdotal price reports, a bump out may cost:

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

How Far Can You Cantilever a Bump Out?

Grade-level (first-floor) bump outs can be cantilevered or have simple foundations. They can rest on a concrete slab or on foundation walls, which can create a crawlspace. Upper-level bump outs are often are small that they can be cantilevered from the second-floor framing.

When you choose to cantilever a bump out, the dimensions of your house's joists dictate the depth of the bump out. Generally, bump outs can be cantilevered a distance equal to four times the size (depth) of the floor joists. For example, a 2x8 joist has a depth of 7 1/4 inches; you can cantilever from 2x8 joists a maximum distance of 29 inches.

Nominal Joist Size Actual Joist Depth Distance of Bump Out Cantilever
2x6 5.5" 22"
2x8 7.25" 29"
2x10 9.25" 37"
2x12 11.25" 45"

In addition, for every foot that you cantilever outward, the cantilevered joists must extend 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 inside the house.