Bump Outs: See How This Low Cost Micro-Addition Adds Room Space

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

Everyone wants more space.  But who wants to spend the money for a complete addition?

When you want multi-room space--bedroom, bathroom, and office, for instance--you build a full-blown addition.  One problem:  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.  How about even less space?

 Consider building a bump out.

What Is a Bump Out?

Full-size, multi-room additions get most of the attention. Builders favor them because they generate the most revenue.  Homeowners love them because they increase the home's property value.  Except for the cost aspect, 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':  Bump outs may extend as far as 10-15 feet from the house.  But they can also be tiny "pop-outs" as short as 2 feet long.
  • Cantilevers:  On an upper story, a bumpout 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% to a 1,500 square foot home.

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

So, a 150 square foot kitchen, when bumped out another 45 sq. ft., receives a 30% 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.
  • 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.


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

Cantilevering an Upper Level Bumpout

Grade-level bumpouts have simple foundations:  they rest on a concrete slab or are elevated over a crawlspace.

But upper-level bumpouts 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 bumpout, the dimensions of your house's joists dictate the size of the bumpout.

Generally, bumpouts can be cantilevered a couple of feet. More than two feet is unusual.

Stated Joist DepthTrue Joist DepthDistance of Bumpout Cantilever

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. By depth, we mean the vertical measurement. For example, if a joist is a 2x10, it is the 10" part we are referring to. Stated and true measurements (also called "dressed dimension") of dimensional lumber are not the same. In this case, the true depth of a 10" joist is 9.25".
  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 bumpout.

For every foot that you cantilever outward, you need to sister twice that length along an existing joist. For example, if your bumpout cantilevers 2 feet, you have a minimum of 4 feet running alongside an existing joist.