Extend a Room:
Bumpout, Build-out or Dormer?

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Your house is perfect except that you just need a little more space in the living room, kitchen, bathroom or dining room.

An economical option is to extend a room, rather than add-on. Three alternatives to extending a room are: a bumpout (cantilever extension), a larger build-out, or a dormer on the second storey.

Bumpouts (Cantilevers)

extend a room:  sketch showing a bumpout A great option if you only require a little more space for a kitchen pantry, corner bathtub, office space or closet, but can't fit it into your existing structure.

The maximum extension of a bumpout depends on the floor joist size and the species of lumber. A general rule of thumb for construction grade spruce:

16" max cantilever for 2x8 floor joists
24" max cantilever for 2x10 floor joists
Check your local building code to determine the maximum size that you can extend a room.


extend a room:  sketch showing a buildout To significantly extend a room size, a build-out may be the answer.

For a 1st storey build-out, you can use either posts and footings for support, stacked concrete blocks, or poured concrete.

For a 2nd storey build-out, you must use posts and footings for support. You can combine a build-out with a porch underneath to increase the outdoor living space and make your home more attractive.

Consult your local build code to determine the structural requirements.


sketch showing a dormer to extend a room For gable or sloping roofs, a dormer can add character to your home, not to mention increase the size of a bedroom, bathroom, sunroom, stair landing, office or create an alcove.

The maximum dormer size is determined by the strength and size of the existing framing in your home. You may have to increase the size of your existing posts, beams and joists if you have a dormer that is larger than what your home can support.

Check your local building code and consult with an experienced builder or structural engineer to determine the appropriate dormer size and roof slope.

extend a room:  add a dormer extend a room: add a dormer extend a room:  add a dormer extend a room:  add a dormer


When you extend a room, you should pay close attention to the architectural details so that the new extension fits in with the style of your home.

For some great examples of well-designed additions, see curb appeal. You can also review the architectural style of your home.

Here are some good examples of room extensions, attractively designed. For images of ideas for the interior, see redecorating.

extend a room at front of house second story room extension over new porch two story extension at side of house extend a room at side of house extend a room at side of house second story room extension over existing porch extend a room with double dormers