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Bump out addition on slab foundation

7 years ago

We are considering a bump-out addition in our kitchen. The cantilevered style seems like a rather simple way of doing this, however we are on a slab foundation in the south. Am I correct to assume that for a house on a slab, that the only way to do a bump-out is to repour/extend the slab foundation, just as you would with a full room addition? This seems like it would always be more expensive and labor intensive than a cantilevered bump out. Is there any other method of doing a bump out for a house on slab? We would only be extending out about 2-3 feet by about 12 ft wide. We actually already have a small bump out in the kitchen that was built with the house, but we'd like to extend that small bump out so it goes the full length of the kitchen.

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7 years ago

last modified: 7 years ago

It needs a foundation. Cantilevered requires a counterweight and conventional framing. Small additions cost as much as large additions. The first few feet of interfacing the new and the old are the most expensive. It might be 50K for a 4'x20' addition with plumbing pipes, and marrying new to old in the external materials and framing. But adding a 20x20 addition might only be 80K.

On additions, go big, and without plumbing, or don't do them, if a limited budget is a concern.

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Comments (7)

klem1
7 years ago
I'm in total agreement with S. Wheeler. Pouring the needed concrete isn't complicated,expensive nor unusual "If people doing it know about construction". You weren't planning on doing it with inexperience anyhow,,,,,were you?
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Not sure if the OP is still following this or if I'm just talking to myself. We had some friends over last night, one of whom is a GC. I was telling him about my project and he said that the rule of thumb for cantilever is that you have to go back 6' into the house for each 2' of cantilever. As mine is going on a gable wall perpendicular to the floor joists, I'll need to support the bump out with either a full block foundation tied into the existing footings or two piers. This project just got more complex than I thought it was going to be.
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This is EXACTLY what we are in the midst of, and no, the costs aren't anywhere near that low. This is our diningroom wall. The plan was to bump out 24", using 18" of the depth for built-in wall to ceiling cabinetry, with a granite topped buffet under the window in the center. This would allow us to do away with a bulky china cabinet and make the very small diningroom function a lot better. To make a long story short, we are in California, and in order to get permits, a structural engineer had to be consulted. This added 2 months to the project and several thousand dollars. The builder was required to use steel 'strong walls' and hurricane straps, bolted every inch along the back of my house. And of course we needed a foundation. So, what 3 companies bid on as a reasonably priced solution to our storage and space problem became a lot more complicated and expensive than anticipated. On a positive note, I know where I am standing the next time the Bay Area has an earthquake. And, It will be gorgeous when it is competed.
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Your toilet requires a 3" drain which is probably the size they would run over to your house to tie into the existing. Both the sink and toilet would also be tied into this same drain under the slab. With regards to your concerns about the slab cracking from saw cutting. No, this shouldn't be an issue. As it relates to your slab and the foundation, you are correct, this we could not tell without seeing pictures or having a better understanding of what your outbuilding is. In the grand scheme of things as it pertains to the plumbing specifically, the only thing the foundation will really play a roll in is the plumber will either have to drill through the wall itself to get out of the garage, or dig under the footing itself if there is one. The second scenario may not be feasible as you most likely would have issue getting grade over to the existing drain you will be tying into. One thing to keep in mind is the number of existing toilets you have in the house. Depending on the code your local area has adopted, if you only have a 3" drain going out to your sewer/septic, you can only have 2 toilets on that drain (some jurisdictions will give you a pass and allow a 3rd. If you already have 3 toilets inside the house, make sure your drain out to the street or septic is a minimum of 4" or you will be dealing with up sizing the main drain as well from your house.
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Emily A
Original Author
7 years ago
We're hiring people. When I mentioned labor intensive, I was referring to the cost of the more labor intensive work, if the cost would be significantly higher for slab vs cantilevered. I'm not worried about the difficulty of work on the guys, as you said, this is their profession.
klem1
7 years ago
" I'm not worried about the difficulty of work on the guys, as you said, this is their profession." I see,but where I come from labor intensive always equate's to more expensive. As you said,this is their profession so maybe you should rely on their advise on best way to support the addition based on conditions for this job.
Emily A
Original Author
7 years ago
This is a home renovation advice forum, so I'm asking a general question. When it's time to get exact bids, I'll call the professionals. My main question was about options for slab homes, which poster # 1 answered - thank you!
Amy Lemley
6 years ago
How did your bump out go? I have a similar problem, I have a very small full bath-it is only as wide as my bathtub and only as long as the stool, a wall-mounted sink, and the narrow end of the bath, with 12 inches or less separating each fixture--and would like a few more feet so I could add a walk-in shower and some kind of towel cubby. House is on a slab. If it is true that adding such a small bit of space is as expensive as adding 20/20 addition, then I guess I will continue to use the yucky shower head on the wall above the tub drain with a plastic shower curtain. Not ideal, but functional. :(
Emily A
Original Author
6 years ago
After getting some unofficial construction opinions, we have opted to wait a while and reconfigure the interior of the house to make the kitchen larger. A larger addition would be preferable, but due to codes in our city, we would not be able to extend the house much further than a bump-out, and as you mentioned the cost/benefit just doesn't make a ton of sense on slab foundation.

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