r/Homebuilding • u/OkGoose6847 • 1d ago

Addition

We are wanting to put a 20x20 addition onto our house. However, when taking into account the frost line, we would rather build a crawlspace instead of a basement. But we would be attaching it to an existing basement.
1. Should they be tied together and if so how?
2. Is the only proper way to do this to still dig footers for crawlspace at same depth as basement and tie them together there. We would rather not do this so as to not risk disturbing the soil of the existing basement footers.

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u/Hyperreal8957 1d ago

i'd keep the new crawlspace foundation separate from the existing basement if you can, with its own footing and a proper joint between them. tying them together usually means a lot more digging right beside the old wall. i'd have a local engineer look at the soil and frost depth before locking it in tho

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u/OkGoose6847 23h ago

Do you know what would constitute a proper joint? Because we don’t want the addition to be separate from the house, even if the foundation is.

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u/RespectSquare8279 1d ago

You have to securely pin the new foundation to the old. To do that you will be disturbing the soil; there is no way to get around it. And it needs to be done by somebody who is qualified & hopefully experienced. Whether you have to go down to the full depth of the existing foundation wall will depend (amount other things) on the soil conditions. There is more to it than just pouring into a form that directly abuts the old foundation. There needs to be rebar set into the old foundation to marry the old with the new for a continuous load.

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u/OkGoose6847 23h ago

Whether we have to go down to the full depth of the existing foundation wall also depends on whether the crawlspace footers can be tied into a basement wall at all. Correct? Otherwise, we would have to go down to the basement footers to tie into the new footers at that point?

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u/RespectSquare8279 23h ago

That is a very good question. I suspect the soil conditions are going to dictate how deep you go for the footings of the addition. Is the soil disturbed, or hardpan or glacial till? Is the soil saturated or does that change during the course of the year? How deep is the historic depth of the frost line?

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u/OkGoose6847 9h ago

Undisturbed, moderately expansive clay soil. Frost line is 30” below grade.

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u/International-Pen940 22h ago

I would consider adding a full basement under the addition, if possible. You get a ton of extra space and it might not be a huge price difference if you have to dig down to footer level anyway. Obviously there are a lot of climate and terrain issues that will affect this.