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House extension foundations lower than house foundations
Extension question please:
We are having a small extension added to our 1820s cottage. The extension foundations are, as expected, deeper than the foundations for the original cottage, and building control / surveyor has specified that the builders need to shore up the old foundations with concrete before pouring the new foundations. It’s not extensive, just in the hollow under the original walls where the new extension will touch.
I think I’m looking for reassurance that this is standard practice, and that the relatively small amount of concrete touching the original walls won’t cause any issues for the old walls?
Thank you in advance.
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12 Replies
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It’s normal practice, not going to be an issue. You might need to dry pack with slate under the existing wall once the concrete has set.
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I don’t really think you have much choice? But see what others say….
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yes you do have to underpin/ shore up, there will be extra wight going through that part of the building. I had to do that on my extension, its good practise to do it.
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It’s called a ‘toe’ to support the old corner. Quite common request from building regs
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Hi Sally, yes this is standard practice when building extensions onto something that has very small/ no foundations.
I’d also get a temp prop under that corner then get it built straight up as soon as possible.
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We had this done under the corner of our Victorian house when we built our extension . It just cost us extra for concrete .
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You’ll have more issues from differential movement tbh. There are foundation designs that can accommodate this but the problem is getting it past BC
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I’ve have often designed the new footing to cantilever over existing footings do that no load is transferred between old and new. Then the end of deeper dig is kept away from the shallower existing footing.
Also making sure that there is a movement joint between new extension and old building.
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As others have said, it’s good practice to do so. Someone also mentioned dry packing between the new concrete and underside of the foundation, which would be essential to wedge the two together.
It looks like you’re on either weathered sandstone or decent sand for the foundations. Assuming that’s the case, any settlement of the new foundations should be minimal and substantially complete by the end of construction.
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Yes we had to do it when we built our extension. The foundations for the extension were superior to the whole of the house. Cages, 1.2m deep and the house is built on a raft but BC insisted.
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I wouldn’t worry about that. Of much larger concern is what the new floor will be? You don’t want a concrete slab, which can push damp into the old building side. A suspended floor would be a better idea.
Also the junction of wall is possibly some concern. If the new walls are to be of entirely modern non breathable type construction.
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thank you. Yes, suspended floor. The walls will be brick with a cavity. Will that cause a problem do you think?
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