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I would like to make an extension for our house entry. Our home is a split-level type of house, and the entry is super small (no space for a closet. You enter the house, and there's around 3 feet of space before having to go up or down). That's why we are thinking about extending the entry a little bit (we would like to "extrude" our entry around 4-5 feet towards the outside of the house...so that would be a very small extension.)

We have a concrete slab in front of the door, on the outside that is 125 inches wide X 48 inches, and 6 inches thick. Around 3 inches of it is in the ground. The slab has been there for decades and never cracked.

My plan was to extend the current concrete slab so that we have a bit more space for the extension. Thing is, I don't know how concrete slab works in climate like mine. I live in Canada-Quebec, and during winter, it can get really cold with a lot of snow.

My guess is that I shouldn't build on top of that already existing slab, since it's not deep enough. I've also heard that I should insulate the slab or something like that, to prevent it from moving as much as possible since we want to build on top of it.

So my question is...should I remove the current slab and poor a new one? Or can I simply extend the current slab, even if it's not very deep in the ground, and build on top of it?

If I must poor a new one, how deep should it be, and do I need to add some insulation material around it so that the ground doesn't touch the concrete directly to prevent it from moving or cracking during winters and on spring when it gets hotter and the ground start to unfreeze?

Hopefully it's not too confusing. I've seen a lot of videos on Youtube on how to make house extensions, but most of them were kind of big extensions. Like full rooms. Mine would be around the size of the current slab (125"x48") more or less. That's why I'm confused about the type of "foundation/concrete slab" I need for this.

Thank you very much!

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    Think this is a floating slab not attached/anchored to the house. It will rise and sink depending on the seasons, therefore building an extension on it will be a bad idea. For an extension will probably need to remove and put in proper footings below frost level.
    – crip659
    Nov 15, 2021 at 16:33
  • Thanks! So as long as my new slab is below frost level I should be good to build on it, correct?
    – larin555
    Nov 15, 2021 at 17:33
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    Quebec frost levels usually start at three feet or more below ground level.
    – crip659
    Nov 15, 2021 at 17:37
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    Like crip659 said, you can't build on a simple floating slab. Any attached structural extension must be on full frost footings. Period. Of course, you could put it on a ledger and two posts, like a modern "four season porch", but they'd need to be properly supported below frost level as well.
    – isherwood
    Nov 15, 2021 at 17:48
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    By the way, frost footing depth in Minnesota is 42-48". You may be at 60" up there. Happy digging.
    – isherwood
    Nov 15, 2021 at 17:51

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You can't avoid the removal of the existing slab, as you will need a new foundation around 3 open sides of it, and the slab needs to be tied to the existing foundation on the building side. Also, you shall take the opportunity to provide better insulation and vapor barrier. Yes, it is essential to check out the local frost depth, as well as the permit for the addition.

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  • Thank you! You guys answered my questions.
    – larin555
    Nov 15, 2021 at 18:26

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