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This concrete slab has a size of 9x20 square feet. It's 1 foot deep. I would like to build an ADU or room on it. I have included a 3D model of the room in the attached image. Can I re-use the existing slab or will I need to remove it and pour a new reinforced concrete slab? The current slab has not been reinforced; no steel was embedded in the slab.

I was thinking I could first drill into the cracks and then drill in or attach some rebar segments into the separated pieces of concrete slab and finally, re-pour some concrete into and around the cracks.

Sort of how it's done in this video.

The property is in Austin, Texas.

enter image description here

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  • Is there a frost line at your location? Chapter 3 of the IRC requires foundations below the frost line unless you do some special detailing under Chapter 4's frost protected shallow foundation. It's already too late to implement Chapter 4's alternative.
    – popham
    Jan 16 at 20:17

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Any structure attached to a dwelling must typically be supported by suitable frost footings in accordance with local requirements. Chances are a simple slab won't suffice unless:

  • The edge is thick enough to support construction. A foot might be enough, though, assuming it remains that thick for at least 20 inches or whatever your local code requires.

  • It's below frost depth (if present in your area), or it's insulated properly against frost. Since the slab is mostly above grade it likely isn't unless you're in a no-frost zone.

  • It's reinforced to local code standards. If it was poured by a layperson or an unscrupulous mason it may not have any reinforcement.

  • It's fully supported by unexcavated, inorganic soil or properly compacted inorganic fill.

Note that it may only be that thick at the edges. It's common to drop with grade for support or appearance, but only at the outside.

Also, stitching together broken concrete as you propose is not viable. It cracked for a reason, and you don't remedy that with a bandage. Chances are the substrate is inappropriate.

When in doubt, run your plan past your local inspection office. They're there to help and will steer you straight.

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    Just as a side comment: I know folks who bypassed the requirements for attached structure by building a sunroom enclosure that was technically not attached to the house, just flashed to prevent water infiltration. Gods know whether any other inspector would let you get away with that.
    – keshlam
    Jan 17 at 15:52

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