I hired a local contractor to install a concrete base for a garden office and has manage to form the base 100mm too narrow! (4 x 2.4m instead of 4 x 2.5m)
I have suggested he break out 200mm, insert some lengths of rebar and cast another 300mm of fresh concrete. He is more a landscaper than builder and is reluctant to do this for one reason or another. He is proposing installing 150mm shuttering and doing a fresh pour of concrete over the correct length / width. I am concerned that a 50mm layer of fresh concrete over the top will result in cracking.
Leave the ground works as-is. Build the structure above using timber framing (not sure if you were planning a timber or masonry structure). Resin anchor the soleplates on top of a DPC then put treated joists to your desired room width and floor over that.
This way you can cantilever your floor out over the original width of your footings. Suggest centreing the building on the footing, then the overhang is only 50mm each side - nothing really.
I guess you'd need to cover the overhang in membrane / cladding and dig out little trenches to make sure none of the wood was within 150mm of the ground...but all of that could be made decorative and like a design feature!
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