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Joined: Jul 2006
Location: Milwaukee area
Well the first question is, why does she want them raised? They're typically where they are because that height has shown to provide the best illumination/shadow compromise.
If she insists, and if your home inspector will allow, there are these devices:
https://www.homedepot.com/p/Tyco-Electro.../202204326
To answer your question, you're correct in that you cannot bury a box inside the wall. But those splices I linked are allowed in some areas, apparently.
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The easiest thing to do being you say the wires come up from the basement, if you can get enough slack in the cables going to the sconces then cut a box in downstairs and abandon the old cable and fish new cables up to the new location and tie into junction box. Now the question is will the old boxes get covered by some mirror or something???
John T.
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Location: Wapakoneta, OH
We did a kitchen remodel some years ago and I had licensed electricians do the electrical work. At one point they used a splice (similar to the one linked above) to extend the wire that became buried in the wall. I wasn't subject to inspection and if you are it pays to ask.
I started with absolutely nothing. Now, thanks to years of hard work, careful planning, and perseverance, I find I still have most of it left.