The builder should be able to "box out" an area for the medicine cabinet, by cutting the studs and framing a box to fit the cabinet. This can be done even if the wall is load-bearing, but the builder should determine first if it is load-bearing in order to properly frame it because a header may be needed. You also have to make sure that there are no other obstructions, such as a plumbing vent or electrical wiring. The wiring may not be too difficult to move, a plumbing vent would be more difficult and probably more costly. Have you already opened the wall to see what is inside or is this new construction?

Wow, sue_b. I found a local dealer and I'm going to take a look at the Nutone medicine cabinet today. They don't have the triple recessed one on display, but the smaller one instead. This will give me an idea of the quality. Hopefully, if I like it, I will be able to find one locally. I actually heard one other horror story of buying a Nutone medicine cabinet online. Evidently it went to the wrong address and it was pretty badly mangled when it finally arrived to its destination :-(. I also like the Ikea Godmorgan triple medicine cabinet, but if I want to put sconces, the Ikea one will be too big and it also juts out a couple of inches from the wall. Details...details...

Well, creating storage niches is simple, then. Plan how big (wide and tall you want the niches. Presumably they will be less than about 14.5" wide (16" minus 3/4" times 2), otherwise you'll have to remove 2' x 4's. If it's a non-loading bearing outside wall you can remove studs; but check with an engineer or carpenter. It's likely load bearing if on outside wall. With the age of the house your studs may be farther or closer apart, too. You may also want to pack around the inside of the box (niche) with hard styrofoam insulation, which has greater R value than fiberglass. May also want to use canned spray foam to further seal? This way after removing some insulation you won't have as much of a cold spot on outside wall where niche is. There's a number of pre-made (pop-in) solutions like this: https://www.wayfair.com/storage-organization/pdp/wg-wood-products-bal-harbour-155-w-x-495-h-recessed-shelving-wgwp1493.html This example is interior wall. See here for a DIY: https://www.diynetwork.com/how-to/rooms-and-spaces/walls-and-ceilings/how-to-make-a-wall-niche Second approach, similar to above but narrower: https://www.familyhandyman.com/storage-organization/build-a-wall-niche/

That's pretty much the worst place you could have a medicine cabinet. Medicine stability data is generally obtained at room temperature (60-70 degrees) and low hunidity. "Accelerated" stability is obtained at 90+ degrees and high humidity (just what you have in Florida). Most medicines are not particularly stable under those conditions so I suggest you fine a different place to store them.