External buried conduit to run power around a structure?

Hey all. The design of my house is a little weird, and the main electrical panel is located in a spot that makes it nearly impossible to add additional circuits to the house without doing major sheetrock demolition. Slab on grade, 12 foot high brick-clad exterior wall, flat roof with no crawlspace access. A great example of Mid Century Modern at its least maintainable. Electrician told me that he'd need to take parts of our living room ceiling and wall down just to get wires to the panel.

We need to add a subpanel to our garage, which is on the exact opposite side of the house from the main electrical panel. My electrical contractor gave me a very steep quote for the run, citing the difficulty we're going to have running the power through the flat roof, the full length of the structure. He's also said that I'll need to hire a sheetrock and painting contractor to fix the damage after he's done.

Two days ago I was talking to my neighbor, a retired commercial electrician, and he asked why we aren't just running a buried metal conduit around the house, since we have very little concrete around the house and it would be a straightforward trenching job. His suggestion was to bring the power through the exterior wall at the panel, run it around the house, and then bring it back into the garage to the new subpanel through the exterior wall again.

I called my contractor and asked about the idea, and he told me that it's "against code". I spoke with my neighbor again last night and he told me that it's not and that I need to find a different contractor.

Who is right? California, if that matters.