Like /u/someothherguyy said it depends on the cable type. There are shielded and jacketed outdoor cables that don't require PVC/EMT, but usually you'll have to terminate them yourself so there's additional tooling required.

I went with PVC because it's the easiest to work with, NEC only required it be buried 12in below the surface for residential, but I went to 24". I oversized the PVC a bit so I had room to run more cables as well. I filled the trench with gravel, ran the PVC over that, then filled over it with some more gravel. A thin layer of dirt, marking tape and then the rest of the dirt. Rent a trencher if you have hard ground.

If you're having a licensed electrician do the work of connecting house and office power then you can ask them to run the conduit for it at the same time.