Dove, we have clay 8 inches down. Solid, orange heavy clay until you hit bedrock with only a shovel-depth of topsoil on it. We have to double dig everything, put in TONS of biomass AND build raised beds on TOP of that or nothing grows because there's no root space.
As for greenhouses, this is a trying-hard-to-be-snooty suburb on top of that (used to be old agricultural land, now is pretending to be on-par with a near-by neighborhood with a bunch of old Rockefeller mansions) so everything needs a permit. You need a permit to buy a pre-built shed from Home Depot and drop it in your back lawn. The only reason I don't need a permit for my chicken coop (and an inspection with a yearly $150 licence!!!) is because I had chickens 2 years before they passed "chicken laws" in my neighborhood "allowing" chickens. (They were previously allowed only on large lots like mine with no other regulations, now they're allowed on smaller lots but heavily regulated.) I'm pretty sure a greenhouse, even a cheapy thing from home-depot might require a permit.

(Maybe an inspection too.) It's just impractical for us.
And yeah, a walipini is an underground greenhouse. It uses geothermal heat to keep things above 50 even on a cloudy winter day and earthen walls to help retain higher temperatures when the sun is shining. You dig a hole 8' down (or so) and put in a slanted glass roof facing the sun (so south for us northern hemisphere-ers) with a berm (mounded earth wall) an extra 2' on the tall part of the slant. The thermal mass of the earth helps maintain warm and steady temperatures and you build raised beds in the bottom and plant right into the floor. It's VERY interesting.
https://en.wikipedia.org/wiki/Walipini
so your wanting a fairly good size greenhouse does sound very cool yes I agree sorry much to old here I would fall just getting in or out but encourage you as it is a very cool idea and way