Eve though your are out of Code territory, good on your for using a nearby code as a foundation. You are likely in similar terrain and still have to be concerned with proper percolation, nearby wells, etc.
I get that it may be undersized for current use, but I wouldn't call a 20-year septic 'old'. Properly designed and used they should last for decades. Also, I'm not getting how adding a hot tub/pool impacts things, unless you need the space.
You're pretty close actually. The existing is like this:
Area behind the shop when we moved in and the pipes at the end of the line or whatever - tank connects up on the other side/end:
This is what I want to do:
Now I'm pretty sure the new shop footprint doesn't go past the green area in the behind the shop picture. I need to measure to be sure, but it's WAY to cold to go out there atm. I was hoping to extend back about 25-30 feet so I can have 16' clearance between the shop and the house. We have 13-14' now and I feel like the shops too close to get equipment through if they ever bring the city sewer line close enough to connect up to or something.
This shot gives a better perspective because the end of the field and the power line post look way "closer" than they really are in the above behind the shop picture:
*The "hill" behind the shop is allegedly mostly a combination of the excavation of the attached garage/dining room addition on the house sometime in the late-70s, the shop's perimeter/slab sometime in the mid-80s, and the current leach field's installation in 2000 and NOT part of the field itself. Who knows if that's true, prior owner couldn't remember a lot of the work he'd done to the place (was a building contractor so no doubt it just blended into his actual jobs,) but I do remember he was quite confident we could make the shop bigger if we wanted. (We had immediately started fighting over who got it when we looked at the place lol)
I was told that adding a hot tub required an upsized septic system at some point in the past, no idea if it's true. Also, we had some "not our guys" pumping our tank for a few years, and also had a 7.3 quake 13 miles away and shallow that probs settled things. When our guy came out last summer he said the tank was half full and solid. Said he "back flushed" it three times and got basically nowhere... He thinks the other folks might have been sucking the wrong half of the tank - the sucker pipe is short and near the deck, vs the taller ones out past the grill are much more "hey look at me!" I actually have no idea if the other guys did the right pipe as I didn't pay attention, they'd just come do it and leave a bill on the door :/
Anyway, I want to bury the power lines to the house/shop which may or may not disturb the field AND we need to move the tank to the other side of the house so our pumper guy doesn't have to buy yet another longer hose to service it (we still feel bad about it lol) when we put on the new front garage -- all of which probably requires us to redo the field regardless of the required size and shape of it. So we might as well make it fit into the new lot layout.