Turns out that for our 60-yr old house, we have copper pipes being attached to galvanized pipes (interior). Apparently not ideal. I assume you mean the developer con-game where all the below-grade and beneath-slab runs are cheap galvanized, while all the risers and stub-outs are copper, so the happy home buyer THINKS they have "all copper" plumbing, then decades later the galvi completely rusts away... SURPRISE!... the phookin' Bastiges! ASIDE: I'm always surprised to hear about concrete slab construction in a seismically active area like SF Bay... always thought it would be too "brittle"...? Do I understand that you also have a concrete ROOF, with zero attic space? Do you also have hollow concrete block walls (aka CBS)? These will really limit your options... OTOH, sunlight and weather will never ever damage exposed copper, painted or not. Careless roofers, maybe, UV rays, nay way. Anyway, concrete slab is the norm here in So-Fla (Hurricane Alley), with a shallow peaked roof, and at least a small attic space, PLUS we don't have the freeze issue mentioned by first respondent. Here, when the original (galvanized) pipes beneath the slab give out, the new replacement copper is typically just run straight up the exterior wall from the service entrance (which is outside the footer/foundation/walls), through the soffet (copper pipe painted to match stucco walls), and then branched off in the attic to where it needs to go. So except for that one main feed, the rest is out of sight. Same with output of the water heater, except it's vertical output is interior. This exterior pipe run can be avoided if service entrace is outside of a utility room--or relocated there--since you can then punch straight through the block, and go vertical inside, typically right near the water heater, i.e. do all your ceiling punches and exposed piping in one spot. New copper feeds for interior fixtures are fished down interior walls, but of course you still have to knock out some drywall to tie them in, and retire/cap-off the old feeds coming up through the slab. (Plumber ain't fixin' the drywall either!) Outdoor hose bibs and pool showers usually result in more of the "dreadful" exterior pipe runs, again, unless it's outside a utility/laundry room. Essentially, with Florida CBS construction, you can't run/hide pipes inside the hollow vertical channels of the block walls, because the wall is topped off with a solid poured-in-place concrete header, which starts below attic level. (In fact the roof trusses rest on top of--and are tied to--the top surface of this header.) SUMMARY: 1) I would be VERY wary of Option 3, and it sounds like you are; 2) I would like more details on your roof, but I'd be very reluctant to punch any new holes in it... OTOH, you probably have stack vents and all sorts of other "penetrations" already, so... 3) Exterior pipe runs, if horizontal, and low to the ground, could be camoflauged with landscaping...? Paint to match in any event. And there's always those "architectural" (aluminum) channels used to cover bundles of conduits and cables. 4) You GOTTA have indoor plumbing, or the neighbors will say bad things about you, LOL!

What does your municipality say? What does the structural engineer/soils engineer/geologic engineer say about the idea? Someone has to physically design your foundation based on your specific site conditions. That foundation design has to get the approval of your municipality. A slab on grade must have a foundation perimeter and load points that goes into undisturbed subsoil. A concrete contractor should understand that, even if he is not a structural engineer. Or undisturbed subsoil with approved and inspected compaction material. A concrete slab isn’t an approved and inspected compaction material. Not without core samples proving what is under it, and the compaction of the fill that’s under it, and the depth at which you reach undisturbed soil. It also will complicate the required termiticide treatment required under slabs. Doing all of the work and reports to prove that you can use it may cost you more than demolition and rebuilding. Time is money in that regard. Working around the old parts of a building is what makes remodeling cost more than new construction. The input of a concrete contractor is valuable, but he is limited to a single perspective that revolves only around his job portion. He doesn’t understand the interconnected complexity that goes into making his job “arrive and pour”. There’s a lot more engineering that happens before that happens. And the GC is the one that coordinates all of those moving parts that the concrete contractor does not see. Talk to your municipality.

Your toilet requires a 3" drain which is probably the size they would run over to your house to tie into the existing. Both the sink and toilet would also be tied into this same drain under the slab. With regards to your concerns about the slab cracking from saw cutting. No, this shouldn't be an issue. As it relates to your slab and the foundation, you are correct, this we could not tell without seeing pictures or having a better understanding of what your outbuilding is. In the grand scheme of things as it pertains to the plumbing specifically, the only thing the foundation will really play a roll in is the plumber will either have to drill through the wall itself to get out of the garage, or dig under the footing itself if there is one. The second scenario may not be feasible as you most likely would have issue getting grade over to the existing drain you will be tying into. One thing to keep in mind is the number of existing toilets you have in the house. Depending on the code your local area has adopted, if you only have a 3" drain going out to your sewer/septic, you can only have 2 toilets on that drain (some jurisdictions will give you a pass and allow a 3rd. If you already have 3 toilets inside the house, make sure your drain out to the street or septic is a minimum of 4" or you will be dealing with up sizing the main drain as well from your house.

There are businesses that “trace the plumbing lines”. I recommend hiring one of them. Your need is very common in areas with similar construction. If this is not DIY, I recommend allowing the business you hire to do the work hire the subcontractors.