Costs vary by region of course, but our detached 2+ car garage with interior stairs and upstairs unfinished space was about $35k, about 4 years ago. (That doesn't include the cost of the driveway or any plumbing or HVAC, but we did put a separate electric panel out there in case we want to add more at some point.) So $25k sounds reasonable. However... be sure to check your local zoning. Ours doesn't allow for plumbing in a detached garage (they want to avoid things like rental units); even a spigot for a hose outside is not allowed from the garage. There are also considerations like egress (think you'll need more than one exit for the space to be considered safe) and also building an envelope between garage space and living space so that fumes don't carry. Also, not sure if your garage is currently existing or to be built. Interior stairs will take up some space inside, so space for your car will be shortened by several feet, even if the staircase turns. (We handled this by bumping forward one half of the garage to be a few feet deeper than the other half. I wanted this anyway so that it wasn't one big flat expanse across the front, and it looks nice.) To be honest, I think you'd be happier with an addition that's part of the house, especially if you'll use it frequently.

I agree that this discussion has no doubt gotten far more complex than what they originally intended but you cannot blame me. If all they originally wanted was a hose bid near the garage I would have began by suggesting a lawn hydrant, however the original post was a bit more complex than that: Quote from the original post:- "I will be installing a slop sink, house bib and possibly a small water heater to start. Might also add a half bath in the future" I am merely pointing out that if they intend to install a sink or perhaps a half bath in future the code requires an approved drainage system as a pre-requisite to install any fixtures inside the structure. Taking this to the next level, we have no yet even begun to discuss the problems of installing fixtures I an unheated structure in a cold climate. If you were asking my personal opinion I would say the whole idea will soon prove to be cost prohibitive. NOW FOR THE NEXT QUESTION: "What is the rationale for requiring a separate sewer tie-in for outbuildings? ANSWER: All of the DWV (Drain, Waste & Vent) lines inside a structure eventually combine into one line before passing through the footer wall to exit the structure. That single line is properly defined as the house or building "Main Drain". Code requires that we must install a "Main cleanout" on that line within 3 of the footer wall. For structures built over a basement the main cleanout is generally installed immediately inside the footer wall in the basement. For structures on slab it is generally located outside the structure within 3 of the footer wall. For structures on a raised foundation with a crawlspace the cleanout may be inside or outside the structure. The main cleanout officially designates the end of the "Main Drain" and the beginning of the "house or building Sewer". On a residential property the "House Sewer" is generally consider the primary sewer line for the property and it would discharge directly into a "Municipal Sewer" or a "Septic Tank. Code only allows one line to connect to the municipal sewer or the septic tank, therefore when we have additional outbuildings that are equipped with plumbing fixtures the "building sewer" from the outbuildings must tie into the "House Sewer" before tying into the municipal sewer or septic tank. It must also be mentioned here that all DWV and Sewer Lines are sized by the DFU (Drainage Fixture Unit) load calculation method. The code gives us a list of all the fixtures that may be connected to a drainage system in both residential and commercial plumbing systems. The list then gives us a minimum size of drain opening or pipe and the DFU load value for each fixture. (IRC table 3004.1 & table 3201.7) Example: Lavatory 1-1/4" pipe & 1DFU, shower/tub 1-1/2 pipe and 2DFU, Shower stall 2" pipe and 2DFU, watercloset with 1.6gpf or less flush min. 3" pipe and 3DFU. A watercloset with greater than 1.6gpf = 3" line and 4DFU, kitchen sink 1-1/2" pipe and 2DFU, A Baptismal pool in your church is a 2"line and 3DFU while an office water cooler is 1-1/4" line and 1DFU and an Autopsy table at your local coroners office is a 2" line and 3DFUs. The code then gives us another table that lists the maximum number of DFUs that may be conveyed by each size of pipe. (IRC table 3005.4.1) Example: For the DWV lines within a structure the table lists the following 1-1/4" =1DFU 1-1/2"=3DFU 2"=6DFU 3"=20DFU 4"=160DFU When designing a DWV system we must begin at each fixture and attach the code required size of drain line then at every junction where two or more pipes combine we must compute the total DFU load at that point and select the appropriate size of pipe from that point onward. When we have more than one structure equipped with plumbing on a building lot we must do basically the same thing. We begin by calculating the total DFU load for each structure. At every point where two or more building sewers combine we must add the DFU load for each structure to determine the total combined load on the sewer line at the junction then we consult another table that defines the maximum number of DFUs permitted on each size of pipe. Example from IRC table3005.4.2 3" with ¼"/ft pitch 42DFU 4" with ¼"/ft pitch 216DFU 3" with 1/8"/ft pitch 36DFU 4" with 1/8"/ft pitch 180DFU (The IRC permits a 3" line to run with a 1/8"/ft pitch only with the expressed written consent of the local authority having jurisdiction)

I posted before about using tigerfoam. I used in in my addition. My garage ceiling was sheetrocked by my builder, and I'm sure he did the least he could get away with. Part of my bedroom is over the garage. I'm thinking that they didn't used to build living space over garages before. Now we all see why. If someone could figure out how to retrofit insulation onto a sheetrocked garage ceiling, that would be great. For stockrex, you don't have to fill the entire space for the Tigerfoam to be effective. Just a few inches will probably stop the air leaks. Then, you can fill the rest with the same batting that you take out, if it's clean. I think that it's best to figure out where the airleaks are coming from first. My powder room shares a wall with my garage. It was always cold before we renovated, but now it's freezing. When I had a wall mount sink installed, I decided to have an access panel behind it in the garage. Cold air was flowing in through the electric plug. I sealed that. It's better, but the walls are still very cold.

"So, in the current setup or as I have diagramed above still maintaining a single circuit, is this system inadequately grounded?" As a single circuit, it is presumably grounded just fine, as long as there is a proper grounding electrode system back at the main house panel. "I had assumed the circuit being fed consisted of a live, neutral and ground (the twisted bare). " As it is set up at the current time, that is correct, but it wasn't originally wired that way when the actual wire was installed. They didn't DO ground wires back then, and a "bare wire" does not infer that it is a ground wire, just not "ungrounded" (HOT). That is a cable with a concentric neutral. I doubt that either of the insulated wires are "identified" as a neutral (white insulation). "200.2 (A) Insulation. The grounded conductor (neutral), WHERE INSULATED, shall have insulation that is (1) suitable, other than color, for any ungrounded conductor of the same circuit on....." "200 .6 Means of Identifying Grounded Conductors. (A) Sizes 6 AWG or Smaller. An INSULATED grounded conductor (neutral) of 6 AWG or smaller shall be identified by a continuous white or gray outer finish or by three continuous white stripes on other than green insulation along its entire length. Wires that have their outer covering finished to show a white or gray color but have colored tracer...." "Regarding the disconnect... " This depends on the final decision of how you wire it. As it is wired right now, you would only need a standard toggle switch (just like the ones that turn on the lights). If you make it 2 circuits, you may only need 2 standard toggle switches, (just like I cut and pasted them out of your original drawing). You really only need something that looks more industrial if you need to install over current devices. Do you know what the actual wire gauge of the cable going from the house to the garage is? Back then, a common thing to do was to put a dryer receptacle in the garage. (Funny enough, I just noticed that there is a documentary on the outlawing of clothes lines in many parts of the country when the dryer became popular.) In other words, that may have originally been a 30 amp wire and a gas line to make provisions for either type of clothes dryer.