If by 'convert' you mean 'tear down and build an actual structure', then you're on the right first step. Even then, I'm guessing a 4" slab is nowhere near code for Seattle.

Without diving too much into details, I don't see this being a cheap or easy endeavor, especially when you say you don't have experience at this scale.

Because I don't know the area all that well and maybe I'm completely wrong, maybe just get a couple of quotes. That will at least give you a ballpark of what to expect

You can put metal siding on the outside and drywall on the inside optional insulation.

You might have better luck asking about converting it to a shed... In this area, they treat any detached structure with water or power as a possible accessory dwelling (read: living space) and make you follow the codes for living space no matter what you say. If you don't have water or power, you often can follow the much more basic rules for a shed. These rules vary wildly, if you don't want to risk being ordered to tear it down by modifying it without a permit, start first with a chat with the local zoning people about the rules. As I said, you may want to first ask if you could even turn it into an enclosed shed...

As someone else pointed out, the stud spacing appears quite far, but that can be rectified by adding extras.

No, you will need footings (as others have pointed out the concrete is insilufficient) at a minimum to build a permitted gargare.

Depending on your municipality, a demolition permit may be required along with construction and electrical permit.

4" isn't enough to sit a garage on even away from seismic. Need 6" around under the structure here in middle canukistan.

Not without adding lateral bracing! Once you add siding this structure becomes a sail and a strong wind will crumple it. For the front opening you need substantial lateral bracing (big header, strong moment connector). You can look up a portal frame wall for an engineered detail. You need a bottom plate that is connected to the slab (1/2” wedge anchors), then full framing and sheathing. This is not a small job, but doable. Most cities make you pull a permit for this type of garage conversion.

With enough time and money you can do anything.

That being said, 4" not enough to support the weight of a building. You could pour a foundation around the slab and build off that, leaving the slab the inside floor.

Personally, I’d just work to improve its wind and waterproofing. Add as little weight as possible. Put more bracing supports in. As others have said, there’s too many obstacles to making it a ‘garage’. Best to consider what functions you want it to perform and what the problems are currently and work towards fixing some of them, ie, “I’d like to be able to keep stuff dry” or “it would be good to be able to work on stuff in winter”. If “I want to securely store stuff out of sight of others” is your goal then that might be a stretch.

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Start with SDCI

https://www.seattle.gov/sdci/permits/common-projects/garages

Let me just get this first thought out of the way. If you are renting the property you will need to get permission from the owner first!!! Then you’ll need plans, county permits, you’ll need to check with the county about what codes you have to meet, you do not finish then as the inspector to come. You have to get to a certain point get the inspection, go to the next point get an inspection, so so forth. And pay all the fees. Probably looking at around $8000 to $10,000 for the whole thing.

Would like to see the finished product. Good luck, you can do it.

Looks doable. Make sure any framing that touches the concrete is pressure treated and you should more or less be good to go. Getting a garage door to sit under a slanted roof might be a bitch but it should be possible.

You will need to go to the city and see if its even allowed if you want to be proper. But id run electric first. And you should be able to do everything you wish. But idk if you can with whats there. As whats there looks like 2' on center and not 16" on center. Which can be a problem. But as it's not a dwelling it'd probably pass. Again ahj is the one who has say here. Ahj=authority having jurisdiction. Which is the city.

I did something like this once with mixed results. I'd suggest doing PVC siding for the outside and cementboard not drywall on the inside for weight/weather resistance. To save yourself excessive hassle with electricals just get some heavy gauge extension cords and run each one from a different breaker from inside the house.

You would be able to turn this into a garage no problem. You have your foundation, you have your main support beams for the whole thing already there. Piece of cake!!!!! Frame the walls for a little more support, slap metal R-panel on outside insulate, drywall and bamo!!!! GARAGE!