Called "Room in roof" trusses. All truss manufacturers make them, they are surprisingly cheap and they are great for big spans, and the truss companys stamp the floor and roof drawings for free. They also go up really fast, 2 framers did the whole roof system and floor on my garage, including 2 dormers, in 2 days - would have been <1 day if didn't have dormers. .
Some of your assumptions regarding the building department don't apply here in South Frontenac. I paid nothing like $2K for the sheets of styrofoam I placed under the concrete. I agree that the full dig was added expense that's hard to quantify. I guess I needed to add the amount of extra concrete which I didn't take into account, so add another $2500 for concrete (20 X 20 X 1 ft) I still don't get anywhere near your numbers. Maybe it's the advantages of living in the woods and lower costs for a lot of stuff away from the GTA. Also, lots of the labour was done by me, so that wasn't factored in. If I had to pay someone to do it, maybe I wouldn't have liked the cost.
If you design it right and spray foam insulate, you can turn those small attic spaces into room temperature storage areas
The rafters are insulated right to the floor, we put kitchen lowers into the spaces inbetween.If you design it right and spray foam insulate, you can turn those small attic spaces into room temperature storage areas
that's what my wife did, she is a freak for storage spaces.
What is the ceiling height in the garage part?
Conversion of attic to storage(or a room) is somewhere between impossible and ill-advised. The floor of the attic was not designed to take much weight (just the insulation and drywall). It is amazing how much weight you can build up in a space once it has a floor. Mike did it the easy way and had the trusses designed to take the weight (and at the same time arranged to maximize useful space).
You can't really do it if your house has engineered trusses, they are designed to carry about 10lbs/sq' if I recall correctly. The main issue is load the floor can handle. If you have a stick framed roof, you might have 2x6 joists holding up the top floor ceiling, if so there is a chance you could finish some storage space up there. You would need to measure the span, joist height and spacing then pull up a span table to calculate the max floor load.
If you had originally excavated 4 ft to get your footings for a normal garage, the additional excavation should be (20 X20X 5)/27 or 74 cu yd by my calculations. Damproofing and insulation included with ICF. Going with blocks or traditional poured concrete may be cheaper, but then, as you note, additional costs for those. What is the steel beam for? I used a heavy pan specified by the engineer, and built temporary supports under the pan, left in place for s few weeks, until the concrete was properly cured. The slab was about 11" thick at the back and about 8" thick at the front. Technically this is all additional cost as the floor under the room would normally be the floor in the garage. I'm not trying to be dismissive, just making sure those contemplating such a thing know the options.
I wouldnt be too concerned with a 4 post. The floor loading isnt grossly different than with a vehicle. Slope may be an issue with a four post. I havent looked at how wildfire solved that (my guess is you adjust cables for level platform on tilted legs). It's rating for 9000 lbs at full height, the most I would ever put on it would be ~5000.
