Foundation considerations for a garage addition

I believe my existing two car garage is a slab on grade.  Likely with a thick section around the perimeter but I'm not sure.  I want to add a third car bay to it but want to avoid pouring a big slab for that addition.

I'm thinking of just doing a shallow footer (12" wide by 8" tall with rebar) around the perimeter.

I'm in zone 4 with a 4' frost depth.  If the garage is heaving with the frost, I assume that my garage slab is just moving as a unit.  It's quite sandy soil so maybe there isn't much heave anyway?  So my addition would ideally also heave with the frost.  Or is it unlikely that a footer based addition would move in sync with the slab and would thus cause issue?  

Is there another way to approach this addition?  The new bay would be for equipment storage and limited butchery so I'm thinking of just having a clean/washed gravel floor in it.

Techno-Metal post footings?
concrete pier footings? either cast in place or precast.

Not sure if a grade beam would follow the slab equally, mostly since it isn't as massive, and not coupled to it in any way. The main garage might not move much due to it being an umbrella to keep the soil below dry, so no water to freeze. The roof might extend that to include the grade beam, or not, if water can flow in from the sides.

Is this a lean-to roof? or supporting a gable wall?

Thanks Kenneth, there is plenty of water getting back in under the "umbrella" of the current roof in a couple places.

I would likely hammer drill the rebar for the footing into the slab where it meets in the front and back corners.

This would be a fully enclosed 3rd car garage bay with a door on the front and walls on all three sides.

The roof would be a 2:12 pitch or thereabouts, but metal so it might slide off.  I'm not sure how much snow would be on it but it could be substantial.  I'd want a proper/sturdy foundation to carry that weight.  

In Australia I have built many of these, but we dont have snow loads nor frozen ground.
Basically follow what has been down before.
Questions;
- do the footings need to go below the front line
- snow loads as mentioned
- roof structure requirements
- will you insulate under the roof?
- slab thickness across the slab.

Here we would either have footings separate for the columns and a 100mm slab with reo.
Or the same slab with 300mm edge beams.

From; https://www.jlconline.com/how-to/insulation/frost-protected-shallow-garage-slab_o
'An Easier Alternative Foundation
Section R403.3 of the 2009 IRC provides details for a “frost-protected shallow footing.” Instead of digging down below the frostline, you insulate below the edge of the slab and out from the building a certain distance with rigid foam. The distance depends on your climate. The foam captures the interior heat from the building and the earth’s geothermal heat, preventing the ground from freezing below the footing. The catch was that the IRC’s method applies only to heated buildings, and my garage/shop would not be constantly heated. However, the details in the IRC come from ASCE Standard 32-01, which also details how a frost-protected shallow footing can be used under an unheated structure. I showed the standard to my local building official, and he green-lighted the detail right away. In short, I had to insulate under the slab (which I planned to do anyway) and the footings and 4 feet beyond the perimeter of the building with 2-inch rigid foam. The foam needed to be laid over a 6-inch-deep layer of crushed stone for drainage, then covered with at least 10 inches of soil.'

Thanks John, I did do a frost protected shallow footing on my greenhouse.  In this case, the garage is likely not frost protected so I'm not sure how to address that with the addition that will be attached to it.

Seems funny, protecting the green house but not the garage!

Well, the greenhouse was new construction and I also wanted to keep it warm inside.