My question would be whether those existing 6 x 6 have sufficient bearing and footing to carry the extra weight?

 

They should. Since there are 15 posts total on the main building, those 2 corner posts bear 3.125% of the weight of the building each. The center post that you see bears 6.25%.

The footings are 15-16 in. diameter concrete and 10" thick.

 

Are the footings wide enough to add in 6x6s right next to the existing ones so you can put your ledger on top of new 6x6s? I'm not sure if it's okay to put two posts on one footing but the greenhouse isn't exactly a 3 story house.

If you bolt a ledger to the side of those 6x6 posts, you definitely want to make sure you put the same sized footings on the right side of the greenhouse (near the wheel barrow) so the greenhouse can't frost heave/sink at a different rate than the tyvek building and possibly buckle the 6x6s. (if it's even code allowed to bolt up to posts like that).

You can also just make 3 new footings about a foot away from the three existing 6x6s and cantilever the floor joists a foot or so over so it meets the tyvek building. Or just bolt your ledger to the tyvek building's rim joists and have it be ~10" higher than you wanted (and make sure to put same size footings near wheel barrow).

ps. I don't like when posts are below grade, even though they're thick 6x6 PT, the front corner post I see can create a pool of water, I would dig around that so water doesn't pool around the posts.

 

In NJ and was told (town inspector) to use 18" footing for 1 story addition. Your footing may be undersized for additional building.
Your 2x8 ledgers are undersized and in contact with ground. Ledger, as usually thought of, is attached to a solid wall, foundation or 16" oc stud wall. Even double 2x10 beam requires 8' supports with no joint in between, although that is for longer floor spans. 12' span may require triple 2x10, for example. Search the beam span charts. PT is also wet until it dries out and I think that down grades the lumber capacity. Also think bolts, not decking screw spikes.
I'm sure you will have crawl space vapor barrier. Same for the sunroom. In fact, that close to ground, I would pour a slab (with slope away). There are at least 4-5 pests that will make a nest under it and the smell alone may destroy the space.
Any post/crawl space foundation I build in future will have vapor barrier on ground as well as some kind of pest barrier, may be expanded wire sheets (name?). I'm also thinking about burying tile cement backers (double layers) between the posts, 3' down. Either go that far, or abandon the space to the pests.:smile:

 

Thanks for the info. The ledger in the picture is just placed there for visual demonstration. Not yet attached, but if I was allowed, I would make it 3-4 inches above the ground. I was going to use 2x8 floor joists between the main building ledger and the 3 posts (covered by wheelbarrow and plywood), which is a 6' span. that should be adequate right? I have rigid foam insulation on the bottom of the floor joists of the main building which acts as a vapor barrier. It is then closed off using solid vinyl soffit sheets.

Wondering if using standard deck skirts or coverings would be adequate?

 

Update - inspector came by and said I could do it, but I have to dig the 3 6x6 posts and attach a 2x6 to the post all the way down to the concrete footings to give the ledger bearing. He also said I can't use structural screws and need to use 1/2" lag bolts that go through the entire post. Also said I need to give about 6" of clearance under the decking for air flow and reduce moisture.