" Is it worth it to do the basement or just stick with the crawl space idea. Would love some advice. Thx! "

Yes,it is absolutly worth doing the basement. I want to be the first to commend you going on line for free advice instead of paying an engineer for advice based on soil conditions. The money you save using free advice gives you extra to spend on nice things for your house.

Well I smell something funny. 800 addition built at ground level on a slab out the back size of the house in Ontario makes me wonder when and how it was built. Even because it's a slab on grade there should be a foundation down at least 3 1/2' to 4'. Better check because someone may have started with a patio, then added a roof and finely boxed it into an addition. happens a lot so check to see if the addition was even permitted.

Is this a 100 plus year old house? Did you just buy it?

Even it a skylight was leaking doesn't mean you need to tear down the addition. (unless it wasn't permitted and doesn't have a foundation.) I hope the roof has been fixed when you first noticed the leaks. Was it? If you found the leaks and fixed the roof did you cut out the sheet rock of the effected areas, pulled out the insulation to dryed out the moisture with fans?

Something doesn't sit right with this story and the request for free information with a cost analysis won't be computed until details to your story line are given and pictures of what you're working with are added.

The decision of crawl vs basement would depend in part on more information not known.

First, if your frost depth is 48", the addition should have full footings and stem walls (mono pours are not typically done at that depth). If so, eliminating the foundation probably doesn't have to happen and makes the decision of crawl space for you. Though if this 800sf addition was put on top a patio slab, then it has to come out regardless.

The existing house is a big factor. If you have a 2300sf footprint (1500 ranch+800 addition) with full 1500sf basement, adding an additional 800sf basement may not add any more amenities. But if you have a 1700sf footprint (2BR 900 ranch+800 addition) with partial basement, then the extra dig could be justified. If you have a walk-out basement opportunity, I would say a basement dig would be justified in any case.

The design and purpose of the 800sf addition, and how it relates to the existing house, also is a big factor. If you have an old small house with a tight back stair to a dungy low ceiling basement, then maybe opening up a new stair to a new usable basement is justifiable for the expense.

And surely other reasons pro and con based on the house/neighborhood/budget not known here . . . .