Assuming the exsting slab has the correct footers for the addition, it wont have moisture concerns w/ the use of proper barriers. I would have them remove the existing concrete and form a new crawlspace for the addition tying in venting w' the existing crawlspace as well. I understand the builders way of thinking as far as running mechanicals in the kitchen/bath area, but you will have a better insulated floor area in the other rooms and if you ever want to run wiring or anything else to those rooms in the future, you can do it all from below w/o interrupting the flooring.

Your first step would be to get a consult from a licensed structural engineer concerning the existing patio. It's no doubt going to have to be re-worked to accept the new load above.

Donna, I am glad you shared the pattern you are seeing. You do have a shrinkage problem and a respect (aka taken for granted) problem. I don't come across this often but it happens. Forgive me for micromanaging a bit but details are important. Ultimately, you will be able to make a informed decisions. Please, have that meeting I advocated for and set up a proper table and few chairs for your office on site. You are the boss. You are the homeowner. You do not read minds. You control the purse strings. Three large signs are needed before you have the meeting. All work must be approved through a signed change order with face to face meeting. The Boss Lady's Office. Anything removed from job site needs to be visually checked and noted by homeowner. You need to have your second sink in your hands by week end or a cheque cut for the amount plus handling costs. Not negotiable. You need to have every change or problem brought to your immediate attention before any work is done. This is normal on most reno sites. Listen, ask questions, and look at all the parts being discarded. Take photos. Educating the client is part of the job. A new QUOTE, not an estimate, should be written up for each additional requirement on the job. i.e. change order. Your heating could have very well gone. That is a separate contract. Ask the GC to give you an estimate and get two more. You are the MRP; Most Responsible Person on the site for the entire home and yard. You choose who gets the work. You pay directly when the contracted job is done with a cheque or e-payment that can be tracked. If you are still challenged, call me on my page. I'm off recuperating from surgery right now but my brain needs a work out. There's no charge. I will put a 60 minute limit on it. But please call PST office hours. One of my recommends is from a woman whose project I was pressed to take on with a manufacture I represented. SOS.

Last night I watched Selma (free rental on Amazon Prime and YouTube). We're discussing it in a Zoom class today and I'd left it till the last minute. I was totally unprepared for its impact. Some of it was difficult to watch but I needed to see this. I went through a lot of Kleenex.