You can't really consider a fireplace a 'heat source' for living purposes. Supplemental to get romantic or maybe to take the chill off in the fall or spring, that's about it.
I've come across these situations before. I've found that it is very important to explain this properly to the client. If you fail to do so, your client might be calling their attorney next December.
I looked at a house last year that had a new heating system installed but not properly. The system couldn't bring the house up to temperature. The client ended up getting somewhere around an $8000.00 credit to have the system redone.
I don't know Utah prices but around here... a typical basment 90+ install will run around $4500-6000. plus another $2000 for AC depending on variables. Beyond that they can expect to put out another $2000-5000 in related costs, drywall, trims, soffits, demo work, etc.
I'd suggest reporting it as you feel comfortable. You could also recommend your client have an hvac guy come in for a ballpark bid.
Another possibility is if the existing unit is large enough to handle the extra square footage the client may just need to add ductwork.

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Accurate Inspections & Consulting, Inc.
773/844-4AIC
Markus Keller