I know it is a little too late, but for others, I always check the zoning records 1st then call the zoning office to confirm if I am considering making a change.
As for the fire rating, it has been a few years since we put up new walls to separate units.
1st I would get more quotes.
2nd I thought fire rated could be achieved with a double thickness of drywall.
So if the new unit is in the basement and you have to do all of the ceilings, and the place is 1200 square feet, you would need to hang 1200 square feet of drywall. At $32-40 for a 4 x 8 sheet hung and finished. Plus the light fixtures need to be moved and you need to fire caulk and paint.
$1 a square foot to drywall
$1 a square foot to paint.
That is $2400
How did they get from $2400 to $25,000.
I spoke to York City the other day and to apply for the Variance to change the zoning for a property was $500 and a 2 month process and it was not a guaranteed change.
Although I doubt a Doc was living in the city.
I have rooms In rent and they rent in hours.