Maybe, why not ask an HVAC contractor?

Do the rooms have individual return ducts? I'd guess not, if so, you wouldn't be asking the question. Would putting those in be feasible?

What is rating of door? I think you have incorrect information. I believe you are allowed 3/4". Your door supplier should be able to verify that.

Thanks for your responses. There doesn't seem to be anyway of responding to a response, so I will just address them here.

@Elmer J Fudd: Well, when I posted it here, I thought I was asking a HVAC contractor. Of course, it is possible to add returns but the idea of adding new bulkheads and columns and running returns back to the furnace room seems expensive and daunting. However, I am contemplating that as it would provide 2 points of control for an individual room.

@Seabornman: The rating is 20 minutes. According to the Ontario building code: A door described in Sentence (1) [sentence 1 defined a 20 minute rating] shall have a clearance not more than 6 mm at the bottom and not more than 3 mm at the sides and top.

@sktn77a: The 1/4" can be achieved with a sill, which would still allow the door to clear the floor by 1/2" or more.

Ah, Canada.... OK. Yes, a sill would work. Tripping hazard, though (?)

Don'tcha just love these codes!

Tripping hazard? Wow!

Yep. Student trips, sues the condo owner and gets a handsome payola!

Doors commonly have sills. Every bathroom door, every door to the outside, and a few of the interior doors in my home have sills - including the door from my kitchen to the basement - have sills.

Student trips on a carpet edge or wet bathroom floor or on a slippery kitchen floor and who gets sued?

You think this is a Canadian thing? lol

Don't make stuff up.

A sill isn't going to do the OP any good. I read the Ontario code. There's a chance a transfer grille could be used somewhere in the wall, but I couldn't tell if it would need a fire/smoke damper.

A sill doesn't have to be a tripping hazard