So you bought a 2 car garage house that had an option for stone elevation at X $$$, which you agreed to. Then at some point you say, Oh, I also want a 3 car garage. They say that's 11k. Then they come back and say, that was just the price for the 3rd car garage bay itself, if you want stone on that too, it'll be $4900 additional.
They quoted you the price of a stone elevation on a 2 car garage--you agreed.
You wanted a 3rd car bay for $11k--you agreed, but you never specified to the builder that you also want stone their too, you just assumed. They also never specified you wanted stone their too.
BTW, $11k for 3rd car garage is a good deal.
Now, you want to know if you have to pay it. Sounds to me like they were open and honest in pricing, and you both made assumptions. Builder says let's be clear about what that price includes, $11k does not include stone.
Series of assumptions on both your and their parts.
Pay the $$ and get it the way you want. It's not unreasonable.
Or walk away and lose your deposits over principle. IMHO, that seems ridiculous, but if it makes you feel better.
You're doing business with somebody who is out to try to build you what you wanted honestly. It should be a mutually beneficial transaction. They aren't asking for an unreasonable amount. Nobody in their purchasing dept is laughing at you or calling you a sucker for agreeing to pay for what you really wanted.
Now, as a buyer, I might feel like I shouldn't pay it if I initialed on drawings for the 3rd car garage elevation that included stone on it. That would mean they purposefully planned it with stone, but just screwed up and left it off. If this is the case, I would ask to speak to the manager and state why you feel they should build what they represented at the price you signed for. Otherwise, see above.
Greater love hath no man than this....