Since there are no changes to load-bearing structural elements, nor are there changes to ingress or egress, windows, or the overall footprint of the addition, no plumbing, etc...it doesn't hit any of the elements that would generally require re-submission to the building department.

As to how much it'll cost, that's a can of worms. You're framing one more door opening and adding one more door. That'll be your biggest framing expense. You're adding a few more studs and a couple of more sheets of drywall. The amount of extra trim could change your finishing costs. Which depends on quality of materials and labor costs, of which I have no idea.

Give your builder a call. Could be $500. Could be $2500. But I agree, the scope is not significant, so the additional charges should be reasonable.

I already emailed the request to my builder. I mainly want to know if his response is reasonable.

I know there's an extra door to be framed, but instead of framing a double door, there are two singles. I would guess framing a single is easier than framing a double. Two singles costs about $50 less than one double. I can't imagine framing a prehung door for new construction is that much labor. It has to be much easier than doing a replacement door and installation costs for an interior replacement door with frame typically runs maybe $200.

I think the only extra drywall will be for the 18" wide opening, which should be under 8' tall, so there may only be one extra piece of drywall. But anyway, the cost of a sheet of drywall is probably under $10, so it's not a big deal.

Plus there's wider shelf boards (unless a support is added to the middle of the closet which would allow 2 boards per shelf).

All in all, I would really question them if the extra cost came to over $500, but you're right, it's not my decision.