This is something that just happened to my associate recently. The seller of the property said he was sure that a garage could be built on the subdivided property he was selling. Since there are several things that have to happen to be able to close on properties that cost the buyer money, we had all contingencies such as inspections, testing and loan commitment begin on the day the seller got the blessing from the city. I would request that he be the one to get the survey since I am sure that there are a ton of buyers passing up this property due to the lack of a garage. It would behoove him to at least have that and a couple quotes on hand.
If you are requesting that the seller put up a garage at the seller's expense, be specific in what you are asking for unless you do not care too much about what he puts up. Do you want it to architecturally to match the home? 2x6 v. 2x4 construction? Concrete slab or is gravel OK?
Also - if the seller is doing this at his expense, expect him to negotiate from you a deposit that he would say is non-refundable once the city give the OK to build. You may not want to do it, but it is fair. Expect to pay a little closer to his price, too unless his price is WAY out of whack for the area.
As far as wording it in the offer goes, a good agent or RE attorney will be able to draft it for you.
Lastly, either you or your agent should be double-checking the facts with the municipality. "Oh, sure you can build on it" from the seller would not give me a good feeling. He and his agent should have had these facts at the ready. Some cities, like Brookfield, will only allow the construction of an attached garage, for instance. Obviously those tend to run significantly more.
Best of luck!