Interesting predicament. I've been involved with something similar, where the building being constructed was inadvertently expanded into the setback, and similar types of relief were required from the neighboring party.
Nope-- no laws (Well, none that I've ever heard of-- in any jurisdiction.) It's their property, they can put the structure anywhere they want within the current Zoning Ordinance... and "interior" setbacks (e.g., those that would theoretically be up the middle 'property line' that now runs through the house) are generally dissolved due to the fact that they're not relevant. (Again, I'll say in MOST Jurisdictions... just to put the disclaimer cherry on the cake one last time. Every area is different as we all know. )
My first bit of advice is to visit the County/Town. Speak to the Building Inspector-- or better if you're in a larger town, the Zoning Administrator. Come clean. Make it sound like you were duped... because in at least one sense, you were. The first thing you need to know is what you can & can't do to the property "as it currently sits." E.g., if you re-build a porch, and that new porch is one inch larger-- do you trigger setback issues? What do the local regs say about legal non-conforming use (because in my opinion that's the scenario that you've purchased yourself into.) Can you even re-blacktop the driveway? What can you do and not do? Forget a Zoning & Planning action for a moment-- can you even get a Building Permit?
After you figure out all of those things, now let's look at the property line, in concert with your intentions for the property-- DO you intend to add anything onto the footprint? Think about porches, decks, patios, drives, walks, pools, and sheds; answer the same questions about these. Does the driveway encroach on the now-neighboring land? Any fenceline added? Figure out what you intend to do, and ask your Town/county contact how much land-- and where on the property-- you would need to acquire in order to proceed with your plans. Then ask this question: If you receive a Perpetual Easement for that land from the Owner, instead of owning it outright, does that give you any form of Zoning relief or do the actual boundaries still rule? (In 99 out of 100 cases, the boundaries will rule, but it's still worth the question.)
Here's another issue: Does any of the now-neighboring land affect your access to the premises? I'm not saying this is your case, nor that your seller is this diabolical, but often you'd hear cases of exactly this happening and it's the (let's say "Front") of the Land that's retained by the Seller... the new property line goes right across the driveway, no "easement" is mentioned in the Deed, and now your access is cut off. Now you have to pay another fee, to own, use, lease, license, permit, grant, or manage/maintain-- your OWN Drive to your OWN house. Many states, however, have statutes that prevent land locked parcels-- in these states an Owner has a RIGHT to access his property-- the abutters must offer a reasonable access. in your situation, I'd argue that it's also the case that the parcel must be SOLD with same rights in a state like this. If this is your situation in any way, then figure that out and memorialize it with the neighbors; don't be surprised if you need to get the Courts involved for this, but it would be your right in those areas.
Finally-- no actually FIRST. First, seek legal counsel. If there's some way to compel the Seller to give you what you need to make you whole, whether it's access-- or an Easement-- or a boundary line adjustment around structure-- or a boundary line adjustment around structure PLUS setback-- and/or all of the aforementioned plus enough land to make it a legal lot on its own (e.g., if the acreage is shy from the zoning requirement in the neighborhood,) then a good lawyer will know it. Please, don't just go to "Bob the Conveyancer" for this-- find someone who does subdivision regs, development, HOA and/or land use for at least 50% of his or her business, and they'll likely be able to tell you your next steps in that juris immediately.
Whatever solution you ultimately wind up with, pay the money, get it surveyed, and get that survey recorded in the official records.
Hope that helps.