What's happening here is that the lender can't or won't lend on vacant land, which is why they're trying to combine the two parcels together to push the loan through. Lending terms for vacant land usually require a higher down payment and shorter time frame to pay off the loan than the typical 30-year mortgage that you'll see on loans for residential properties.
If you combine the acreage of both parcels and find similar properties with similar double-lots as comps, you still wouldn't be appraising the property at it's highest and best use. The vacant lot would then be encumbered under the same mortgage as the residence, and the only way that the buyer would be able to split out and sell that vacant lot would be if the house and lot that it sits on would appraise for approximately 20% or more above the existing loan balance, effectively keeping the loan collaterol at an 80% LTV. At that point the owner could request that the vacant parcel be released from the mortgage, and depending on the lenders terms they will determine if that is possible.
That being said, you've already determined that the house and lot that it sits on would not, by itself, appraise for the contract price of $330,000. The vacant lot, if encumbered by the same mortgage, is technically little more than excess land. A new construction loan could not be gotten on it since the lot is encumbered by a mortgage tied to the existing residence, and it can't be sold as a vacant buildable parcel.
Many buyers don't understand how this all works. They're trying to get the smallest mortgage at the best interest rates possible, but by doing so they've effectively cut off the possibility of taking advantage of the properties at their highest and best use, which is to keep the parcels separate. The buyer in this case may be thinking that as soon as he closes on this loan that he'll sell that vacant lot and stick $65,000 in his pocket, but the fact is that unless he's put one heck of a big downpayment on this loan he'll be prevented from doing so until the house and lot it sits on appraises for $330,000 all by itself, or he can negotiate with the lender that a portion of his profits would buy down the existing principal balance on the loan to bring the LTV into line.
I would never ASSume that the buyer understands all of this, or that the loan officer or selling agent would bother to explain it to him. I would assume that if dividing up the properties after the loan is closed is what is on the buyer's mind, he'll be very upset at someone, and that someone would probably end up being the appraiser who didn't value the properties at their highest and best use. I'm sure that any good real estate attorney would point him in your direction if he's not sure who to blame for his dilemma.
You don't know anything about this buyer's financials. It's possible that he's putting as little down as possible and COUNTING on selling that vacant lot just so that he can make his payments. How are you going to explain yourself if the guy ends up defaulting and your appraisal gets reviewed and you didn't value the properties at their highest and best use? Even if the buyer and/or mortgage broker swear on a stack of Bibles that the buyer has no intentions of doing anything with the extra lot, you're still on the hook.
Dave, if I were you I'd stick with the separate parcel appraisals. The contract can be renegotiated to be two separate parcels and sales, and financed separately. It may not be the best loan scenario for the buyer if they had no intention of doing anything with the vacant parcel, but that's not your problem.