The lowest gain would be if you threw up a trailer/mobile home on the 2 acre lot, lived in it for 2 years and sold it. (no capital gain) then built and live in the house on the 11 acre lot.
Other than that, you would need to show the IRS that you used a reasonable process to come to a basis for the 2 acre and 11 acre lot.
I would use a percentage of value (.85 for the 11 acres, .15 for the 2 acres or 84,615 and 15385 or 7,693/acre ) to allocate the expense to separate the lots.
The value of the lots could be allocated by an appraiser or Real Estate Broker if some of the land is more valuable that the rest.
The best answer is that you would need to have "Justification" of what values you use to answer to the IRS if needed.
