New construction home offering double lot

[Joseph Labit]

My wife and I are looking to buy a new home or lot and have a new home built. I found a new construction home in a subdivision that is in the final close out. One is the national home builders. They have 7 lots left and they are putting spec homes on 5 of the lots. The other 2 lots they are offering as a double lot option to the adjacent lot. They are only charging 13k extra for the double lot option. Each lot is .33 acres. A lot that size in this worth around 75-80k. I am wondering what the catch is? They have 2 pins. Homes are being built on the left, right and rear of the home so I assuming the lot is buildable. The sales lady did tell me that they did a soil test and found higher levels of sand in the soil so it would be more expensive to build, but she said they will build on it if the double lot option is not taken. They did offer the same deal on 2 other lots in the subdivision, but they ended up splitting them up and building on both lots. Wondering if I should take both lots and try to sell the second one off or possibly build on it later? How would I find out the extra cost of building on a lot with sand? Would it be closer to 10-20k extra? Or something crazy like 75-100k extra?