when you tear down a building on property its tax basis gets added to the tax basis of the land along with the demolition costs.
as to the "vacant land sale" that's covered by reg 1.121-1(b) so you need to discuss this with your CPA
(3) Vacant land—(i) In general. The sale or exchange of vacant land is not a sale or exchange of the taxpayer's principal residence unless—
(A) The vacant land is adjacent to land containing the dwelling unit of the taxpayer's principal residence;
(B) The taxpayer owned and used the vacant land as part of the taxpayer's principal residence;
(C) The taxpayer sells or exchanges the dwelling unit in a sale or exchange that meets the requirements of section 121 within 2 years before or 2 years after the date of the sale or exchange of the vacant land; and
(D) The requirements of section 121 have otherwise been met with respect to the vacant land.
(ii) Limitations—(A) Maximum limitation amount. For purposes of section 121(b)(1) and (2) (relating to the maximum limitation amount of the section 121 exclusion), the sale or exchange of the dwelling unit and the vacant land are treated as one sale or exchange. Therefore, only one maximum limitation amount of $250,000 ($500,000 for certain joint returns) applies to the combined sales or exchanges of vacant land and the dwelling unit. In applying the maximum limitation amount to sales or exchanges that occur in different taxable years, gain from the sale or exchange of the dwelling unit, up to the maximum limitation amount under section 121(b)(1) or (2), is excluded first and each spouse is treated as excluding one-half of the gain from a sale or exchange to which section 121(b)(2)(A) and § 1.121–2(a)(3)(i) (relating to the limitation for certain joint returns) apply.
