Buildable is not one rule but several, and a lot has to pass all of them. The first is the zoning: the district the lot lies in must allow the use you have in mind, by right or through a hearing, and the lot must meet the district's minimum area, width and frontage on a street.
The second is whether the lot was created lawfully. A lot carved out by deed without the subdivision approval the law required may not be recognized for a building permit. A lot that was legal when it was made but is smaller than today's minimum is usually called nonconforming, and many codes still let a house be built on it, sometimes with limits.
The third is the land itself. Wetlands, open water, a flood zone, steep slopes, protected trees and recorded easements can each take part of a lot out of use, and the setbacks the code requires from lot lines and from those features take more. What is left has to hold the building, its driveway and, often, its stormwater.
The fourth is service and access. A lot needs legal access, usually frontage on a public street or a recorded easement, and a way to get water and dispose of sewage: a public connection, or a well and a septic system the health authority will permit, which depends on the soil. The answer for a given lot comes from its own zoning, maps, survey and records; in South Carolina, for example, as elsewhere, those are held by the city or county that governs the land.