Zoning usually caps how many homes a site may hold, often as units per acre. A density bonus raises that cap when the project provides something the community wants, most often homes priced for households below a set share of the area's median income. The larger the affordable share, the larger the bonus tends to be.
Most bonuses are local and voluntary: the code offers the extra density and the developer chooses whether to use it. A few states require local governments to grant one; California's state density bonus law is the best-known example. Some places instead require affordable units in every project above a certain size, usually called inclusionary zoning.
The terms matter as much as the bonus. Codes set which income levels qualify, how long the homes must stay affordable, often recorded as a deed restriction, and whether the bonus brings other relief, such as smaller lots, less parking or a faster review. Some bonus paths are by right; others need a hearing.
A bonus only helps if the land can hold it. Wetlands, easements, trees, roads and lot-size rules still apply, so the realistic gain is found by fitting the bonus to the buildable area, not the acreage. In Charleston County, South Carolina, for example, the zoning code allows higher density where a share of the homes qualify as affordable or workforce housing.