Density

Density describes how many homes fit on a piece of land. Zoning codes control it directly, through units-per-acre limits, minimum lot sizes, and caps on the number of units per building, and indirectly, through height limits, setbacks, floor area ratio, and parking requirements that constrain how much building can physically fit.

Density matters for housing costs because land is a fixed cost that is divided among the units built on it. The more units a site can hold, the smaller each unit's share of the land price, and the smaller the rent needed to cover it.

Land cost per unit = land price ÷ number of units

Density is not free. Beyond a certain height, construction shifts to more expensive structural systems, and higher density can require more elaborate parking, elevators, and fire protection. There is a range in which adding units lowers cost per unit, and a point past which it starts to raise it. Where that point falls depends on the site, the building type, and local rules, which is why the same density limit can be harmless in one place and costly in another.