A growable is a kind of building which the player does not place directly. Typically, the player designates a zone and appropriate growables appear when their needs are met. For example, the player might place a high-density residential zone in an area with moderate land value. The growables that would appear would mostly be middle-income apartment buildings.
Growables are distinct from "ploppables", which are buildings placed directly by the player. Growables are privately owned buildings and the player does not pay for their upkeep. Growables are the primary source of income for the player (via taxation) in most city-builder games.
Multiple growable assets need to be created for any given zone. At a minimum, this includes size - each zone will need at least ten growable assets ranging in size from an 8m × 8m lot (0.02 acres) up to a 32m × 32m lot (0.25 acres). New architectural styles may develop as HONs do more research and as land values increase; each of these will need its own set of assets.