In the classic Monopoly board game, what must a player do before building houses?

Before building houses in classic Monopoly, a player must own the entire color group.

This rule is called owning a monopoly: the player must possess every street in that color set. Once the set is complete, the owner may buy houses from the bank at the listed building price. The player must still build evenly, so one street cannot become heavily developed while another street in the same group remains undeveloped.

Owning a complete group also doubles the rent on unimproved streets in that group, unless the property is mortgaged. This makes group completion valuable even before construction begins. Railroads and utilities do not use houses or color-group building rules.

Players sometimes assume that landing on a property or collecting enough cash grants building rights. It does not. The deed set must be complete, and the bank must have the required buildings available.