Air rights are rights to develop the empty space located above a physical property site. As with other types of rights associated with real estate, it is possible to transfer these rights to other parties while retaining ownership of the property itself. In highly developed areas where the only possible direction to build is up, air rights are a hot topic of conversation and debate as developers fight to extend their projects while still remaining within the confines of zoning law.
Historically, property owners retained essentially infinite property rights. As human societies developed and began instituting zoning laws, people began to be limited in terms of development by only being allowed to build to a certain height, or by having their total floor area limited under zoning regulations. Such laws may be implemented for safety concerns, to meet aesthetic goals, and for other reasons. They limit air rights by restricting what people can do with the empty space above their properties.
Another complication that changed the way people thought about air rights was the development of aircraft. Property owners do not control the air space over their properties, as otherwise planes would be in a constant state of trespass. Instead, regulatory agencies control airspace and mandate the way that planes use airspace to keep air travel safe and efficient. The limitation of control over airspace is a bone of contention in some regions, especially around airports.
When people buy property, they usually buy the rights that come with it. Leasing and renting also entitles people to some rights, depending on the nature of the agreement that comes with the property. It is possible to merge lots to transfer air rights, as seen in some urban areas where people with unused air rights may merge with a neighboring lot and transfer the rights that aren't being used. Likewise, people can lease, sell, or donate air rights to other parties. Historic buildings, for example, may be protected with donations of rights to keep the space above these buildings undeveloped.
Air rights are not unlimited. Building codes restrict not only the amount of development people can engage in, but the nature of that development. Controlling the space over property does not give people the right to develop whatever they want there, and people must also comply with safety regulations when making new developments. This is designed to promote public health and safety by standardizing construction in order to limit the risk of building collapses, fires, and other preventable disasters.