Air rights are a type of development right in real estate

Real estate

In general use, esp. North American, 'real estate' is taken to mean "Property consisting of land and the buildings on it, along with its natural resources such as crops, minerals, or water; immovable property of this nature; an interest vested in this; an item of real property; buildings or...

, referring to the empty space above a property. Generally speaking, owning or renting land or a building

Building

In architecture, construction, engineering, real estate development and technology the word building may refer to one of the following:...

 gives one the right to use and develop the air rights.

This legal concept is encoded in the Latin phrase Cuius est solum, eius est usque ad caelum et ad inferos

Cuius est solum eius est usque ad coelum et ad inferos

Cuius est solum, eius est usque ad coelum et ad inferos often appearing in the shorter form Cuius est solum eius est usque ad coelum, omitting et ad inferos "and to hell", is a principle of property law, stating that property holders have rights not only to the plot...

("For whoever owns the soil, it is theirs up to Heaven and down to Hell."), which appears in medieval Roman law

Medieval Roman law

Medieval Roman law is the continuation and development of ancient Roman law that developed in the European Late Middle Ages. Based on the ancient text of Roman law, the Corpus iuris civilis, it added many new concepts, and formed the basis of the later civil law systems that prevail in most...

 and is credited to 13th century glossator

Glossator

The scholars of the 11th and 12th century legal schools in Italy, France and Germany are identified as glossators in a specific sense. They studied Roman Law based on the Digestae, the Codex of Justinian, the Authenticae The scholars of the 11th and 12th century legal schools in Italy, France and...

 Accursius; it was notably popularized in common law

Common law

Common law is law developed by judges through decisions of courts and similar tribunals rather than through legislative statutes or executive branch action...

 in Commentaries on the Laws of England

Commentaries on the Laws of England

The Commentaries on the Laws of England are an influential 18th-century treatise on the common law of England by Sir William Blackstone, originally published by the Clarendon Press at Oxford, 1765–1769...

(1766) by William Blackstone

William Blackstone

Sir William Blackstone KC SL was an English jurist, judge and Tory politician of the eighteenth century. He is most noted for writing the Commentaries on the Laws of England. Born into a middle class family in London, Blackstone was educated at Charterhouse School before matriculating at Pembroke...

; see origins of phrase for details.
Unlimited air rights existed when people began owning real estate

Real estate

In general use, esp. North American, 'real estate' is taken to mean "Property consisting of land and the buildings on it, along with its natural resources such as crops, minerals, or water; immovable property of this nature; an interest vested in this; an item of real property; buildings or...

.