‘Ancient Lights’ is an English property law giving house owners the right to receive natural light through a window – if that particular window has been receiving light uninterrupted for 20 years.
Once a window wins the right to ancient lights, adjoining land owners cannot obscure them by erecting a building, raising a wall, planting trees, etc.
In England, the rights to ancient lights are most usually acquired under the Prescription Act 1832.
In American common law the doctrine died out during the 19th century, and is generally no longer recognized in the United States.
Japanese law provides for a comparable concept known as nisshōken (literally “right to sunshine”).
In our modern times we can still find what one might consider a hybrid concept of the Ancient Lights rule.
In 1984, voters in San Francisco passed Proposition K, which prevents construction of any building over 40 feet that casts a shadow on a public park, unless the Planning Commission decides the shadow is insignificant.
Massachusetts has similar laws against the casting of shadows on Boston Common, the Public Garden, and other important public open spaces.
In 2016, the Eneref Institute launched the Right To Daylight campaign to promote the idea that daylight is a natural right.