What happens if you spend $25K covering your roof with solar panels and somebody builds a tall building that throws them into shade? We used to have rights. When working with a real estate developer in the early nineties we had a neighbour oppose our project, saying that their building had a "right to ancient light" (extinguished in Toronto in 1880 but their building was older. As you can see here, it is now completely in shadow) and had us all scratching our heads. According to Bldgblog, they still have it in the UK but you have to work to preserve it.
The Royal Institution of Chartered Surveyors says that "many people are allowing adjacent buildings to block their natural light, unaware that they have a legal right to it. Light blocking can be classified as a ‘nuisance’ alongside noise and air pollution and culprits range from large new commercial developments to a neighbour’s building extension or a new garden shed. Even a tall hedge can be a problem." The tone of the RICS abruptly shifts at this point, however, as they begin to explain how you can actually prevent your neighbours from acquiring ancient light rights. There is a "need for vigilance to prevent neighbours acquiring a right to light," they warn; after all, such an acquisition "may hamper future development and investment possibilities" on your own property. "It is possible to prevent a building acquiring a right to light," the RICS explains, "but despite the procedure being simple, it is rarely used."
Wikipedia says that the right died in the USA in 1959: "Under American Tort law, in Fountainebleau Hotel Corp. v. Forty-Five Twenty-Five, Inc. (1959) the Florida Appellate Court stated that the "ancient lights" doctrine has been unanimously repudiated in the United States." ::Bldgblog
and see the google earth explanation of the Fountainebleau Hotel Corp. v. Forty-Five Twenty-Five case here.