Some legal experts in the United States have called for a clarification or change of laws or even the formulation of new laws if solar access is to be guaranteed. This raises an interesting question of legal precedents.
The most commonly cited law outside the United States is the English Doctrine of Ancient Lights; but there are problems with its application.2 Roughly, the doctrine states that if in living memory no one has overshadowed your property, they cannot now do so. However, this doctrine has been repeatedly disavowed in US courts.
Some legal experts have suggested that American water law, especially the doctrine of prior appropriation, may offer a more useful precedent for sun rights.3 Both sunlight and water are used rather than captured and sold; both may be consumed, but both are renewable. In addition, there is an equivalence between upstream and downstream in water law and the geometry of solar shadowing. But, like the Doctrine of Ancient Lights, there are problems with the application of water law.
The doctrine of prior appropriation is a formalization of the general practice among early Western settlers of appropriating available water according to who first put it to beneficial use. Simply put, "He who gets there first, gets the most." It was the American frontier's answer to the exigencies of pioneer settlement.
Prior appropriation is not likely to be applied to solar allocation in any simple way. Future access would not be assured for structures without present energy-conversion systems. Several permits acting on different, adjacent properties (as well as those on distant sites) may conceivably act to stop development completely on one of them. This point has been made abundantly clear in the writings of legal experts who point out serious weaknesses in any attempt to move directly from water law to solar law.
The difficulties in applying water law have led to arguments for straightforward
zoning as a more appropriate approach to the problem.4 First, it offers
the possibility of more local administration of rules affecting the allocation
of sunlight. Second, zoning is traditionally applied to all properties in
a district thus assuring future access and bypassing the problems of preference
based on prior use. Finally, existing zoning limiting heights and setbacks
is already based on the concept of an envelope of buildable volume. These
reasons have been found compelling and have led to development of the solar
envelope.