I think I'm safe in presuming that your starting point is that there should be a presumption against the overshadowing of solar panels by new development. However, my concern would be what the public interest is in restricting the right to develop land in order to maintain the amount of sunlight hitting a 3rd party's solar panel. I don't see that it would be any more valid that restricting development on the basis that it affects someone's view, which of course is not a public interest matter. Solar panels are installed with the risk that someone may carry out a development that could reduce the amount of light hitting them - the owner of the solar panels may not know that when they have them installed, but the risk is there. Just as there's a risk that climate change will (ironically) reduce the amount of sunshine we get and (less ironically and just as annoyingly) feed-in tarrifs may change with little notice from the Government. It's not a question of spoiling the enjoyment of the dwellinghouse as such - we're not talking about overshadowing habitable rooms and garden space. A house with a solar panel is still going to have regular mains electricty and some sort of hotwater backup if the panels are solar thermal, and it's not as if they wouldn't work - they'd just generate less power/hot water. There's also a choice of wind, ASHP, GSHP, biomass, micro-CHP, etc: no-one's forced to choose a technology that might be overhsadowed. Indeed, if the protection of solar panels from overshadowing were valid then the principal would have to be extended to all other forms of renewable generation. This does, though, draw attention to the need to design future-proofed rewewables on new developments wherever possible (eg. avoid a landscaping scheme that would one day overshadow solar panels or dampen the wind for a turbine), or allow for technologies to be retrofitted to a scheme (eg. future GSHP potential could be negated by badly planned services).
