Solar panels

Hi everyone, we have a high south facing wall on the outside of our property which is also the boundary wall. Today we had a solar company installing panels for the house opposite us and we asked them for a quote, assuming we could only have a few panels on top of our storage unit on the roof terrace. He said, for maximum benefit, he would install them high up on the south facing wall, and therefore sticking out about 30cm from our boundary. Although we have an empty plot next to us at the moment, we know it will be built on at some point. We've done our homework on the height of the sun and the impact of a taller building on the panels and that wouldn't be a problem. Does anyone know if legally there's a problem with having the installation? Or any other issues we should know about?

Many thanks, Helena