When planning permission is granted it does not confer all of the other legal rights required to actually go ahead with the proposal, they are only assessing its impact on the neighbourhood. For example i may get planning permission to build a house in your garden because the council thinks it would look ok, but without your permission i couldn't actually build it as you own the land. When it comes to light, you may in some circumstances gain a right to light over a period of time, and if this is breached your action would be agains the homeowner building the extension and not the council. Talk to a solicitor.