If they had planning permission then there is not much you can do about the extension blocking your light. You should have been asked by the planning department at your local council if you had any objections, not that that will do any good as a friend of mine in a similar position found out.
Regarding the elder you are entitled to cut back any branches that grow over the boundary but you should offer them back to the neighbour although they can decline and you then have to get rid of them. Elders can be cut back every couple of years so have a chat with your neighbour about doing this as it will keep the bush rejuvenated but not so tall and straggly and you will have a nice bush instead. On the plus side the flowers keep the flies away and can be used for elderflower cordial, champagne, and sorbet which are all very delicious. Also the more flowers you use the less the berries in the autumn, although they are enjoyed by the birds or can be made into a lovely elderberry wine.
Did they put an addition onto their house? Did they get a building permit from the city/town? If not you can complain to the building inspector. There is something called "setback" that says you can only build so close to the lot line. Everyone has to follow the rules.
WE now have reduced light due to neighbour’s recent extension and an elderberry tree between our houses which has grown very tall, if left alone it will touch our walls, this blocks a lot of light during spring summer not to mention all the black deposits from berries, I try to keep it all there side as its so fast growing ,I’ve cut back a huge amount of branches but they haven’t said anything to me but I could do without all the hassle
