Why not ask them to plant shrubs instead? What are they trying to obscure? The slope, or your house as well. Working with them, and if they are amenable, might help find a better solution. There are some great shrubs that grow quite quickly that could fit the bill. But first, your zone, your soil and is it windy (I know it is north facing and in the shade) but a bit more info helps.

First, if there is an HOA, check & see if your covenants address this issue. Secondly, there are areas around where I live in NoVa where the county/city/town has rules about obscuring views & I know a lot of other places have the same rules. Finally, I agree withe ASVInteriors to try to talk with them & come up with a mutually plan if they are willing to work with you. The caveat to this final point is to have all of information you can gather from points 1 & 2 at hand before you meet so you are well educated about your rights.

Thank you for your comments. We are trying to work with them but they are adamant about the trees. They say privacy is the issue. There is a fence at our property lines and a hedge planted on the top of the slope by the fence about 3' tall. We cannot see their home unless we go to the edge and look over. I believe the real problem is our second story. They want palm tress but will end up with no coverage when they get tall just fronds in the view. Yes there is a HOA with rules but the neighbor says once they put them in they don't have to take them out. We live in CA. Soil is more clay like and yes there is wind. And most unfortunate they are friends with the representative for the HOA. Was hoping if I came up with some alternate solutions we would be able to come to a middle of the road