You can't put a gutter on top of a roof, without it being some mickey mouse job, which will most likely put future buyers off.
If it needs adapting, then you have to fit a box gutter which remains on your side of the boundary, and the roof reduced to discharge into the gutter. It's a lot of messing about.
Or you remove the windows at the side, brick it up (or just block up on the inside), and then adapt the roof. This will be a much better option as it removes future problems with the neighbour.
This is the price attached to moving to your "dream home", and you have to pay it.
If it needs adapting, then you have to fit a box gutter which remains on your side of the boundary, and the roof reduced to discharge into the gutter. It's a lot of messing about.
Or you remove the windows at the side, brick it up (or just block up on the inside), and then adapt the roof. This will be a much better option as it removes future problems with the neighbour.
This is the price attached to moving to your "dream home", and you have to pay it.