Semi detached property where Neighbour is replacing only their roof.

Most competent roofers are quite capable of doing a roof on a semi without damaging the adjoining roof whether that be with the same tiles fitted together or having to fit a separation valley to separate the roofs the only thing you can do is to have a look when they finish each side and point out anything your unhappy with, building control should be signing of the roof so if in doubt have a word with the local building inspector signing off the job who will be happy to alay any fears you may have. Good luck Alex