Trellissing is considered a temporary structure which is probably why it is commonly higher than the regulated boundary wall height of 1.8m (6'0"). If properly and securely fixed to walls it is rare for neighbours to object, as trellissing can be very attractive addition to garden boundary providing clear definition of perimeters as well as very useful support for trailing plants and discreet screening without interfering significantly with light levels. It is this latter point, ie, blocking sun which can cause upset with neighbours.
Anyways if you intend to go up (900mm/3' 0") above boundary wall, it would be wise to at least discuss with neighbours before commencing works.