It makes practical sense e.g. having a defined space but I would say that the decision ought to be agreed in full by both parties not one telling the other.
We lived in a terrace of four cottages and had a “firewall” between us and our neighbour (we were an end terrace so only one side needed) – oddly there was a door in ours into their roof space!
If they go ahead without your contribution make sure it is in the right place.
Further down our terrace someone got the line “wrong” and so one neighbours roof hatch went into the neighbouring property’s roof space not theirs …