Whose Roof Is It?
In a lot of older terraces the roof void is not divided at the party wall, or is divided only by a wall that stops short of the tiles. A colony that entered at number 12 can be sitting over number 14, and the person hearing it is not the person whose elevation the wasps are using.
That produces the common and awkward situation where the noise is in your bedroom ceiling and the entrance is on a wall you do not own. It is worth establishing before booking, because treating somebody else's elevation without their agreement is not something we will do.
Having the Conversation
In practice most neighbours agree quickly, because it costs them nothing and the nest is as much over their heads as yours. What helps is going round with the facts rather than the complaint: where the entrance is, that the treatment takes a few minutes, that nothing is drilled or opened, and that it does not touch their property beyond the point the wasps use.
Where a neighbour is a rented property the conversation is with the landlord or agent rather than the tenant, and that is worth establishing early because it adds days rather than hours.
Shared Entrances and Party Walls
The other terraced pattern is a shared entrance point: a gap at the junction of two roofs, a valley gutter between them, or the top of a party wall where it meets the tiles. Those are genuinely joint, and it is usually simplest for one household to arrange it and settle up between them afterwards.
One visit deals with one nest whichever side of the boundary the entrance sits on. It is a single fixed price, not one per house.
What Not to Do About a Neighbour's Nest
Do not treat, spray or block an entrance on a neighbour's property, however accessible it looks from your side. Beyond the obvious, a blocked cavity or roof entrance sends the colony chewing inward — and that damage lands in somebody else's house.
If a neighbour will not engage and the nest is genuinely on their property, the environmental health team at the local council is the route rather than anything done from your side of the fence.