Handled early it is administrative. Handled late it delays the start of work and sours a relationship you will live with long after the builders leave.
When notice is needed
Cutting into a shared wall for beam bearings, raising or rebuilding it, building astride the boundary, and excavating within the prescribed distances and depths relative to a neighbour's foundations all trigger notice requirements.
There are statutory notice periods before work can start, which is why the process belongs at design stage rather than the week before the first delivery.
Consent or an award
A neighbour can consent in writing, which ends the process quickly and cheaply. If they dissent or do not respond, surveyors are appointed and an award is produced setting out how the work will be done.
The award also records the condition of the neighbouring property beforehand, which protects both parties if damage is later alleged.
Keeping it civil
Talking to the neighbour before the formal notice lands makes a very large difference to whether they consent. People react badly to legal paperwork arriving unannounced.
A photographic schedule of condition is worth doing regardless of the route, and being straight about working hours, parking and deliveries prevents most disputes on site.
Common questions
Do I need a party wall agreement for a loft conversion?
Usually yes in a terrace or semi, because steels typically bear into the shared wall.
What if my neighbour ignores the notice?
Non-response is treated as dissent, and the surveyor route follows. That is why notices are served well before the intended start date.
Who pays for the surveyors?
The party carrying out the work normally bears the reasonable costs, which is another reason to seek written consent where possible.
Can I start before the notice period ends?
No. The statutory periods have to run, which is why the process is started at design stage.