Serve the notice first
An unserved neighbour can injunct the site.
Extensions and excavations near a neighbour need a party wall notice first. Skipping it can stop your build.
The Party Wall etc. Act 1996 is the most frequently ignored piece of law in domestic building, mostly because it feels like bureaucracy between neighbours who get on. Cutting into a shared wall, raising it, inserting a beam into it or excavating within the distances the Act specifies of next door's building all require notice to be served on that owner before work begins, with a statutory period for them to consent or dissent.
Ignoring it works right up until it does not. An adjoining owner who was never served can apply for an injunction, and a stopped site with trades booked and materials delivered is expensive in a way no party wall surveyor's fee approaches. It also poisons the relationship with the person living on the other side of the wall for the rest of the project, which matters more than people expect during months of noise.
The certificate at the other end of the job is the same kind of problem in slow motion. Building work requiring Building Regulations approval should finish with a completion certificate, and the moment it is missed is rarely the moment it hurts: that comes years later, when a buyer's solicitor asks for it and the sale stalls. Both of these are cheap to do at the right time and awkward to fix afterwards, which is the entire argument for doing them.
An unserved neighbour can injunct the site.
A buyer's solicitor will ask for it.
Gas, asbestos and notifiable electrics go to specialists.
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