In 40 seconds
If your building work is covered by the Party Wall etc. Act 1996 — work on a shared wall, building on or astride the boundary, or excavating close to a neighbour's foundations — you must serve a party wall notice at least two months before the work to a party wall starts (one month for some excavation and boundary-wall work). If the neighbour dissents or does not reply, a surveyor is appointed to produce a party wall award. Where one agreed surveyor acts for both sides, that award commonly costs roughly £700–£1,200+ for a straightforward residential job; where each owner appoints their own, the total is usually higher, often around £1,000–£3,000+. The building owner who is doing the work normally pays these reasonable fees. Many purely internal jobs are not notifiable at all — so the honest first question is whether you need an agreement, then what it costs. This is general guidance, not legal advice.
Most party wall guidance is published by firms selling surveying, so the figures tend to be presented as fixed and the 'do I even need this?' question gets skipped. The pages below give honest cost ranges, explain when the Act actually applies, set out how notices and the two-month period work, and explain what happens if a neighbour ignores a notice — citing the Party Wall etc. Act 1996 and gov.uk guidance. It is information, not legal advice.