How the third surveyor is chosen
As soon as both surveyors are appointed, they agree on a third, usually an experienced party wall surveyor. If they cannot agree, the Act provides for appointment by the local authority.
When they are called in
- The two surveyors cannot agree something
- Either owner disagrees with something one of the surveyors has decided
- There is a dispute about fees
Either owner or either surveyor can make the referral.
What they can decide
The third surveyor can decide the matter referred to them and make an award on it. That award is binding, subject to the same 14-day right of appeal to the county court.
Costs
The third surveyor's fees are decided in their award and are paid by whichever party they direct, which may not always be the building owner, particularly if a referral was unreasonable.
A useful safety valve
Referral is far quicker and cheaper than court. Knowing it exists encourages the two surveyors to resolve differences themselves.
Where this comes from in the Act
Links go to the official text of the Party Wall etc. Act 1996 on legislation.gov.uk.