According to the Party Wall etc. Act 1996, if an adjoining owner does not consent within 14 days of receiving notice of the proposed works then the parties are deemed to be "in dispute".
What this actually means is that a Party Wall Agreement or Party Wall Award will have to be drawn up by a Party Wall Surveyor. Party Wall Surveyors have a duty to be impartial and so there should not be any disagreement per se although there are often misunderstandings as to what the Act means.
Basically by not consenting and hence instigating a dispute the adjoining owner is stating that they would like to have their interests protected by an independent person who understands construction matters and Party Wall issues.
Section 6(7) of the Act states that if an owner on whom a notice has been served does not serve a notice indicating his consent to it within a period of fourteen days from the date the notice was served he shall be deemed to have dissented from the notice and a dispute shall be deemed to have arisen between the parties.
If the adjoining owner does not appoint a surveyor, or concur in the appointment of an agreed surveyor, within the 14 day period they will receive a follow-up letter asking them to appoint a Party Wall Surveyor within 10 days otherwise one will be appointed on their behalf by the building owner.
If it ends up that the building owner appoints a surveyor on behalf of the adjoining owner this does not mean it will be an "agreed" surveyor but rather a separate qualified person who will fulfill the role.
A surveyor may serve notice on behalf of their client but as soon as a dispute arises the owners switch from being 'clients' to 'appointing owners'. The term appointing owner re-enforces the surveyor's duty to be impartial.