The honest answer is that the parts fixed by the Act are the easy parts, and they are also the parts people leave too little room for. Two months for a party structure notice, one month for the other two, and a fortnight for the reply are all known in advance and can be counted from a start date on a calendar. Everything after that depends on people. Here is the timeline as it usually runs, in the order the weeks arrive.
The fixed part: the notice period
A party structure notice runs at least two months before work begins. A line of junction notice and an adjacent excavation notice run at least one month. Those periods start when the notice is served rather than when it is posted, so allow for the post as well. The worksheet on this site counts them at their longest, two calendar months as sixty two days and one as thirty one, because a notice a day short is not a notice, and it gives you the days you have left rather than a date you have to work out.
The part that depends on the neighbour: the reply
A fortnight from service. Consent in writing ends the process there and the two months is all you wait. Silence, or a dissent, moves you to appointments. In practice the reply is the single biggest variable in the timeline, and it is the one most within your influence: a neighbour who was told what was coming, in person, before a formal letter arrived, replies. A neighbour who opened a legal looking envelope from a builder they have never met does not.
The part that depends on the surveyors: the award
Appointments in writing, an inspection of the adjoining owner's property, the schedule of condition, agreement between the surveyors on the works and the conditions, then drafting and service. Weeks rather than months on a straightforward job with one agreed surveyor; longer with two surveyors, several adjoining owners, or a design that keeps changing. Then a fortnight in which either party may appeal the award to the county court, which is normally uneventful but is part of the calendar.
Questions people ask about how long does a party wall agreement take
Can the neighbour agree to a shorter notice period?
The periods are minimums set by the Act. A consenting neighbour can shorten the practical wait by replying quickly, but a notice cannot run for less than its statutory period.
What is the fastest this can go?
Notice served today with a written consent by return, and a schedule of condition agreed between you: the notice period, and nothing else. That is why asking plainly for consent is worth the effort.
Does the notice expire if my build slips?
A party structure notice ceases to have effect if the work has not begun within twelve months of service and is not prosecuted with due diligence. A job that slips a year needs serving again.