The cost of a party wall agreement is not one number, it is two very different numbers with a fortnight between them. If the adjoining owner consents in writing, the cost is the notice, the postage and your time. If they dissent, or say nothing at all, the cost is the surveyors: yours, theirs, and a third if the two cannot agree. Understanding which side of that line your job falls on is worth more than any published average, and it is largely within your control.
The consent route, and what it costs
Notices written and served, and a schedule of condition agreed between the two owners with photographs and a date. That is the whole outlay, and it is measured in stamps and an afternoon. Most straightforward loft conversions and rear extensions could take this route, and the reason many do not is that the notice arrived cold, with no explanation, from a builder rather than from a neighbour. A covering letter that explains what consent means, in plain words, is the cheapest thing on this page.
The award route, and what drives the party wall award cost
Surveyor hours, on both sides, at whatever rates apply where you live. The variables that move it are the number of adjoining owners, whether each of them dissents, whether one agreed surveyor is appointed or two, how complicated the works are, and how much of the neighbour's property has to go into the schedule of condition. A basement with three neighbours is a different order of cost from a loft conversion with one, and it is the number of dissenting owners rather than the size of the building work that moves it most.
Working your own number rather than reading one
Take the owners you must serve, the ones you expect to dissent, your surveyor's rate and hours, and the adjoining owner's surveyor's rate and hours. The arithmetic is short and the worksheet on this site does it, keeping owners served and owners dissenting as separate inputs so you can see what a single written consent is worth. What this site will not do is publish a figure and let you anchor on it, because that figure would be wrong for your job.
Questions people ask about party wall agreement cost
Is there a cheap way to do this properly?
Yes: serve early, serve clearly, and ask for written consent. It is legal, it is what the Act contemplates, and it removes the largest cost from the job.
Do I pay if the neighbour appoints an expensive surveyor?
You pay reasonable costs. What is reasonable is itself a matter the surveyors determine, and an award that sets fees can be appealed to the county court within a short window.
Does the cost change if work has already started?
It usually rises. Without a schedule of condition taken before work, more time goes on establishing what condition the property was in, and the neighbour has less reason to cooperate.