The Party Wall etc. Act is a procedure, not a permission. It does not let a neighbour veto your extension, and it is not part of the planning system. What it does is give a neighbour a right to be told, to have the condition of their property recorded, and to have a surveyor decide how the work is done where the two of you cannot agree.
The three kinds of notifiable work
Work on an existing party wall, such as cutting in a beam or raising it. Building a new wall at or astride the boundary line. And excavating within the distances the Act sets out of a neighbouring building, where the depth of the dig relative to their foundations is what matters. Most rear and side extensions touch at least one of these, which is why the procedure is so common and so often discovered late.
Notice, response and what silence means
Notice is served on the adjoining owners with the details the Act requires, and each has a period in which to consent or dissent. Consent ends the process. Dissent, or no reply at all, means a dispute has arisen under the Act and surveyors are appointed. Silence is therefore not agreement, which is the point most homeowners are surprised by, and it is the reason notices belong at the start of a programme rather than the end.
What a party wall agreement for extension work actually decides
The award records the condition of the neighbour's property before work starts, sets out how and when the notifiable work will be done, and deals with access and with making good. It does not decide whether you may extend, and it cannot be used to object to the design. The building owner normally pays the reasonable surveyors' costs, which is the cost most often left out of a budget.
Side extension boundary rules UK councils apply sit on top, separately
Planning rules about how close to a boundary you may build, how high a side extension may be and whether it must be set back are entirely separate from the Act, and satisfying one says nothing about the other. Read the permitted development guidance and the local authority's own design guidance for side extensions, then handle the party wall procedure as a parallel track with its own timetable.
Questions people ask about extension party wall
Can a neighbour stop my extension under the Act?
No. They can dissent, which brings surveyors in to decide how the notifiable work is carried out, not whether the extension happens.
What if the neighbour does not reply?
No reply counts as dissent, a dispute arises under the Act, and surveyors are appointed. Do not read silence as consent.
Who pays for the surveyors?
Normally the owner doing the work, since it is their project that triggers the procedure. Budget for it from the start.
Do I need a notice if I build entirely on my own land?
Possibly, because excavation near a neighbour's foundations is notifiable even when the wall is wholly yours. Check the distances and depths in the explanatory booklet.