There are three notices in the Act and the job of choosing between them takes a minute, but choosing wrong costs a month or two. The party structure notice, under section 3, is the one for work to a structure that already exists and is shared. If the wall is not there yet, you want a line of junction notice instead. If you are digging rather than building on the wall, you want the excavation notice. Here is the boundary between the first two, which is where the confusion usually is.
What work a party structure notice covers
Work to an existing party structure: cutting into it to take the bearing of a beam, raising it, underpinning it, cutting away a projection, rebuilding it, and removing a chimney breast that is part of it. In a loft conversion the steel beams that carry the new floor almost always bear into the shared wall, and that alone is enough. In a rear extension it is often the flank wall being raised, or a chimney breast coming out to make room. If the structure exists and is shared and you are altering it, this is the notice.
How it differs from a line of junction notice
A line of junction notice, under section 1, is for building a wall where none stands on the boundary at present. The two are easy to tell apart in practice: is there already a shared wall there? If yes, section 3. If you are putting up something new on or at the line, section 1. The periods differ too, which is why the choice matters: a party structure notice runs two months and a line of junction notice one, so serving the wrong one can cost a month you did not budget for.
What a party structure notice template has to carry
The same three things every notice under the Act carries: the building owner's name and address, the nature and particulars of the work, and the date it will begin. Where special foundations are proposed the notice must include plans, sections and details of construction together with reasonable particulars of the loads they will carry. Because the adjoining owner may serve a counter notice asking for the work to be done differently, describing the work properly at this stage saves an argument later.
Questions people ask about party structure notice
Can one notice cover several kinds of work?
Yes, provided it describes the particulars of each. What it cannot do is cover several owners: each adjoining owner is served their own notice.
What is a counter notice?
An adjoining owner may serve a counter notice asking for additional work to be done for their benefit, such as chases or foundations built to suit a future extension. They pay for the extra, and the counter notice has its own timetable.
Does removing a chimney breast always need a notice?
If the breast is part of the party structure, which in a terrace or semi it usually is, then yes. Taking one out without notice is one of the commonest reasons a neighbour ends up with a claim, because the remaining stack above needs support that somebody has to provide.