Extensions are where people most often serve one notice and owe two. The wall work and the groundwork are covered by different sections of the Act, with different periods, and a job that raises the flank wall while digging trench fill footings beside the neighbour's house has met both. Here is how to tell which sections your extension touches, and what building right up to the boundary changes.
The wall half of the job
If the extension raises, cuts into or is built off the existing shared wall, that is a party structure notice under section 3, served two months ahead. An extension party wall in a terrace is normally the flank wall being taken up, and the beam over the new opening bearing into it. If instead you are building a brand new wall on or at the line of junction where nothing stands, that is a section 1 notice, served one month ahead. The two are easy to tell apart: is there already a wall there?
The groundwork half, which is the one people miss
Footings for a rear extension are commonly deeper than the shallow foundations of an older neighbouring house, and if you are within three metres of their building and going below their foundation level, section 6 applies and a notice is owed a month before you begin to excavate. The six metre test with its forty-five degree line catches deeper digs further away. Neither test measures from the boundary: both measure from the neighbour's building, so a house set back changes the answer.
Building against a party wall, and what it does not give you
Building your extension hard against the shared wall does not make the wall yours, and enclosing on a neighbour's wall may bring an obligation to pay for the use you make of it. It also brings practical questions the award should settle: how the junction is weathered, whether the wall is raised or built against, and what happens to the neighbour's rainwater goods. Where you are building right on the line, the Act's line of junction provisions decide whether the new wall may sit half on their land at all.
Questions people ask about party wall agreement for extension
Can I build the extension wall astride the boundary?
Only with the adjoining owner's consent following a line of junction notice. Without consent you may build wholly on your own land, as an external wall, at your own expense.
Do I owe a section 6 notice if the neighbour's house is six metres away?
Only if the excavation cuts the forty-five degree plane drawn from the bottom of their foundations. On a normal rear extension at that distance, usually not; on a basement, quite possibly.
Does planning permission change any of this?
No. Planning, building regulations and the Act are independent, and permitted development rights do not remove the obligation to serve a notice.