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 extension party wall: 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 of a party wall extension, 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.
Where a new boundary wall may sit: section 1
A line of junction notice describes a wall you intend to build on or at the boundary, and the neighbour's reply decides where it may go. If they consent in writing to a wall astride the line, it is built astride the line, at a cost shared according to the use each owner makes of it. If they do not consent, or you never asked to build astride, the wall goes up wholly on your own land, as an external wall or a fence wall, at your own expense. Either way the footings may project under the neighbour's land where that is necessary for the construction, provided they are not special foundations, and you compensate the neighbour for any damage that causes. The practical point for an extension is width: a wall wholly on your side costs you the thickness of a brick along the whole length of the room, and a wall astride the line costs you a conversation.
Getting onto the neighbour's land to do the work: section 8
Building against a boundary means somebody standing on the other side of it, to lay the outer skin, point it, weather the junction or take down a scaffold. The Act gives a building owner, their servants and workmen a right to enter the adjoining owner's land to execute work in pursuance of the Act, on fourteen days' notice of the intention to enter, or without notice in an emergency. It is an offence to refuse entry to someone entitled to it who has given that notice. That right is for the work the Act authorises, not for convenience, and it is a poor thing to rely on cold: an award normally settles access dates, hours and what is put back afterwards, and a neighbour who was asked properly rarely has to be shown the section.
Security for expenses: section 12, and when a neighbour asks for it
An adjoining owner who is worried that an extension will be started and then abandoned half way, with their wall opened and their footings exposed, may serve a notice under section 12 requiring the building owner to give security for expenses before the work begins. The amount is agreed between the owners or, if they cannot agree, determined by the surveyors under section 10, and it is held so that the neighbour is not left paying to make their own property safe if the building owner walks away. It is asked for more often on basements and on deep excavations near an old house than on a rear extension, but it can be asked for on any notifiable work. A building owner who has already agreed a programme, a contractor and insurance, and says so in the notice, is asked for it less.
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.