Removing a chimney breast to win floor space is one of the commonest small jobs that meets the Act, and one of the commonest to be done without a notice. In a terrace or a semi the stack is normally built into the party structure, and the breast that is being removed is carrying the part above it. Taking it out without providing support is how a neighbour ends up with a stack resting on nothing, which is the failure this section of the Act exists to prevent.
Why it is notifiable
The breast forms part of the party structure, and cutting away a projection from a party structure is listed work under the Act. That makes it a party structure notice under section 3, served two months before the work begins, on every adjoining owner. The notice has to describe what is being removed and what will support what remains, because the neighbour's interest is in the part of the stack that stays, often including the section in their own roof space.
The support question, which is the real one
Removing a breast at ground floor leaves the breast above it and the stack above that with nothing under them. Gallows brackets are the traditional answer and are not always appropriate, particularly where the wall is of poor brickwork or where more than one floor is being removed. A steel beam and padstones is the usual alternative. This is an engineer's decision, and it is the part the award will care most about, because the consequence of getting it wrong lands on the neighbour's side of the wall.
Where a neighbour has already done it
A neighbour removing chimney breast without party wall agreement is the same problem as any unnotified work, with an extra worry: you may not be able to see what, if anything, is holding up the part of the stack that remains. Photograph your own side, including the loft, and write once saying you have had no notice. If you can see distress in your own brickwork or plaster, say so factually, with dates. Whether anything more follows is a matter for a solicitor and, on the structure, for an engineer.
Questions people ask about party wall agreement chimney breast removal
Do I need a notice if the breast is entirely in my own wall?
If the breast and the stack are wholly within a wall that is not shared, the Act is not engaged. In a terrace that is unusual, and the drawings are worth checking rather than assuming.
Are gallows brackets acceptable?
Sometimes, and it depends on the brickwork, the load and how much is being removed. It is an engineer's call, and an appointed surveyor will want to see the design.
What if my neighbour's stack is now unsupported?
That is a structural concern and a safety one. Get an engineer to look at your own side, record what you find, and put it in writing to the building owner.