Repairs to something two owners share are the oldest argument in this area, and the Act deals with them more sensibly than people expect. The principle is that expenses are apportioned by the use each owner makes of the work and by whose act made it necessary. Applied to real cases that usually gives an obvious answer, and where it does not, the surveyors decide. A garden wall between two properties is a different creature again, with its own name in the Act.
How the cost of a repair is shared
Where a party structure is repaired because it needs repairing and both owners benefit, the cost is shared having regard to the use each makes of it and to their responsibility for the defect. Where one owner's work makes the repair necessary, that owner pays. Where one owner wants the wall raised or thickened for their own purposes, they pay for that, and the neighbour contributes only if and when they make use of the added work. The result is that most disputes about repair are really disputes about cause.
What a party fence wall is
A masonry wall that stands astride the boundary and separates land rather than buildings: the garden wall between two back gardens, typically. It is covered by the Act, but it is not a party wall, and the distinction matters because the work that is notifiable differs. A wall wholly on one owner's land, however close to the line, is not a party fence wall, and a timber fence is not a wall at all.
The party fence wall rules in practice
Both owners have an interest in the whole of it, so neither may simply demolish it, and work to it can be notifiable in the same way as work to a party wall. Repairs are shared on the same benefit and cause principle. The common real case is a wall that is leaning: if it is a party fence wall, it is a shared problem, and rebuilding it is work the Act contemplates being notified. If it turns out to sit wholly on one side, it is that owner's wall and that owner's problem, which is why the position of the wall is the first thing to establish.
Questions people ask about party wall repair responsibility
My neighbour will not contribute to a repair. What now?
If the wall is shared and the repair benefits you both, the apportionment is the sort of thing appointed surveyors determine. If the argument is about where the boundary runs, that is a solicitor's question and not one the Act answers.
Is a fence a party fence wall?
No. The Act is about walls. A timber fence panel between two gardens is outside it entirely, whatever the deeds say about who maintains it.
Who owns a leaning garden wall?
If it stands astride the boundary it is a party fence wall and both owners have an interest in it. If it is wholly on one side, it belongs to that owner along with the problem.