The Act mainly applies to three types of work:
A party wall is a wall you share with your neighbour. If you want to cut into it, change it, or raise it, the Party Wall Act applies.
You need to follow the Act if you’re building a wall on the boundary line (“the line of junction”), or even very close to it. This often includes new extensions.
The Act applies if you’re excavating for new foundations (for example, for an extension) and the work is close enough to affect your neighbour’s property.