The Party Wall etc Act 1996 makes it a legal requirement to notify adjoining owners when you intend on excavating close to an adjoining structure. For notice to be required, the proposed excavations can be for any purpose, and must meet one of the following requirements:
1. The excavations are within 3m of an adjoining structure, and will be deeper than the foundations of the adjoining structure.
2. The excavations are within 6m of an adjoining structure, and those excavations would intersect with an imaginary line drawn diagonally downwards at a 45 degree angle from the bottom of the adjoining structure’s foundations towards the proposed excavations.
The first situation is quite common. For example if you are planning on constructing a building, or an extension, within 3m of another structure then it is often safer to assume that your excavations will be deeper than the adjoining structure’s foundations, unless you can prove otherwise. So if you own a terrace house, or a house within 3m of a structure owned by someone else, and you intend on constructing an extension within 3m, the chances are you will need to serve your neighbour with a notice of adjacent excavation.
The second situation is still common, but usually applies to construction that involves piled foundations which go down to a greater depth.
In both circumstances, if you don’t know the depth of the adjoining structure’s foundations, it is better to assume that notice will be required, and to serve one.
It should pointed out that for the purposes of the Act, a structure can be any building or structure, including a garage, a house, or even a garden wall which is owned, at least in part, or in full, by a neighbour.
You should ensure your notice is served at least 1 month before the intended excavations are started, and in order for it to be compliant with the Party Wall Act, the notice must be accompanied with a plan showing the proposed layout of the excavations, as well as a section through the proposed excavation, including any proposed foundations.