The 3 metre rule
Section 6 in the Party Wall etc. Act 1996 includes details regarding the 3 metre rule for any excavation works you intend to carry out.
3 metre rule in a nutshell
You need a party wall agreement if you plan on carrying out any excavation works deeper than the neighbours foundations and within 3 metres of the their foundations OR perform any excavation within 6 metres if any part of your construction work meets a downward line towards the neighbouring house at 45°.
3 metre rule details
There are no enforcement procedures stated in the Act. However, failure to negotiate and produce required documentation to your neighbour may result in the construction works being stopped by the Court or other legal bodies.
Your neighbour has to be notified if you intend to:
Carry out any excavation works within 3 metres of the foundation of the neighbouring building, if your excavation will go deeper than their. This includes constructing new foundations or making changes to your existing structure (assuming it's of the same kind).
Perform any excavation works within 6 metres of the adjoining building, if any part of your construction work meets a downward line towards the neighbouring house at 45°. This applies to any changes to your building structure, either modifications to an existing foundation or creation of a new one. This excavation rule applies even if the affected party is separated from your works construction by land or another building.
The below images should make it incredibly clear
In the notice to your neighbour you MUST include:
Name and contact details of the owner
The clear purpose of planned construction works.
Plan for any adjustments, including depth and location of the intended excavation.
Location of the planned structure of the building
Date when the excavation will begin
To measure the depth of your and the adjoining owner's foundations, you should get professional advice on where and how deep you can dig. It is not advisable to just figure it out yourself. Inaccurate work may be personally dangerous and also cause damage which you would be liable for under the Party Wall etc. Act 1996. In fact, you may want to consider party wall insurance if you're carrying out excavations.