If you are building or altering a wall on a boundary with another property, you will need a Party Wall Agreement. You will also need one when excavating the foundations for your double storey extension, depending on their depth.
If your proposed two storey extension project affects a shared wall, such as in a terraced house, you should make sure to notify your neighbouring properties before starting work, as per the Party Wall Act.
Any disputes should be settled before starting the project. If necessary, it is possible to consult with a Party Wall Surveyor.