Neighbouring land is 2m lower than my planned side extension.

You will have to ensure that your side extension foundations are at a sufficient depth and specification to avoid impacting upon the stability of the neighbours’ property, both structures and the land. I suggest that engage the services of a structural engineer to design the foundations. If you are building within 3m of the site boundary, you will need to consider the requirements of the Party Wall Act; this is likely to require you to serve notice on the adjoining owner before works commence.