How do I bathroom / kitchen / extension? etc.

We had Party Wall letters to our house when next door were building a home office in their garden. Not even touching the property border; never mind any walls.

Doesn't have to touch the border or walls to fall under the Party Wall Act...

"
 2. What does the Act cover?
...

• Excavation within 3 or 6 metres of a neighbouring building(s) or structure(s), depending on the depth of the hole or proposed foundations (see paragraphs 28 to 30).
  
  

"

Source: https://www.gov.uk/government/publications/preventing-and-resolving-disputes-in-relation-to-party-walls/the-party-wall-etc-act-1996-explanatory-booklet