Assuming the property is in England or Wales.
No you don’t ‘need’ a Party Wall agreemment or Award but you do need notice.
Planning Consent may not be required but Listed Building Consent will. As will Building Regulation Approval (Building Control or Approved Inspector).
Technically, yes you can be charged for half the wall, however, it would be unusual for a competant Party Wall Surveyor to persue this as part of the works.
You may be liable to pay an enclosure benefit to the neighbour should you choose to convert your loft to habitable space.