“ By the London Building Act 1894, § 5 (16) the expression “party wall” means— (a) a wall forming part of a building and used or constructed to be used for separation of adjoining buildings belonging to different owners, or occupied or constructed or adapted to be occupied by different persons; or (b) a wall forming part of a building, and standing to a greater extent than the projection of the footings on lands of different owners. Section 87 regulates the rights of owners of adjoining lands to erect party walls on the line of junction. ”