The usual American theory of the rights of adjoining land owners in a party wall is that each owns in severalty that part of the wall on his land and each has an easement of support in that part on the land of the other. If the structure is erected under an express contract, the rights of the parties are determined by the terms of their contract. And when the easement of support is created by prescription, its scope is measured by the prior user, and no right to remove or replace the wall can exist by virtue of the easement so acquired. Frequently, buildings are erected with a division wall between them and the ownership is later severed without express reference to the character of the division wall. In this situation the theory is that the party wall easement is created by grant or contract implied in law. The courts, under the guise of presumed intention, are making the contract for the parties. This should be noted, not because it is open to criticism, but because in the exercise of this discretion policy considerations are given great weight in defining. the scope of the easement.
Charles W. Allen,
PARTY WALLS - REPLACEMENT AND REMOVAL,
Mich. L. Rev.
Available at: https://repository.law.umich.edu/mlr/vol35/iss6/6