A "party wall", also called a "wall in common", a "common wall", or a "shared wall" is a dividing wall between two connected and mutually supporting buildings of different owners, or intended to be between a building actually constructed and a contemplated one, usually standing half on the land of each owner, maintained at mutual cost of both owners, and subject to use by each owner consistent with the rights of the other.

A party wall does not need to be located exactly on the boundary line between two adjoining properties. Instead, it may stand entirely upon the land of one owner only. It takes the place of two walls that would have otherwise been erected by adjoining landowners, each owner holding in severalty that cross-portion of the wall on his or her tract, subject to an easement (called cross-easement) by the other owner for use of the wall as a perimeter wall of the owner's respective building, and for its support.

By Statutory Authorization

Statutes may expressly authorize a lot owner to erect a party wall and to enter his or her neighbor's adjoining land to do so, provided the builder complies with certain requirements.

By Contract or Agreement

Unless prohibited by statute, a wall may acquire the status of a party wall pursuant to an express or implied contract or agreement between adjoining landowners.

By Conveyance of Property

A wall may acquire the status of a party wall by the conveyance of land on which it stands or which it adjoins and serves or is intended to serve for building purposes.

By Implied Easement

The conveyance of lands separated by a common wall on the dividing land may give rise to easements in the nature of a party wall.

By Use of the Property

A right to an easement of support in a wall may arise through use during the pertinent prescriptive period, whether the wall rests wholly on the land of one of the adjoining landowners or in part on the land for each.

Party walls generally take one of four primary forms:

• A wall in which two adjoining owners are tenants in common.
• A wall divided longitudinally into two strips, each strip belonging to an adjoining owner.
• A wall which belongs entirely to one owner, subject to an easement in favor of the owner to have it maintained as a party wall.
• A wall divided longitudinally between two properties, each property being subject to a cross easement of support and/or division in favor of the other.

• By agreement of the parties.
• By the destruction of the wall as by fire, earthquake, or by any agency beyond the control of either party. (State law must be consulted.)
• By the destruction of the buildings, though the wall remains standing.
• By demolition of the wall, as a nuisance, pursuant to an order of a public authority.
• By abandonment, as where one having an easement in a wall formerly a side of his or her building erects a new wall on a different foundation.
• By failure to comply with a contractual provision, like one calling for a contribution to the cost and maintenance of the wall.