One developer-client once told us, “horse racing is not the sport of kings—litigation is.” During the greatest high end real estate market in the history of our country, the ultra wealthy or the corporate kings have battled in our courts for every inch of land. Because only the wealthiest own townhouses in Manhattan and Brooklyn, these plots­—built 50 feet by 50 feet—during the time of author Edith Wharton, resulted in shared walls, called party walls, supporting each townhouse. Because many kings want to expand their spaces wider and higher, the rules on party walls have been tested. This article aims at educating the practitioner on the not-so-obvious rules of the party wall.

Defining Ownership Rights

A landowner owns the land and all that is on it. Where the wall is partially on one piece of land and partially on the other, the person who owns the land under the wall, owns the piece of the wall on that land.¹ However, each side has the right to maintain the part of the wall on the other party's property and to make sure that it is usable by both sides.² Neither one may interfere with the wall being completely available to the other owner.³ Thus, for all practical purposes, the law treats the wall as completely belonging to both neighbors. The minor support given by a party wall to a perfectly independent wall, where the independent wall is sufficient in and of itself to stand, does not qualify as use of a party wall, therefore not obliging the owner of the independent wall to pay for the upkeep of the party wall under a contract obliging such payments in the event such owner makes use of the party wall.⁴

Use of Party Walls

The whole purpose of a party wall is that it is to be of use and benefit to both owners. Neither owner may subject the wall to a use where it ceases to be continuously available for enjoyment by the other owner,⁵ but each may subject it to whatever uses are proper to a wall, so long as that does not interfere with the other owner's equal exercise of freedom.⁶ The right to use the wall includes the right to use the portion that is on the neighbor's property as well as the right to use the portion on one's own property, provided the exercise of that right does not interfere with the neighbor's exercise of the neighbor's rights.⁷

Neither owner has a right to build on top of a party wall a construction that will be for that party's own purposes only, even if the construction is limited to that party's own side of the property line.⁸ A party wall is for the mutual convenience and benefit of adjoining property owners and the only restriction upon its use by either is that that use shall not be detrimental to the other.⁹

Increasing Height and Length

Either owner may heighten the party wall, provided the existing party wall is not damaged by it, and the addition is available for use by both owners.¹⁰ A party making an addition to a party wall does it at his or her peril; and if injury results he or she is liable for all damages. One adding to a party wall, either as to height or length is an insurer of the damages it causes to the other land owner.¹¹

Replacing a Party Wall

However, where the action taken by one of the landowners is merely to replace a dilapidated wall with one that is in a good state of repair, that party cannot be held liable to the other for damages,¹² for such party acts as of right.¹³

Hanging Signs

As noted, a party wall is for the common benefit of adjoining landowners. Each party may use the wall properly so long as the other party is not harmed by the use. While the right to erect a construction under the party wall easement permits one to erect a construction that sits on both sides of the wall, the owner of one side has no right to erect a construction that sits entirely on the other side. Thus, one cannot hang a billboard on the other side.¹⁴ Commercial use of a party wall, such as by hanging a sign, that is on the owner's own property, however, is permissible so long as it does not weaken or encroach on the other side of the wall.¹⁵

Weakening/ Harming Walls

Neither party can rightfully do anything on his or her own land, which would weaken the party wall and deprive the other party of their entitled use of the land.¹⁶ However, neither owner can compel the other to make the wall stronger than it was when originally constructed, as by sealing off windows, for example.¹⁷ The law is clear that a party wall does not have to be maintained as a solid wall. It can include piercings such as a flue.

Adding Windows

The placing of windows in a party wall where the agreement for the maintenance of the wall does not provide for such opening is a violation of that agreement.¹⁸

Adding windows to a party wall is generally permissible as long as the windows do not interfere with structural integrity of the wall¹⁹ or cause any privacy concerns that may serve as a valid objection to the window.²⁰