Sharing a Party Wall Isn't Always a Party
Contrary to what you might think, a party wall is not a wall on which you hang party decorations. This clown fellow might be the perfect accompaniment for your 6-year-old's birthday party, but the fact that he's hanging on the dining room wall does not make it a "party wall".
In fact, a party wall is a common wall that divides two adjoining buildings, units or apartments and shared by the occupants (parties) on each side. A party wall usually provides structural support and/or fire protection between the two buildings or units.
A party wall is typically owned by both property owners as tenants in common, but it may also be wholly owned by one owner or the other, subject to easements and rights of use granted to the other owner. At any rate, both parties share the use of the wall - hence the name "party wall" - just like the old telephone "party line" exchanges which were still common in rural areas until the 1960s.
Since both parties share the wall, they must also share responsibility for any necessary maintenance and repair of the wall. This can give rise to some very unneighborly disputes over when work should be done, what type of work should be done, who should do it, and who should pay for it. If a dispute does arise, you could end up paying some hefty professional fees for surveyors, engineers, and even lawyers. That's why it's a good idea to draw up a Party Wall Agreement if any work needs to be done, or if you decide you want to make alterations to your side of the wall.
To misquote Robert Frost, good party wall agreements make good neighbors.