What is a party wall? The wall standing on the boundary between two attached homes, owned to the centre by each side, with each side holding an easement of support in the other half.
A party wall is a shared wall built on the lot line between two attached homes. Where the homes are owned fee simple with their own lots, which is the usual arrangement for townhouses and many duplexes inside a planned unit development, each owner owns the wall to its centreline and holds a cross-easement of support in the other owner's half. Neither side can knock it down, cut a structural opening through it or undermine its footing, because the neighbour's house is leaning on it. In a condominium the wall is generally not owned by the units at all: it is a common element and the association is responsible for it.
The duties are written down in a recorded party wall agreement or, more often here, in the CC&Rs. A typical split is that repair and maintenance costs are shared equally where neither owner is at fault and fall entirely on the owner who caused the damage where one did, with each side insuring its own home and neither side allowed to alter the wall without the other's consent. Building code adds a fire-resistance requirement, commonly a one-hour or two-hour rated assembly carried up through the attic to the roof deck rather than stopping at the ceiling, and sound performance comes from how the wall was built: a true double wall with an air gap and separate framing is quieter than a single shared frame.
What a buyer checks is physical and then documentary. Look for whether the separation runs to the roof deck, whether the roof line and flashing are detailed where they cross the wall, and whether the attic is genuinely divided. Then get the party wall agreement or the declaration and read who pays for a repair, who fixes a leak that starts on one side and lands on the other, and whether an architectural approval is needed to touch anything. Those answers belong in the inspection contingency, not after it.
A worked example
Two attached townhouses share a wall on the lot line. A slab crack under the wall needs $9,400 of foundation work. The recorded party wall agreement splits repairs equally where neither owner is at fault, so each pays $4,700; had a neighbour's failed irrigation line caused it, the agreement puts the whole $9,400 on that side.
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Questions people ask
Can I hang a television or run wiring on a party wall?
Fixings into your own side are normally fine, since you own the wall to its centreline. Cutting through it is not: that breaches the fire-rated assembly and the neighbour's easement of support, and in a condominium the wall is a common element you do not own at all. Check the CC&Rs before opening anything up.
Who pays if a leak crosses a party wall?
The party wall agreement or the CC&Rs decide it, and the usual rule puts the cost on the owner whose side the water came from and splits it where nobody is at fault. Each owner's homeowners insurance covers their own home, so a claim on one side does not automatically cover the damage on the other.