Party wall

A party wall stands astride the boundary between two lots and serves the buildings on both sides, each owner holding rights of support in the shared structure — the structural fact underneath New York's rowhouse blocks, where neighbors literally hold each other up. The arrangement arises from history and instruments, not from zoning.

Party walls surface in practice at demolition and excavation, which trigger protection duties toward the neighbor the wall supports; in alterations, where work on a shared wall needs more care and sometimes more consent than owners expect; and in measurement, since walls astride the line complicate the simple picture of a building fitting inside its own lot lines.