A party wall stands on the boundary and serves both properties, most often in attached commercial and residential buildings.

Creation. By express agreement, by grant, by long use, or by statute in some jurisdictions.

Nature of the right. Each owner typically owns the half on their land, with a cross-easement of support in the other half. Neither may remove or impair the wall to the other’s detriment.

Maintenance and repair. Shared in proportion to use in most formulations, with either owner entitled to make necessary repairs and to contribution. Improvements benefiting only one owner are that owner’s cost.

Alteration and extension. An owner may generally increase the height or thickness at its own cost if it does not impair the other’s use, and is liable for any damage caused.

Demolition. Where one building is demolished, the party wall remains as support for the other, and the demolishing owner must weatherproof and protect it.

Agreements. A recorded party wall agreement addressing maintenance, cost sharing, alteration, insurance, and what happens on redevelopment resolves nearly all of this and is standard in urban redevelopment.

Diligence. For any attached building, confirm whether a recorded agreement exists and what it says before pricing a redevelopment.