It is California law that neighbors share the cost of a new fence – hat tip DV:
(a) Adjoining landowners shall share equally in the responsibility for maintaining the boundaries and monuments between them.
(b) (1) Adjoining landowners are presumed to share an equal benefit from any fence dividing their properties and, unless otherwise agreed to by the parties in a written agreement, shall be presumed to be equally responsible for the reasonable costs of construction, maintenance, or necessary replacement of the fence.
(2) Where a landowner intends to incur costs for a fence described in paragraph (1), the landowner shall give 30 days’ prior written notice to each affected adjoining landowner. The notice shall include notification of the presumption of equal responsibility for the reasonable costs of construction, maintenance, or necessary replacement of the fence. The notice shall include a description of the nature of the problem facing the shared fence, the proposed solution for addressing the problem, the estimated construction or maintenance costs involved to address the problem, the proposed cost sharing approach, and the proposed timeline for getting the problem addressed.