You are correct that retaining wall issues can be substantially different than shared fence issues. With an issue such as a retaining wall which was put in place due to an excavation, you should look to California Civil Code Section 832, which governs the adjacent owners right to lateral and subajacent support. (See the text of the code below).
At the time of construction
, notice should have been provided to the adjacent owner by the excavating owner. In such a situation the excavating owner has an obligation to support the adjacent land. In your situation, it appears your land was excavated and supported the adjacent land with a shared retaining wall. If this was done by a builder who owned all the property at the time, notice would not have been needed.
The answer to your question is not as simple as initially proposed. It will depend on any agreement in place at the time of construction. If no agreement is in place and your lot was the excavated lot, there is the potential that you have an obligation to maintain the support of the adjacent owner's land based on Cal. Civ. Code 832, but as you have made it clear this is a shared retaining wall their is a presumption of shared maintenance
Understand that this is speculation based on the information included. To be definitive, one would need to examine all of the contractual documents and understand the sequence of events leading to the construction of the retaining wall. As you can see in Cal. Civ. Code 832, there is a clear obligation on the part of excavating adjacent property owners. I can only speculate, that your property was excavated after the neighbors lot was developed or at the same time by a developer. As such, there are many factors to consider. Just be aware that the issue is not clear and that if this is a shared wall there is a presumption of shared responsibility to care for the wall. Good luck with this matter.
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Cal. Civ. Code 832
Each coterminous owner is entitled to the lateral and
subajacent support which his land receives from the adjoining land,
subject to the right of the owner of the adjoining land to make
proper and usual excavations on the same for purposes of construction
or improvement, under the following conditions:
1. Any owner of land or his lessee intending to make or to permit
an excavation shall give reasonable notice to the owner or owners of
adjoining lands and of buildings or other structures, stating the
depth to which such excavation is intended to be made, and when the
excavating will begin.
2. In making any excavation, ordinary care and skill shall be
used, and reasonable precautions taken to sustain the adjoining land
as such, without regard to any building or other structure which may
be thereon, and there shall be no liability for damage done to any
such building or other structure by reason of the excavation, except
as otherwise provided or allowed by law.
3. If at any time it appears that the excavation is to be of a
greater depth than are the walls or foundations of any adjoining
building or other structure, and is to be so close as to endanger the
building or other structure in any way, then the owner of the
building or other structure must be allowed at least 30 days, if he
so desires, in which to take measures to protect the same from any
damage, or in which to extend the foundations thereof, and he must be
given for the same purposes reasonable license to enter on the land
on which the excavation is to be or is being made.
4. If the excavation is intended to be or is deeper than the
standard depth of foundations, which depth is defined to be a depth
of nine feet below the adjacent curb level, at the point where the
joint property line intersects the curb and if on the land of the
coterminous owner there is any building or other structure the wall
or foundation of which goes to standard depth or deeper then the
owner of the land on which the excavation is being made shall, if
given the necessary license to enter on the adjoining land, protect
the said adjoining land and any such building or other structure
thereon without cost to the owner thereof, from any damage by reason
of the excavation, and shall be liable to the owner of such property
for any such damage, excepting only for minor settlement cracks in
buildings or other structures.