^ Good for you.
Same here, but that doesn’t apply to fence upgrades. Upgrades aren’t necessary: if it was necessary, it would have already been part of the fence.
And my neighbour put fence extensions up, which we like because they spend time in their yard, and asked our consent (because it’s our shared fence), and didn’t charge us. But /next time/ when the fence is replaced, there is going to be a good argument that the shared cost will include a higher fence.
Our other fence just got replaced. We paid half. The neighbor organised it. They got quotes for the work, and we approved the quotation before the work started. because we were going to pay.half.
You didn’t approve the quote. You didn’t set the timetable. The fence wasn’t due for replacement. The maintainance could have been done on a different schedule.
I’d be paying a round number around a 1/3 for those reasons, because otherwise you’re overpaying because you didn’t get the right of approval and scheduling.
(You know you are going to need to replace your car. Your delear comes along and tells you that it’s got to be this month, and the color is black. Do you pay full price? Do you accept the generous discount that’s offered?)
Accept the generous discount that’s offered. Pay something for actual value.
I would say “We agreed to pay half. The TOP half. We’ll throw in our share when you build the rest of the fence!” 
I didn’t see it mentioned but do you know where in relation to the property line this fence is? If it is on the actual boundary, everything above should be considered and the following ignored.
In both the houses that I’ve owned, we’ve intentionally built the fence 6" inside our property line (this requires an actual survey or some pretty high confidence in your own ability to measure the legal boundaries since an actual survey would trump what you measure), so that it is on our lot and therefore our property - which gives us control over it (within boundaries set by local ordinances). Since we own it and can do what we want with it, we pay the full cost.
We however are a construction-oriented family with a tendency to do things from scratch, and I realize that most people buy properties with fences already installed years earlier and just replace what’s already in place. I’m just mentioning this angle as you might feel wronged 20 years from now when you sell the house and then find out that “your” fence was a foot into your neighbors lot and you shouldn’t have paid for any of it. That doesn’t mean your neighbors are trying to scam you… they might not even be aware of the actual property lines themselves.
Wow! Just goes to show how important it is to know the law/custom where you live. (Of course, that sub-E “other equitable concerns” could cover multitudes…)
I’m trying to figure out how that works. I move into a neighborhood that has no fences. Maybe I have a dog or small kids - or just like fences. So I get some quotes and figure my neighbors will have to subsidize 50% of my chosen property improvement? 
And THEY have the burden of proving that they ought not pay for something they may not want? Seems odd.
Here in the frozen north(ish), I can only imagine a couple of circumstances where I would contribute to a neighbor’s fence:
-Maybe my neighbor’s fence is in poor repair, but they are unwilling/unable to maintain it. I might offer to pay some or all of the cost of repairing/replacing what I consider an eyesore - as an alternative to building my own fence 6" on my side of the line.
-Or maybe my neighbor is planning a chainlink fence, but I would prefer wood. Or 4’ and I’d prefer 6’. I might offer to pay the extra cost to have it look the way I prefer.
-I generally perceive a benefit to having fences make clean/sensible lines. So if my neighbor wanted to put up a fence that only covered part of the property line, or that did not butt up to an existing fence or building, I might offer to contribute to make it make sense/be aesthetically pleasing to me.
Pay half. You both benefit from the fence.
I would say that there’s no obligation to pay anything. Your neighbors are the ones who decided that they wanted the fence improved, so they can pay for whatever improvements they want.
That said, it’d be neighborly to pay for half the cost of the new posts, since that’s something that you probably would have wanted to do, if you’d known about them.
But you shouldn’t pay anything for the height increase, since that’s something that you don’t care about, and wouldn’t have done at all if it had been up to you.
An entire unit of my property law class was all fencing dispute cases. Those get ugly fast, wow.
Anyway, were you in NSW Australia, if you were given notice you would have to pay half of a sufficient fence.
Rules: http://www.swslc.org.au/wp-content/uploads/2015/03/Neighbour-fencing-disputes-Fact-Sheet-final.pdf
If you think about it as “I can force my neighbors to pay for my home improvement”, then it seems pretty unfair, sure.
But if you think about it as “property-line fences are really common, and a common area of dispute. Let’s just say that as long as the fence is reasonable, both people split it”, it’s a pretty reasonable rule for society that’s going to work correctly in the vast majority of cases and vastly simplify the dispute process.
It’s like no-fault auto insurance, which is common in lots of places. Sure, it’s unfair that if someone hits you because they were driving like a moron your insurance pays for it. But removing the “figuring out fault” process from the vast majority of vehicular accidents is a benefit for society even though it results in plenty of minor injustices.
Plus, you know, you still have to collect 
In CA, fences are split by both owners.
ETA: I see that the OP is in CA and has already noted that.
It sounds like some of the work was not on your shared property line, right? If the bill is not in the thousands of dollars, I’d just pay half and move on. But I might ask the neighbor if we could discuss this sort of thing in the future before the work gets done. I’d say I’m happy to pay my share, but I’d like a say in how it’s done.
I imagine it COULD be a reasonable system, but it is just so different than what I am used to, that I’m having a hard time getting my head around it.
I was born/raised in Chicago, where each lot was 30’ wide, and had a chainlink fence running the length of the backyard. And I doubt I was 10 by the time I had heard “Good fences make good neighbors.”
As an adult homeowner, I’ve always had dogs, and later kids. So at each of the 3 houses I bought I erected fences to keep my dogs/kids in (and other people/dogs/kids out!) So long as I pulled a permit, had it done on my side of the property line, had the posts face inward, and paid for it myself, no problem. Didn’t even need to inform the neighbors (though, of course, I did.)
I’ve got nothing against neighbors working together, but I wonder about situations such as I described above. What if I live where there are not fences and am perfectly happy not to have a fence? New neighbor moves in and wants to put up a fence - so he is able to do so, and can bill his neighbors for 50% of the cost? That would definitely piss me off.
Or if I agree to the fence, but I’d be happy with a cheap chainlink, but the neighbor wants a 6’ cedar privacy fence. I have to pay 50% of what HE wants?
Like I said, might make sense to someone who is familiar with it. But to me, it sounds crazy.
Round these parts, you can do pretty much anything you want so long as it is on your property, permitted by law, and you pay for it. I guess I hadn’t realized how strongly that had been driven into me, and that other places might do it so differently.
No. If you look at the CA rules linked above, you’ll get a slightly better idea of how it works.
There is still room for argument (and people do argue about fences), but the egregious examples you give all have the simple obvious correct answer: “No”.
Well, I did read the statute (tho admittedly not as closely as if I had been litigating under it). I see there is a 30 day notice, which is good.
I also agree that my descriptions were phrased somewhat extremely - reflecting my surprise at this system. But I’m not sure the answer is the simple “No” you suggest. Instead, the answer is more likely, “Possibly not, maybe probably not, but it is still gonna cost you SOME $, time, and stress.”
The underlying fact is, if my CA neighbor wants to erect a fence, he can notify me that I am expected to pay 50%.
IF I do not want to pay 50%:
-I have to become embroiled in a lawsuit, with all of the costs, stress, and inconvenience involved.
-I have the burden of proving by a preponderance of the evidence that I should be allowed to pay less than 1/2.
-In order to carry my burden, I likely have to disclose personal information, such as my finances which prevent my contributing, or even my personal preferences.
-The outcome is determined by some 3d party/judge.
What did I miss?
Instead of my neighbor volitionally doing what he wants with his property, he involves me in a lawsuit, which might end up costing me significant time and $, and depends on the decision of some third party.
I admit, if that was the system I grew up knowing, it might make sense to me. But I really don’t get it. I also admit that I have ZERO knowledge of how courts interpret the presumptions and burdens set forth in that statute. I also wonder how many CA folk installing fences expect their neighbors to contribute? I can imagine myself saying, “Look, they’re MY dogs/kids, and I want the fence. I’ll pay for it myself.”
I guess if I moved into a CA house with no fences, I would simply have to understand that at some point my neighbors might want to erect fences, and that if that occurred, I might have to pay 1/2 the cost. I would essentially factor that presumption into my expected longterm costs of home ownership. Conversely, if I wanted to erect a fence, I could expect that I might be able to get away with paying less than the full price (while potentially earning my neighbors’ disfavor.)
Thank you for this! I’ve been in a bit of a disagreement myself. My neighbors next door are renters and are friendly, but the owner isn’t responsive at all to our needed fence replacement. The neighbors tell me that the owner also responds slowly to them, and is cheap and minimalistic if he makes any repairs.
I’ve been trying to work with that owner for years to replace our fence. With this law I will now provide written notice with the six “shall” elements defined by that law.
My letter is already drafted. All I need are the estimates and I’ll move on those today.
Thanks again!
<draft letter>
***Hello neighbor,
Our shared fence has been rotting and falling apart for over five years. It is beyond repair and requires replacement. I propose we replace this fence and the reasonable cost to do this is, by legal presumption, the equal responsibility of both you and me.
► 123 Main St., Anytown CA USA — my house
► 456 Main St., Anytown CA USA — your house
The fence in question runs along a northwest-to-southeast line between our houses. Here is a map (gMap link redacted).
I have obtained three estimates for this work. The estimated cost is $XYZ123 and I propose that we share this cost evenly. I propose that we replace this fence in April 2018.
Please refer to the applicable law, California Civil Code, Obligations of Property Owners, §841, for more information:
Law section.
Please contact me at your earliest convenience so we can work together to fix this problem. If I do not hear from you by (30 days — by 31 March?), I will proceed with this fence replacement.
Thank you,
Bullitt
415-555-1212
bullitt@myEmailServer.sdmb.com
</draft letter>
What do you think of this draft? I want it to be polite yet firm, professional and not stern.
Well, you can always refuse to pay, and then, if your neighbor cares enough, they have to become embroiled in a lawsuit to collect.
Seems fine, but remember that, even with the law as stated, if you proceed to build a fence, you still have to figure out how to collect from your neighbor.
You might want to make it a certified letter, too.
After reading more carefully, this section stands out to me “(ii) The extent to which the costs of the project appear to be the result of the landowner’s personal aesthetic, architectural, or other preferences.”
So, I think if you moved to a place that had no fences, the other owners could reasonably say: “hey, we’ve been here for years without a fence, and it suits us fine. If your personal preference is to have a fence, knock yourself out.”
So maybe the law really applies more to cases where the fence already exists, and need to be fixed or replaced.
I’ve never lived somewhere that “backyard” property boundaries weren’t marked by a fence or other barrier of some kind.
I don’t think that follows, since “adjoining landowners are presumed to share an equal benefit from any fence dividing their properties.”
Since the presumption is that erecting the fence will benefit both parties equally, it’s up to the dissenter to prove that he will be unduly burdened or will not derive benefit.
Agreed. Like many laws, this might well be practically applied differently than a simple reading suggests.
Meanwhile, I’ll be better able to tolerate the cold and snow up here in the tundra, with the realization that our fencing conventions are far more reasonable than then hippy-dippy commie types living in paradise! 
Yeah, but only if it comes to an actual lawsuit. It’s up to the fence-wanter to actually push the disagreement into a legal battle. And if they do, it’s not clear to me that they’d win.
If party A that wants a fence to keep his dogs/kids in goes to court and says “I want to build a fence to keep my kids/dogs in”, and party B says “I don’t have kids/dogs, and I’ve lived here for years happily fenceless. A fence doesn’t benefit me”. Then what’s the judge going to say? I have no idea, but it’s not obvious that the presumption will hold here.