I see no problem. Those few inches are the neighbors property and they will have to remove the growth between the fences. Apparently they were hoping you'd mow it and weed their property but whatever. I'd give them the option and time to put in a permanent barrier for weeds and then move forward with your fence.

20 years ago you could have quietly had the lot line adjusted to claim that extra land your neighbor abandoned but now the homeowner is notified and then its Hatfields and McCoys from there.

I had a similar situation years ago. The problem was solved with a three sided fence that had a cantilevered piece that extended from the corner post that was on my property to very close to but not actually touching their fence. Since it was cantilevered, it never touched their ground either. You do wind up mowing a bit of their lawn but it did save me money and they never actually even noticed it was there.

Thanks for the comments, do you have any picture of that cantilevered solution you did? Trying to visualize it and see if we can try something similar. Thanks!

I'll see if I can get a picture tomorrow since it's dark and rainy here but I mocked it up to give you the idea. Remember, you have to fill only five and a bit inches. It's basically a bit of fence that is attached to your post, a bit shorter so it doesn't touch the ground and then just extended off the post for less than 6 inches. It doesn't need a post support as it's very light in weight. This was suggested by my fence builder. Said he does it often to solve this issue. Mine is wooden but I don't know why you couldn't do it with any material.

So now I'm going getting into this. Here is a view with a metal fence like my friend has to have for her HOA. That would be even easier as those fences' panels usually don't touch the ground.

And here is an overhead view. Remember, you have to fill only a bit less than 6 inches. It really isn't noticeable and doesn't impact the fencing functionality as the gap will be only about a half inch.

Awesome, these are great ideas. Thanks for sharing.

If the HOA or neighbor objects to the cantilever you can plant a holly or other dense shrub on your property and it will grow to fill the gap. Unless you are trying to contain a small dog, remember that 6" is only a bit more than the 3.85" spacing between pickets of most metal/wood picket fences. It's not a lot of space.

In any event, I would consider putting in a different kind of fence than the one your neighbor has constructed. That way, the repetition of materials 6” apart (if you don’t do the cantilever suggested) doesn’t look silly. AND, I would plant against that fence, using it as a backdrop and obscuring it (and whatever collects between the fences.)

Thanks, was thinking of making a backdrop too and plant along that line.

I'd put a hedge or other plantings on the property line. If they gifted you with some extra inches, use them.

The space between the fences will end up being very weedy and you won't be able to clean it out. We have a fenced yard for our dogs. Most of our fencing is one shared fence with all the neighbors, but new neighbors built their own solid board fence six inches from ours on a potion of our fence. We had replaced an old part of the fencing which is on a steep hill. I guess they didn't like our fencing. We can see the weeds, but cannot reach them. Needless to say, we hardly spoke. They have moved now and the young couple who moved in are delightful. With luck, they will remove that portion of fencing.

ETA: do not count on the neighbor taking care of the area. Our neighbors could not see that side of their fence, so now we have English ivy, bamboo, and weeds, weeds, weeds coming into our yard (I had pretty much eradicated those in the 20 years we had been here at that point) in an area that is diffiult for me to weed now that I am not as agile as I once was. Also, I do not use hebicides or pesticides.

ETA 2: I would probably plant something prickly at the corners where the gap is and then just plant whatever I wanted with stems/trunks just barely inside the property line. The shrubs and trees would grow to fill the entire gap. If you take care of it for 20 years without permission of the owner(that is timeline where I live), you may be able to go to court and have that strip retitled and declared part of your lot (adverse possession). My son was able to do that for a small area.