How can I close a gap between my fence and my neighbor's that's on his side of the property line?

If you are worried about access around your pool you should be extending your fence as necessary (beginning at right angles to the end showing in the picture) so that it encloses the area of concern. That can all be done on your property with no quibbles from the neighbor.

Plant a bush in the corner. Something with needles or thorns should keep dogs and children out.

If you decide to go the full malicious compliance route:

You can begin to call bylaw on him - to the best of my knowledge, there are usually local ordinances about keeping one's lawn maintained. Eventually the grass will grow as he is unable to cut it. Refuse him access to your property to cut the grass, but tell him that you're willing to do it provided you can fix the gap.

Granted, this will sour your relations with your neighbour more and will take some time to fix your problem, but it may work.

Your options are:

1. Ask him to remedy the fence situation, explaining your interest and offering to do it for him if he's unwilling/unable.
2. Go ahead and fill in the gap yourself without asking permission--repairing his fence for him.
3. Address the hazard itself and raise a complaint with your HOA or town board over the hazard of his unconfined dog--accept mending of the fence as a resolution to this.

As @PlatinumGoose suggested, a thorny plant on both the inside and outside would mostly deter any entrance through the gap.

In addition to this, you could use insulated stand-offs extending vertically along the edge of your fence exactly up to his property line, with an electric fence wire strung across the ends running vertically parallel to the edge of your last fence board, along with the appropriate warning sign.

The combination of the two should ensure that nothing but small rodents would clear the gap in the fence.

Whether this is permissible legally or with your HOA I cannot say.

According to IRC as listed here the key seems to be 4". That is, unfortunately, exactly the size of the gap between the property line and your neighbor's fence. But since you do not need to have a complete barrier - i.e., anything less than 4" is OK, I would add a board to your fence to narrow the gap to 3.5". And then hope that your neighbor doesn't measure it. He knows your fence is not all the way to the edge, so hopefully you can get away with that 0.5" - which would get you legal for protecting your pool without having to build an entire parallel fence on your property.

And as Harper noted in his excellent answer, if you can work out a deal with the neighbor to pay for 1/2 the cost of the fence between your properties, he'd likely then go along with a full 6" panel between the two fences.

If I understand your question correctly, the situation is like this:

---

All you have to do is close the gap by nailing a plank onto your fance outside your properties like this:

This extra barrier would be in communal (Government or "Council" in Australia) property, so he could not demand you remove it. If he complains to the authorities you can say that you don't want to let a child through the gap, who might drown.

---

Having said that, though, it is not your neighbour's job to stop kids from drowning in your pool - that is your job. What if your neighbour added climbing panels to his fence so it could be easily scaled by a child?

What if he lets children onto his property, who subsequently fall into your unsecured pool, and drown? What if he gets a dog?

It is your responsibility, indeed, your duty, to secure your pool on your property, regardless of what your neighbour might do.

---

Edited to add

Isn't the distance between the fences irrelevant, if one has a large gap in it? See my take on your photo:

While the fence is not closed, it doesn't really matter how close they are.

It is your pool. It is your job to guard it.

I imagine your state law or city ordinance is clear on that point.

You are missing an important point here. You are trying to use your neighbor's fence to protect your pool. It's his fence and he has a right to remove it at any time. He could remove a panel right after you go to work, and put it back right before you get back. That's his right. If kids got into your pool and drowned because he took down his fence, that's all on you.

If you think that example is absurd, he could easily put a gate or several gates in his fence, and refuse to lock them. That is his right, and you don't get to say boo about it.

He does not owe you a fence

I suspect part of the tension is that you are trying to get effective use out of his fence, yet haven't paid a dime for it. His feelings are reasonable, and you should respect them. You need to pause here, and realize so far, your conduct has been that of an over-entitled jerk, and his negative reaction would be one you yourself would be having, were the roles reversed.

Buy in

Normally when 2 neighbors want a fence, they split the cost of the fence. And most states have laws around how that is handled. I suggest you research that law.

Now, since this fence is already built, and you want to make use of it, I suggest pricing a comparable fence, so you know what genuine fence costs look like. You can do this on the pretense of doubling the fence, which is your ultimate option, after all.

Then once you have hard numbers, you can have an informed discussion with your neighbor about (instead of wastefully building two fences) "buying in" to the fence he's already built.

That's a bargain for you at $1 less than the cost of your own fence. However, if you are respectful, you can probably talk him into half the value of the fence. If he wants to turn down thousands of dollars, then clearly you've done something else to upset him pretty seriously.

The simplest answer would be in my opinion, to continue your fence precisely inside the boundary of your yard, ignoring that strip. Leave that strip as your neighbor's unfinished property.

This solves your problem by giving you an enclosed yard.

This solves your neighbor's problem by not affecting their fence at all.

This gives your neighbor no say in your fence's design, as it is on your property.

This also means that should say, a child or dog become wedged between the two fences, it is your neighbor's problem to resolve, as they are definitively on the neighbor's property.

It may cost slightly more, but will save you in terms of removing hassle and worry, a trade-off I personally would go for.

I can't see a location for the question, but the answers suggest USA, so this may not be directly relevant (although versions of it may exist in other countries).

In the UK, it's common for houses to have a 'fence clause' in the contract of sale. For example, at a change of ownership, the house comes with a duty to erect and maintain a fence of at least 4 feet high along the entirety of the property on the left hand side (when looking from the street) within some amount of time (eg. 90 days).

In the UK, confusion is common because not all houses have such a clause, and it's not always the left hand side, and the 'direction' of the existing fence panels doesn't guarantee anything either. Even if it has such a clause, it's often not rigorously enforced. Often it's a "shared" responsibility (or unwritten, which usually means "shared"), or sometimes it's set by prescient (ie. the previous owner put it in, so it's your responsibility to maintain it). Also, just because one side of your house does have a clause doesn't necessarily mean your neighbour on the other side has one on that side of your property. Some solicitors point all this stuff out when you buy houses, and some don't.

A slightly different situation also occurs in the UK with regards to extensions on semi-detached houses. In order to avoid a 'party wall' (where both neighbours own the wall and have to share maintenance by fairly gnarly rules), often extensions are built a few centimetres away from the boundary (eg. 15-30cm). This causes the wall to be inside the boundary, and so the solely owned by that side. It makes maintenance difficult (as you only have a narrow gap), but often avoids problems, especially when the neighbours decide to build an extension of their own and use your 'party wall' to do so in a way that you disapprove. Thus the 'air gap' is common on UK extensions, loft conversions etc.

So in short, in the UK you, or your neighbour may have a responsibility to put in and maintain a fence (along the boundary). Or it may be shared, or it may be 'unwritten'. The sort of 'cold war' situation with both neighbours building a fence inside their boundary are rare here, although I'm sure must have happened somewhere.

If the question revolves around meeting requirements for fencing off your pool, then the only solution is for you to finish off the fence on your property so that it encloses your pool.

This particular gap is irrelevant. Consider this. Your neighbor caves and lets you completely cover the gap (great). The next week, he feels spiteful again, takes down a panel of fencing on the other side of his property, and calls code compliance to report you. Since you have no fences between lots, your pool is now unsecured. You have no capability to fix the fence since it's nowhere close to your property, yet all the legal consequences will still be yours.

Since you are legally liable for restricting access to the pool, the only way you can be certain that you can meet legal standards is if the entirety of the enclosure is under your control (or is a shared fence such that you can perform repairs unilaterally).