I live in a 2-unit town home. I have installed an AC on the side of my unit. My neighbor would like to install an AC for their unit. However the city codes will not allow them to install it on the side of their house since it is too close to the property boundary. The front and back of their unit is also prohibited by other codes.

One solution is to allow them to install their AC on my side of the house (shared property) and run the refrigerant lines thru my garage. Are there any problems I should be aware of for this kind of arrangement? Are there any alternatives that folks can think of that doesn't involve going thru my garage?

diagram showing layout of buildings top-down view of the properties

  • 33
    If it helps to frame your thinking, put away the "I'm being a nice neighbor" hat for a minute and ask yourself how you'd feel about buying a place where the neighbors AC was run though your space. It could knock 10s of kilobucks off your resale, I'm thinking.
    – Ecnerwal
    Commented Dec 19, 2021 at 0:38
  • 11
    Why not run the refrigerant lines on the outside of the building? Have you or your neighbour asked the city? Sometimes you can get permission in special cases by asking the city directly to waive the bylaw preventing installation when you have no other options. The driveway obviously isn't an option, but the "too close to the property boundary" is something they might let them get away with if you get permission in advance. The space between the buildings seems otherwise the ideal spot so I wouldn't give up on that without at least asking the city first.
    – J...
    Commented Dec 19, 2021 at 12:17
  • 4
    As an example, I have a sun porch on the side of my property that was added by the previous owners and which is also "too close to the property line" - they applied for special permission to build it anyway (I have the documents) and it was granted with no objections. Usually building close to the line is disallowed in general, but for special cases it can often be waived. This should be your first objective since it is by far the best solution. You may need permission from the neighbours on their other side, but it's way better "too close" to theirs than running straight through yours.
    – J...
    Commented Dec 19, 2021 at 12:32
  • 4
    If this is a two-story unit then your neighbor could hang a min-split condenser unit high enough over the driveway. It's possible to have multiple rooms run off of a single mini-split condenser. Would the "other codes" prohibit such an installation?
    – Dave D
    Commented Dec 19, 2021 at 16:33
  • 4
    And if everything OP said is true, the local jurisdiction is basically telling OP's neighbor that she can't have an AC unit on the ground outside her house, but OP can. That just doesn't sound right to me. Maybe OP's neighbor just hasn't tried hard enough, like seeking a variance from the planning/zoning people.
    – SteveSh
    Commented Dec 19, 2021 at 23:09

9 Answers 9


No, you should not run refrigerant lines through your property. There are many reasons:

  • They will require access to the unit to repair lines.
  • A split system, like you are suggesting, has two lines, one of which is often cold. This causes condensation on the line, and if the insulation isn't 100% it's your property the line will be sweating into.
  • You can only damage the lines, there is perpetual risk to you, with no benefit.
  • The lines being damaged, can only harm you; again there is no benefit.
  • The agreement you and your neighbor make will have difficulty surviving new neighbors, as well as complicate the sale of your property.

There is an alternative. Have you neighbor look into a roof top mounted AC stand.

  • This keeps the property lines where they are, without encroachments.
  • The stands themselves are rather cheap (under $200)
  • Access to the AC is a bit more complicated, but not "enter my neighbor's house with a stranger technician complicated."
  • A crane will have to lift the unit in place, which typically adds $100 to $200.

However, if anything goes wrong, it's not going wrong in your home. Having something go wrong in your home, even if you had nothing to do with the failure, will put stress and pressure on the neighbor and your relationship. Even if you are completely helpful to the neighbor, there will be ways that you could have been more helpful; and, if you neighbor is pushing for access, even once, you might find yourself in the position of not wanting to help them.

  • 10
    Incidentally, half of these issues can be solved by running the lines outside the building rather than through the garage. In any case, quoting the guy 20kbucks for the easement will probably change his mind and go with roof mounted.
    – Joshua
    Commented Dec 20, 2021 at 4:59
  • 1
    Also, I believe this creates an easement through or around your property where your neighbor has access rights.
    – Arluin
    Commented Dec 20, 2021 at 16:58
  • @Joshua It only moves the issue outdoors. They might have to dig up flower beds, uproot plants, and cut tree roots (which can injure or infect a tree) to gain access to the now outdoors lines (which I hope are buried). If the lines are not buried, then they must still be insulated, sweating against the buiding they are attached to. Yes, outdoor access is more easily arranged, but the damage of work isn't avoided. Also, this is "encroachment" which, depending on jurisdiction, will make transfer difficult (without destruction of the work) or possibly open grounds for property line dispute.
    – Edwin Buck
    Commented Dec 20, 2021 at 18:56

Problems that come to mind:

  1. Liability if you damage the lines.
  2. Liability if the lines damage your property.
  3. Access to the lines in the event of repair or replacement.
  4. What if the property is sold and the new owner is a some sort of a jerk?

I'm sure any or all of the above are manageable but this is something that you may want to discuss with an attorney and have the appropriate documents drawn up.

  • 4
    5. What happens if at some point in the future you want to do work on your garage?
    – avid
    Commented Dec 19, 2021 at 10:56
  • 19
    Note that these kinds of issues may scare off potential buyers (= lower price) when you come to sell your house, even if they don't bother you.
    – avid
    Commented Dec 19, 2021 at 10:57

Everyone is correct, you should not allow it. In fact, it’s your duty to not allow it.

Look at your deed and it will say something about NOT allowing other improvements to encroach onto the property. This includes buildings considered duplexes , tri-plexes, etc.

Also, I’d check with your title company. They certify that your building is “only on” your property and “no other improvements encroach “.

  • 15
    Also, not ok to run electrical lines from a different shut off (panel) in a different dwelling. What encroachment laws/principles apply depends on the OP's jurisdiction, which we don't know (yet). Nevertheless, I think this the reason mentioned here is why OP should not do it.
    – P2000
    Commented Dec 19, 2021 at 4:39
  • @P2000 'OP's jurisdiction, which we don't know (yet)' OP's profile places them in Washington State, USA. Commented Dec 19, 2021 at 11:28
  • I think OP should allow it - after negotiating an easement that includes compensation for the reduced value of the house and other costs (including the not insignificant costs of hiring the lawyers and other experts to negotiate the easement).
    – emory
    Commented Dec 20, 2021 at 1:37

Don't run the lines across or through your property. Your neighbour should consider something more compact like a side-discharge A/C unit. These are available for both mini-split and central A/C units and would certainly fit on your neighbour's side.

enter image description here

Side-discharge units can even be mounted directly to the side of the building and won't even extend past the eaves, so there should be no issue with it being too close to the property line. This is also much less of an eyesore than anything roof mounted.

If your neighbour is dead set on a top-discharge unit, you could still try applying to the city for a variance to allow them to install it anyway. Often the rules about how close to the property line you can build can be waived in special circumstances. Those rules are there to keep generally sane margins around a property, but there are many special circumstances where building up to the line is necessary and most cities are reasonably understanding in those cases. Here it's clearly a better idea for your neighbour to put their A/C on their side rather than some sort of nightmare of wiring and refrigerant lines crossing through your property.

  • 2
    You haven't seen small until you've seen the eaves extend past the curb. (I have.)
    – Joshua
    Commented Dec 20, 2021 at 5:01
  • 1
    @Joshua Yes, in high density downtown areas, sure that happens. Those are exactly the types of installations that side-discharge A/Cs are there to solve. Usually you bolt them to the side of the building high enough up that they're out of the way of whatever's below.
    – J...
    Commented Dec 20, 2021 at 17:30

Are there any alternatives that folks can think of that doesn't involve going thru my garage?

  • On the front or in the front yard (unusual)
  • On the back (usual)
  • In the backyard (usual)
  • On the roof (fairly usual, especially if yard space is limited or subject to damage/vandalism - though unusual in much of suburbia)
  • Inside the attic (if there's an attic)(standard some places, unusual in others.) There are vents to the outdoors, of course
  • 3
    All of the sides are off limits except mine. But the attic is an interesting idea. I'll ask about that.
    – V Maharajh
    Commented Dec 19, 2021 at 4:00
  • 6
    The roof, +1. Not my problem. IDT you can (or at least you shouldn't) put a condenser in an attic. Sure as hell not if half of it's mine. The only option you're missing is the ugly one : mounted on a rack on the side of the building.
    – Mazura
    Commented Dec 19, 2021 at 6:29
  • @Mazura Condensers should not be in the attic, if only for the primary reason that they should be cleaned occasionally, which often involves spraying water thorugh the cooling fins. Also, it's a really bad idea to vent heat into a space that tends (on the warmer months) to collect heat.
    – Edwin Buck
    Commented Dec 19, 2021 at 18:22

If by ducts you mean the air supply and return ducts, then no, you should not allow that. That creates an unnecessary fire risk between your homes.

Instead, your neighbor should install a split system so that only the refrigerant lines and wiring pass through your house. They could even be run around the outside of your house.

  • 2
    Sorry I mispoke. It is the refrigerant lines that would have to go thru my garage, not actual ducts. I updated the question.
    – V Maharajh
    Commented Dec 19, 2021 at 3:55
  • If it were ducts they would also pose a risk of leaking foul smells or even airborn disease. Commented Dec 19, 2021 at 13:46

Not an answer, but this is the only way I could post a picture of a friend's house in the city, with the AC unit sitting on the sidewalk next to the house right behind the mail box. Yes, he had to go to the city to get a waiver to do that.

OP - Has your neighbor applied for a variance? That should be the first approach. Like others have said, almost all jurisdictions have a process whereby bulk requirements (like building setbacks) can be waived.

enter image description here

  • 2
    Upvoted because there is an answer hidden in here that I wanted to give: the neighbor should see if they can apply to the city for a waiver. Under the circumstances, the city is likely to grant one on condition of preapproval for the plans. I would suggest rewriting your answer, as that is what you're pointing out, i.e. cities often grant waivers even in worse circumstances.
    – Bloodgain
    Commented Dec 20, 2021 at 23:16
  • Yeah that is a really good option mentioned elsewhere as well. I can't mark them all as answers, so I've updated the one I marked with an answer to capture these good options as well.
    – V Maharajh
    Commented Dec 21, 2021 at 18:01

For 'Are there any alternatives': if you're not in a location where you need active cooling 24/7/365, your neighbour could consider ditching the chiller component of the AC, and instead using phase change materials for temperature regulation. (On a careful analysis, they might even find that the moisture content of outdoor air in your area is low enough that the dehumidifier component of the AC is also unnecessary, and/or that they can get all the fresh air they need using displacement-mode natural ventilation, rendering the fan component unnecessary.)


Yes, just use suitable conduit for some extra peace of mind.

I can't comment on the legal/contract details of how to set this up in the US, but technically there should be no issue doing it the way it's done in my house in the EU. Just install a pair of straight 40mm(-ish) steel pipes as a sort of conduit and then run the AC lines inside those.

  • No risk of you damaging the AC lines as it's pretty much impossible to accidentally drill through several millimeters of steel
  • No risk of the lines damaging your property (again, any leak will be contained and carried away by the conduit), same for anything nasty possibly infiltrating from your neighbour's property (fire, bugs)
  • If the AC lines have to be replaced later, it can be done without opening walls or entering your house at all (just uncap the conduit, pull out old line, pull new one in). (But I suspect this isn't going to be necessary at all, chances of a leak developing in the middle of a straight copper pipe are likely negligible.)
  • If you can manage to run this through a wall that won't ever be moved (load-bearing/external) or in the ceiling, it's not going to limit you in the future.
  • I take comment with that "pretty much impossible" as my brother was an AC technician for many years. He's seen, and I've heard about, nails being driven through a line when attempting to hang a picutre. Yes, there should have been a "nail plate" but there wasn't. At least the person damaged their own AC system. And as for stringing through the conduit, this is new installation, they'll have to install the conduit first. It will likely be messy or at least messier than you are advertising.
    – Edwin Buck
    Commented Dec 20, 2021 at 2:59
  • 1
    @EdwinBuck I'm talking about the outer steel pipe making it nearly impossible to puncture the copper line inside. And I'm not saying it's not going to be messy once,but that's a matter of agreeing on a fair compensation,not a long-term liability as some other answers suggest.
    – TooTea
    Commented Dec 20, 2021 at 7:20
  • Generally speaking it makes it more difficult; but, I assure you that nails do go through pipe. It is incredibly rare, but less so now that nail guns exist. The typical path here is not to enclose in iron (or steel) pipe; but, to place nail guards (sheets of steel) behind the sheetrock. It's often cheaper than pipe. Perhaps the flat sheet makes nail puncture more likely than a pipe; but, there are occasionally people who simply think a stubborn nail just means hammering harder (who are blissfully unaware that such barriers exist or a reason).
    – Edwin Buck
    Commented Dec 20, 2021 at 19:12

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