I just had a friend that went through this about 8 months ago. This is not really a DIY question if you want a long-term answer. I will get back to how his situation turned out...
Your answer is to stop your neighbor from smoking. Let's just take weed off the table. Let's talk cigarettes or vaping which is not considered a controlled substance. It is illegal in every single state to emit smoke (cigarettes) onto another dwelling. Second hand smoke can cause major health concerns and if you can smell it it can harm you. I mean you know the fart smell is just you taking in tiny vapors with poop in them right? Same thing for cigarette smell, vaping smell, pot smell.
So pot would be met with double the attention because more than just the negative health side effects you are in fact giving someone a dose of an illegal substance. It does not matter how small it is, it is illegal.
Here is what my friend did:
- Talked to neighbor. This did not help and neighbor continued.
- Warned neighbor they were going to get sued if they continued.
- Wrote a letter to the HOA. Advised them that if they deemed it legal to do in their facilities then the liability of the home owner would be at least partially passed to them.
- File a suit in small claims court against home owner. Filed a motion asking board to act as witness and to testify on their HOA rules. The suit was to pay them 20k+ for renovations that would allow for zero smoke to enter their home.
- Friend called cops each time smell got bad. After the 5th time the cops issues the other home owner a public nuisance ticket.
After the ticket was issued his case was basically won for him - judge would look incompetent not favoring him and he could just keep sueing as each light up is a different offense. So HOA lawyer basically told other homeowner this. She put her condo up for sale within a few weeks and moved out... HOA made it against their rules to smoke weed (as much as it can be smelled).
These steps were outlined to him by case law that he found on home owners with similar issues. He did not think of any of these things himself. He also researched remedies like positive pressure and filling in gaps. Most of the people experienced the same thing - they spent tons of money for some remedy but still smelled the smoke. My recommendation is that you talk to neighbor and HOA board before doing anything. I have worked on condos and apartments with lots of smoke damage. There is no way you are sealing off an adjoining wall from a non-commercial building without ripping the whole thing apart.
Here are some other things to think about:
If it is weed or cigarette smoke these smells can basically get trapped by your house. In severe cases I have had to rip out drywall and start over. Someone other than the homeowner is liable for this for sure (HOA and other home owner). This is destruction of property. Let's just say the homeowner seals their home "adequately" where it isn't making them upset. There is a high chance that they just got used the smell. What if that smell has infiltrated the walls. Think about the dog/dog-pee smell you smell when at a friends house and they smell nothing. So now even if homeowner is sort of happy short-term they could be out potentially tens of thousands if trying to sell.
What would your reaction be to a house that installed positive pressure or some funky air traps and filters? Cliffhanger... I would buy something else. Most people would react the same. Spend a bunch on fixing house so 80% of the smoke smell doesn't get in and prove to potential buyers that the condos were not made well in the first place AND that there is/was a big problem.
This is not a new problem. Don't let uninformed people fool you. There is no right to smoke marijuana even for medical purposes if smoking will effect others. The law is crystal clear in every state.