There is one slight issue, the property has an open permit from 1992!

They will tell you the next steps which likely will involve DOT in your area depending on where the property is located and what type of street or highway you are on.Assuming you can proceed the next step is to hire a civil engineering firm to do all the survey work, site plan, actual construction details, traffic studies if needed and help get all the permits for you etc. 

That sounds like a more elaborate version of what's happening in Nashville, namely, condos that are appropriately zoned and STR-permit eligible, with HOAs that explicitly allow STRs.  

I have a detached and permitted ADU (400 SF) which currently is a Studio.

We'll pay for permit and dumpster, then funds come after work completed weekly/biweekly as stated.

You could get the city involved as they may stop any work/permits they may have planned.

Does anyone have any experience with permitting triplexes, workarounds or any thoughts?

@Justin Clemmer - It has to do with the fact that proper permits may not have been taken out when they made additions.

You would need a permit to add / build it.  

When we renovate the property we will be pulling permits to make it a duplex.