Real Estate Professional status is explicitly not recognized by California, among many other beneficial tax rules that exist on the Federal level.

But to stay in compliance with NYC Department of Buildings (DOB) rules, every detail must be handled precisely.Let’s walk through how it works under DOB’s Technical Policy and Procedure Notice #3/97 (TPPN 3/97), and why having the right professionals on your team is key.C of O Not Required—When You Follow TPPN 3/97TPPN 3/97 allows apartments in multi-family buildings to be legally combined—without triggering a C of O update—if:The overall number of primary living spaces (such as bedrooms, living rooms, dens, etc.) stays the same or decreasesYou don’t alter building egress (staircases, corridors, exits)The work is limited to interior constructionThis provision applies to all multiple dwellings in NYC, including old law, new law, converted, and new code buildings.What You’ll Need to FileArchitectural Drawings from a Registered Architect (RA) or Professional Engineer (PE)Alteration GC Application for general construction items like partition changes or internal stairsAlteration PL Application for plumbing work—particularly when repurposing or capping kitchen fixturesLetter of Completion issued by DOB when the work is signed off, confirming no new or amended C of O is neededTPPN 3/97 Requirements—SimplifiedSame or Adjacent Floors Only: Combinations are limited to apartments located on the same level or directly above/below each other.

Quote from @Wale Lawal: I closed on three identical lots next to each other last week, with each lot around 7,150 SqFt with 65 feet of frontage.

Would I be breaking any epthical rules?   

I haveheard of certain investors doing it and finding success doing it but am warygiven the restrictions on short term rentals (under a month) on RSO propertiesand generally how sensitive these rules are.From what Iunderstand, they are "hotel-like" bookings.

@Jonah B. it would be customary and appropriate to do a move in/move out condition inspection, and to have any specifics like house rules, quite hours, parking arrangements etc all down in writing and they can be exhibits to the lease or have it in a lease addendum. 

There are many variations of using chapter7 and chapter 13 to remove debt but keep the primary home.While bankruptcy has federal rules, there are also state rules and above all, the court allows for lenders and borrowers to have considerable latitude in dealing with the problem, depending on what each side wants to accomplish.

What are your specific HOA rules and are you on a waiting list with your HOA? 

The house rules and maintenance procedures will also be in there (kind of similar to what you may have seen in the past with Airbnbs) and a list of emergency numbers, closest medical, fire, and police, etc.

Clarification on 27.5 vs 39 - for @Joseph Coleman and other investorsYour previous accountant who used 27.5 did so (I'm 99.9% sure) not because he went deep into an analysis of the tax rules, but because he was unaware that STRs were treated differently from regular rentals.