HELOCs have a couple of different flavors to them and running them through a lender might be helpful.

@Marty Gold it is important to make the distinction between a traditional mortgage compared to a commercial real estate loan offered by a portfolio lender.  

Also, there is a distinction in some states of animals versus Emotional Support Animals, so make sure your lease has writing pertaining to that if needed and if your state works with ESA's.

I view it as a distinct pot of money that shouldn't be unplugged, and loans against a 401k are bad news, especially if one gets fired, laid off, etc. 

This may seem like an ignorant distinction to point out, but if you do form an entity, make sure your business name is on everything that has to do with the reason you formed it.

For me there were 2 very distinct moments in my life.

Thanks for the mention @Brett Chupka 😊, I usually make the distinction on most podcasts about being a real estate professional (REPS) versus not, as I did on the BP episode as well with @Terrance Doyle at around 22:00 minutes in.

That third unit needs to be a regular dwelling unit (and comply with related parking) and can't be a companion unit.I understand what Tyson's saying about intent of the state-level ordinance, but that's not how the San Diego ordinance is written.On the financing, I've seen no distinctions between companion units and dwelling units.  2 dwelling units + 1 companion unit = successful conforming refinance in my experience. 

@Jason Kudo that's a good point to make the distinction between the city and the county.

Have you heard of this distinction between appraisal prices for owner-occupied vs investment property?