So when you build your pocket neighborhood, which is an interesting concept, what's to stop an undesirable person from moving in.

You have to find a zip code that has a prevalence of multifamily and a pathway to build multifamily.

Do you know how I should go about to find  a trustworthy and fair GC for ADU build in North Hollywood? 

Quote from @Matt McNabb: I'm a solid $400k/yr job in NYC and have $250k to begin investing as my first step into building a portfolio, although I don't have time to be a landlord so I have to account for management fees.  

Building an ADU is not like building a structure on a vacant lot.

No matter where you settle upon, your idea to build really comes down to your goals.

I invest in land and have done entitlement/zoning and other DD work successfully for years.A broker called me this morning on a property I have on a busy interesection and has a QSR client who likes the location, but does not want to do a ground lease, only a build to suit. 

Many of our borrowers/clients have one LLC that owns the building and another than own the operating business.

Real estate investing can be an effective way to build long-term wealth, and among the various strategies, one has gained significant popularity over the years: BRRRR.

Since you said you are selling your building I’d argue you already have been a real estate investor.