@Quito Keutla here in California they are making accessory dwelling units easier to add to a property.

“vaping”).

Your proposed situation would not be legal under the tentative plan“The proposed city rules would allow short-term rentals as a permissible accessory use in all city zoning districts that allow residential uses.

San Francisco recently started the ADU program which is one way of adding a legally unit relatively quickly:http://sf-planning.org/ACCESSORY-DWELLING-UNITSIf you go through their site it actually explains pretty clearly how to pass the legal requirements.

Luckily in California we have the option to build an ADU (accessory dwelling unit).

My first question would be will the zoning for the parcel, density of the surrounding parcels, and other matters allow for an accessory dwelling unit?

Only one principal dwelling and itscustomary accessory building may hereafter be erected on any lot, except for Planned UnitDevelopments approved by the Gatlinburg Planning Commission.504.

The updated legislation in 2018 "exempts single family dwellings with an Accessory Dwelling Unit and a single rental unit in a condo, co-op or homeowners’ association."

RESTRICTIVE COVENANT FOR ACCESSORY DWELLING, DETACHED HISTORICAL IN CONSIDERATION of the issuance of a building/use and occupancy permit by the Metropolitan Government of Nashville and Davidson County for a single-family accessory dwelling, detached residence on certain property herein described,The question regarding the double mailboxes - I assume so, but that never came up. 

The other company is also a brand more based on accessories.