I was paying $660 for my 1 bedroom apartment in 2012,
i was paying $950 in 1990

it was considered very low for the area i lived in-had no central heat or air, no laundry facilities, no parking but was less expensive than anyplace else (at least it was safe).
They changed the law in California to do that. The legal name for those residences is ADUs (auxiliary dwelling unit) but most folks call them Granny Flats. There are even companies that have the blue prints all drawn up, and will come build them on your site*. Here in Sacramento the city has changed the law to allow these in older neighborhoods that are build with alleys. They have given the alleys names and will assign you an individual address for the ADUs as they are built.
there has been talk for some time of doing that in the larger city near us but then some issues have been raised that (from what i know) are still being researched-
the older neighborhoods also have the very old water and sewage lines that were placed for homes to be within a specific distance to the existing streets-there are concerns that adding putting adu's in the back of these homes would put a stress on the lines and cause stress that could bring them down (an issue people have encountered with renovations on existing homes that entailed higher water pressure needs/additional sewage due to added bathrooms),
the alleys are not wide enough for snow plows or emergency vehicles to access so that makes for a safey concern,
parking-unless the primary home owner is willing to take on the cost/has the land available for additional parking ON their lot there is the concern that older neighborhoods that already have issues with parking shortages would be overwhelmed (as it stands during snowy months residents are supposed to alternate parking on the right and left sides of the street so snow plows can get through but there's not enough parking so many streets can go without plowing for days on end until the city steps in, starts towing, gets it cleared only to be in the same situation a few days later).
now, i live rurally and i know some folks that would consider doing adu's BUT the issue we run into here is-
a well for one's home can only provide so much and the cost is prohibitive to drill a second (that's IF you have sufficient water and pressure to accommodate it),
septic-all the homes around me were built with limitations on how many bedrooms/baths can be on their existing system. an adu that was hooked to a septic system would entail a larger piece of property than the adu would occupy, environmental studies ($$$$$$$$$) and the cost of creating the second system ($$$$$$$$$$$$$$$).
i don't know what the solution is but it will entail either private or public funds to address the horrific costs of the infrastructure.