If you live in California, you can potentially build one of these as an ADU (Accessory Dwelling Unit). Local agencies are required to approve an ADU within 60 days, and recent state bills (AB 976, 434, 1033, and 2221) make it easier to build/sell/rent ADUs on your property. You'll have to wait until 2025 before every locality is forced to have a process for pre-approved ADU plans (at which time you could potentially have an "open-source permit" to construct one of these very quickly).
Surprisingly enough, Los Angeles is actually ahead of this timeline and rolled out a program more than a year ago to develop pre-approved ADU plans with private architecture firms. As of now, there are about 65 standardized plans that have been approved, ranging from ~300 ft^2 (28 m^2) to 1,200 ft^2 (110 m^2).
These do not make sense if you are looming for similar build quality to match your home. If you have been inside of a pre manufactured home (a “mobile home”) then you’ll know how everything has to be made of oddly hollowed plastic in order to keep shipping weight down.
The frame is the cheapest part of a permanent structure, and a 300-500sqft ADU is not going to have much in the way of plumbing, electrical, and hvac. Paying a plumber, electrician, and hvac tech for new construction on a tiny building is not going to represent a significant chunk of your budget for what amounts to 1-3 days work maximum.
required to approve or required to decide, and they can reject? is the approval criteria checking mechanistic? (ie. every criteria is quantified and "objective"?)
