The shift
For a long time, whether you could build a second unit on your lot was largely a local decision, and many jurisdictions made it effectively impossible. A series of state laws changed that by capping how restrictive local ADU standards are allowed to be.
The practical effect in Los Angeles has been dramatic. A great many lots that could not carry a second unit a decade ago now can, and the approval process is faster and more predictable than it was.
What changed most
Parking is the big one. The requirement to provide an additional off-street space used to kill projects on smaller lots, and state law now removes it in a range of common situations - including proximity to transit and conversions of existing space.
Minimum permitted sizes and setback limits also matter. State standards set floors that local rules cannot go below, which means a small detached unit is achievable on lots where local setbacks would previously have squeezed it out.
Review timelines are the third. Authorities operate under defined windows for ADU applications, which makes the process less open-ended than general permitting.
What still constrains you
Physical reality, mostly. Lot coverage, height limits and setbacks still apply within the state floors. On hillside parcels, slope and construction access frequently decide feasibility more than zoning does.
Utilities are the practical constraint people forget. An older electrical panel often cannot carry a second dwelling without an upgrade, and on a detached unit the service trench is a real cost line.
And on parcels in or near the Wildland-Urban Interface, fire-resistive construction requirements come into the design - assemblies, vents, and sometimes sprinklers depending on the property.
The order to do things in
Feasibility first, always. Establish what your specific parcel permits before you commission a design, because an ADU designed first and zoned second usually gets redrawn.
If the financial case rests on rental income, confirm the rental rules that apply to your property at the same time. Long-term and short-term letting are treated differently and the rules change.
State law removed most of what used to block ADUs in Los Angeles. What remains is physical - slope, coverage, utilities - and it is all knowable before you design.