01
The California floor
California sets minimums that a local ordinance cannot undercut. The often-quoted "800 square feet, 16 feet, four-foot setbacks" floor is real, but it is assembled from three separate provisions — § 66321(b)(3), § 66321(b)(4)(A) and § 66323(a)(2) — and quoting it as a single rule is how people end up applying it to the wrong ADU type.
Two more that change feasibility more often than people expect: under § 66323 an agency may not require the correction of nonconforming zoning conditions as a condition of approving an ADU; and under § 66314(d)(12) an ADU does not need fire sprinklers if the primary residence is not required to have them.
02
Septic is the one that stops projects
A sewer connection is usually an engineering and cost question. A septic system is a capacity question, and capacity is finite.
Adding a dwelling can exceed what the existing system was sized and permitted for, and the answer comes from county environmental health rather than from the building department. Where the system needs replacing, it is frequently the largest single cost on the project.
Find this out early. It is the item most likely to change whether the project happens at all.
03
What a feasibility tool can and cannot tell you
Our feasibility checker will never tell you that you are approved, because no tool honestly can. Approval is a determination made by a permitting agency against your specific parcel.
What it does is turn your situation into the specific list of questions to put to the counter, with the office that answers each. That is the useful output, and it is what turns a vague first phone call into a productive one.