I would have to agree with this from Grok:
Grok:
"A detached casita is generally not considered part of the primary dwelling’s living area (Gross Living Area / GLA) in California for appraisal purposes.
Key distinctions
In California (especially Southern California), “casita” is a common informal term for a small detached dwelling or guest house. When it has permanent provisions for living, sleeping, eating, cooking, and sanitation, it is typically permitted and treated as a
detached Accessory Dwelling Unit (ADU).
Under ANSI Z765 (the measurement standard required by Fannie Mae since 2022 and adopted by Freddie Mac):
- Gross Living Area (GLA) is finished, above-grade, heated, contiguous living space of the primary dwelling with direct interior access.
- Detached structures (casitas, ADUs, guest houses, pool houses, etc.) are not included in the primary home’s GLA because they are not contiguous with the main living area.
How appraisers treat them
Fannie Mae’s Selling Guide is explicit: when reporting the living area of an ADU,
it should not be included with the finished above-grade square footage of the primary dwelling. It is measured (to ANSI standards) and reported/adjusted for on a
separate line in the sales comparison grid based on its contributory value (or, if qualifying, potential rental income).
- The exception is limited: an ADU that is contained within or part of the primary dwelling, has direct interior access, and is entirely above grade may sometimes be included in the primary GLA.
- Attached casitas with true interior access and contiguous finished space can occasionally qualify; purely detached ones almost never do.
MLS listings and county assessor records sometimes lump the casita into total square footage (especially in certain markets such as the Coachella Valley). Appraisers for lending work generally do not, and USPAP expects the report to be clear and not misleading about the distinction."