How an appraiser values a casita, and what the data says

Three ways an appraiser can value the unit

Why the unit's square footage is reported separately

On the standard appraisal forms, ADU living area is not added to the main house's gross living area. It is described and valued as its own feature. That is the mechanical reason the casita rarely gets the same price per foot as the house: it is adjusted as a line item, from whatever the comparable sales show, rather than multiplied by the neighborhood rate. On the math page we model that as a credit of 50% to 100% of the main-house rate.

What pushes the credit up

• Permits and a final inspection. An unpermitted unit may be given little or no value, and can block the loan entirely. This is the single biggest factor.
• A complete unit. Full kitchen, full bath, private entrance, its own address or meter where the city allows. A "casita" without a kitchen is a bonus room.
• Comparable sales. Neighborhoods where casitas have sold give the appraiser something to point at.
• A lease or market rent. Documented rent supports the income approach and, since 2023, lets a buyer's lender count the rent (see rental math).
• Quality consistent with the house. The law says it does not have to match; buyers still pay more when it does.

What the largest dataset shows

The Federal Housing Finance Agency looked at every purchase appraisal submitted to Fannie Mae and Freddie Mac in California from 2013 to 2023 and compared homes reported with an ADU to homes without one.

Two cautions, both from the FHFA itself: the ADU flag in appraisal data has entry errors, and faster appreciation is a correlation that "requires further analysis". Homes with ADUs are also larger and sit on larger lots, which explains part of the gap. Still, it is the best evidence available that a legal second unit is something the market pays for, and Arizona's 2024 law puts Valley homeowners on the same footing as the California owners in the study.

Source: FHFA: Trends in median appraised value for properties with ADUs in California (Jan 2, 2025).

Resale in practice

Our two completed casita projects both sold on the MLS, one in Maryvale with a converted-garage studio and one with an attached two-bedroom casita. Both are written up as case studies, with the public sale records. We do not present the whole-home renovation budgets as the casitas' cost, and we do not publish prices; what we can show is that both homes sold above their neighborhood's median price per square foot. The numbers are on the math page.

Build for the appraisal from day one

1. Pull the permit and get the final inspection. Keep the paperwork with the house.
2. Give the unit a full kitchen and bath and its own entrance.
3. Keep a record of the scope and the invoices so the cost approach has something to work with.
4. If you rent it, keep the lease. If you do not, know the market rent anyway.
5. When you sell, make sure the listing agent flags the ADU and that the appraiser is given the permit history and comparable casita sales.