What is an ADU in real estate? Value, appraisals and selling

What is an ADU in real estate?

California law defines an accessory dwelling unit as an attached or detached residential unit with complete independent living facilities for one or more people, on a lot with an existing or proposed main home. It must have permanent provisions for living, sleeping, eating, cooking and sanitation. Efficiency units and manufactured homes can qualify. Our guide to what an ADU is covers the four types.

What does ADU stand for in real estate?

ADU stands for accessory dwelling unit. You'll also see granny flat, in-law unit, backyard cottage and casita; see granny flat vs. ADU for how the terms line up. A JADU, or junior ADU, is a smaller unit of up to 500 square feet inside the main house.

Lenders use a similar test. Fannie Mae calls a space an ADU only if it has its own entrance, kitchen, sleeping area and bathroom. A second kitchen alone doesn't make one, and neither does space you can reach only through the main house.

ADU property listings: what to verify

A listing that says “ADU,” “guest house” or “income unit” is the seller's description, not proof the unit is legal. Before you count on it, check:

Ask the seller for the permit number and look it up with the city or county. If there's no permit, read the section on unpermitted units below.

Does an ADU add value?

The best public data comes from the Federal Housing Finance Agency, which tracks appraisals on homes bought with Fannie Mae and Freddie Mac loans. In California:

Source: FHFA, January 2025. FHFA cautions that the data may contain appraiser entry errors and that the faster growth “requires further analysis.” Homes with ADUs may simply be larger lots in pricier areas, so this shows correlation, not the value an ADU adds to your home.

Builders use rules of thumb. Snap ADU, a San Diego builder, says a new ADU adds about 100 times its monthly rent, so roughly $350,000 for a unit renting at $3,500. That's a seller's estimate, not an appraisal method. For the rent side, see ADU rental income in San Diego; for the cost side, what an ADU costs in San Diego.

How an ADU appraisal works

Loans sold to Fannie Mae and Freddie Mac must follow their appraisal rules, which is why they shape how appraisers handle an ADU property.

• Separate line, not added square footage. Fannie Mae says an ADU shouldn't be included in the main house's above-grade square footage. It's reported and adjusted on its own line, unless it's above grade, inside the house and connected by interior access.
• Comparable sales. Freddie Mac requires at least one comparable sale with an ADU for a zoning-compliant unit. If none is recent, the appraiser can use an older sale or a competing market. Ignoring the ADU for lack of comparables is “an unacceptable appraisal practice.”
• No market reaction, no value. If the comparable data shows buyers don't pay more for an ADU, the appraiser may give it no value.
• One ADU per parcel. Fannie Mae's definition allows one ADU with a one-unit home, and none with a 2–4 unit property. California lets many lots have more than one, so ask your lender early how it will classify the property.

Can the ADU rent help you qualify for a loan?

Yes, with limits. Fannie Mae lets a lender count rent from one existing ADU on a one-unit principal residence, for a purchase or a limited cash-out refinance. The ADU rent can be no more than 30% of your total qualifying income. On a purchase, the lender needs a comparable rent schedule from the appraiser, and the lease if one transfers to you. Our ADU financing guide covers loans for building one.

Property tax

Under Proposition 13, building an ADU doesn't reassess your whole property. Only the new construction is assessed at market value and added to your existing assessment; the land and house aren't reassessed. The statewide base rate is 1% of assessed value plus local voter-approved bonds. A buyer is different: a sale is a change in ownership, so the whole property, ADU included, is reassessed to market value.

Selling a house with an ADU

In most California home sales, the seller fills out a Real Estate Transfer Disclosure Statement. It asks directly whether the seller knows of:

• “Room additions, structural modifications, or other alterations or repairs made without necessary permits”
• “Room additions, structural modifications, or other alterations or repairs not in compliance with building codes”
• “Any zoning violations, nonconforming uses, violations of ‘setback’ requirements”

If the ADU or garage conversion was never permitted, say so. A tenant in place is also worth flagging to buyers early, along with the lease terms.

Unpermitted ADU when selling

You have two paths: legalize it first or sell it as is.

• Legalize it. For an ADU built before January 1, 2020, a city can't deny a permit just because the unit breaks building standards or ADU rules, unless the fix is needed for health and safety. You generally won't owe impact fees to legalize it. The City of San Diego explains the steps in Information Bulletin 242.
• Sell as is. Disclose it. Expect buyers' lenders to look hard at it: Fannie Mae will finance a home whose ADU isn't allowed by zoning only if the lender confirms it won't jeopardize an insurance claim and the appraisal uses comparable sales with the same non-compliant use.

ADU condo sale: selling the ADU separately

By default, an ADU can't be sold apart from the main house. State law allows a separate sale in two cases: certain affordable units built by qualified nonprofits, or as a condominium where the city has adopted an ordinance allowing it. The City of San Diego has opted in. A junior ADU can never be sold separately.

Source: Government Code § 66342. Lender consent is often the hardest step, so call your mortgage servicer before you spend on a condo map.

Buying a home to add an ADU

If you're shopping for a lot with ADU potential, check these before you make an offer:

• How many units the lot allows. A single-family lot can generally have a conversion ADU, a detached ADU of up to 800 square feet and a JADU. From January 1, 2027, a single-family lot can have two detached ADUs.
• HOA rules. An HOA can't effectively ban or unreasonably restrict an ADU on a single-family lot, but it can impose reasonable rules.
• Parking. No parking can be required within half a mile walk of public transit, including a bus stop. In the City of San Diego, parking applies only in the Beach Impact Area outside transit priority areas.
• Fire zone and setbacks. San Diego requires a 4-foot side and rear setback in high and very high fire hazard zones, even for a low ADU that otherwise needs none.
• Slope, sewer and power. Snap ADU puts a separate SDG&E meter at $10,500 to $15,000, retaining walls at $20,000 to $30,000 or more, and a sewer pump at about $5,000. A flat lot with a sewer line that runs downhill to the street is cheaper to build on.
• Existing unpermitted structures. Factor in the cost to legalize or remove them.

Our California ADU laws guide has the full rules. When you're ready, compare verified ADU builders in San Diego and read how to hire an ADU contractor.

Common questions