Selling a Campbell Home With an ADU: The Permit Line That Decides Its Value

An appraiser walks the backyard of a Campbell listing, notes the detached unit behind the main house, and pulls the county permit file before writing a single number into the report. If there is no certificate of occupancy on record, the appraiser typically leaves that square footage out of the home's Gross Living Area entirely. The unit is still standing. Someone may be living in it. None of that matters to the number that determines what a lender will finance.

This is the moment that catches Campbell sellers off guard, and it has nothing to do with the quality of the construction. It has to do with a piece of paper.

The Disclosure Comes Before the Offer

California Civil Code Section 1102 requires sellers to disclose known unpermitted work on the Transfer Disclosure Statement, and the obligation applies even if the seller didn't build the addition. If a prior owner converted the garage or added a backyard unit without pulling permits, the current seller who knows about it still has to put it in writing. Waiting until after an offer is accepted to reveal that status is not a strategy. California law gives buyers three days to rescind after receiving a TDS if a material fact surfaces late, and an unpermitted structure discovered during the buyer's inspection period tends to reopen price negotiations from a weaker position than disclosing it upfront ever would have.

Once the disclosure is on the table, the appraisal and the loan file are where the practical consequences land.

What the Appraiser Won't Count

A permitted ADU with a finaled certificate of occupancy shows up in county records as legal living space and gets full credit in an appraisal, either through the sales comparison approach or, for rental units, an income approach that capitalizes achievable rent. An unpermitted unit doesn't get that treatment. Lenders following Freddie Mac guidelines generally will not count rental income from an unpermitted unit toward a buyer's qualifying income, and appraisers routinely exclude unpermitted square footage from the valuation used to underwrite the loan. That doesn't stop a sale. It narrows the buyer pool toward cash buyers who aren't constrained by an appraisal contingency, and it usually means the price gets adjusted to reflect a structure the lender won't recognize.

Here's how the two scenarios compare at the closing table:

The gap between those two rows is the difference between an ADU that adds documented value and one that sits as a pricing question mark until someone resolves it.

What Campbell's Ordinance Actually Allows

None of this means building an ADU in Campbell is complicated on the front end. The city permits one ADU plus one Junior ADU by right on any single-family residential lot, with no minimum lot size and no requirement that the owner live on site to build or rent the unit. A detached ADU can run up to 1,200 square feet for a two-bedroom or larger unit, or 850 square feet for a studio or one-bedroom. Permit fees for a Campbell ADU typically fall between $5,000 and $15,000 depending on scope, and units of 750 square feet or less are exempt from development impact fees, a threshold that state law under SB 543 locked in statewide effective January 1, 2026. That same law also exempts units of 500 square feet or less from school impact fees and shortened the completeness review clock local agencies get before a permit application is deemed accepted.

None of that changes what happens the day a Campbell homeowner decides to sell. The rules above govern whether you can build the unit and how much it costs to permit. They say nothing about whether you can sell it separately from the house.

The Sale Campbell Can't Do Yet

Assembly Bill 1033 opened a path in California for cities to let homeowners sell an ADU as its own condominium unit, splitting the primary house and the accessory unit into two separately deeded properties under the Davis-Stirling Common Interest Development Act. The law took effect statewide on January 1, 2024, but it is opt-in. A city has to adopt its own ordinance before any ADU within its limits can be sold that way, and adoption has been uneven. San Jose became the first California city to complete a transaction under this framework, closing the sale of a unit at 410 Josefa Street in August 2025 after an 89-day combined review of the application and parcel map, a timeline the city's dedicated ADU review staff helped compress. San Diego, Santa Monica, Santa Cruz, and San Francisco have since adopted their own ordinances as well.

Campbell has not joined them. As of 2026, the city is still developing its local implementation framework for AB 1033, which means a Campbell ADU, however well built and fully permitted, cannot currently be sold as a separate unit from the primary residence. It can be rented. It can add documented value to the whole property when that property sells. It cannot yet become its own deed.

That distinction is the one worth sitting with if you're pricing a Campbell home with an ADU, or deciding whether to build one before you list.

What This Means for Pricing

A permitted ADU in Campbell functions as a contributory improvement to the main sale, not a second transaction waiting to happen. Appraisers typically price it through documented rental income or comparable sales with similar secondary units nearby, and in 2026 a well-finished detached Campbell ADU with quality appliances and flooring has been commanding somewhere in the range of $2,500 to $4,200 a month in rent, with basic units toward the lower end and architecturally considered builds toward the top. Construction costs for a turnkey detached unit in Campbell have run in the neighborhood of $300,000 depending on size and finish, which is the number a seller is weighing against whatever premium the market is currently paying for documented ADU income at resale.

One detail works in a seller's favor regardless of AB 1033's status here. Adding an ADU does not trigger a full reassessment of the primary home under Proposition 13. Only the new construction, the ADU itself, gets a new tax basis. The existing home's assessed value stays where it was.

A Few Questions Sellers Ask

Can I sell my ADU separately from my house in Campbell right now? No. That option only exists in cities that have adopted their own AB 1033 ordinance, and Campbell has not done so as of 2026. A Campbell ADU sells as part of the single-family transaction, not as its own deed.

Does building an ADU raise my property taxes on the whole house? Only the ADU itself gets a new construction tax basis. The primary home's existing Prop 13 assessment is unaffected.

What if I suspect my ADU was never permitted? Pull the permit history from the city's Community Development Department before you list. If there's a gap between what's built and what's on file, you'll want to decide whether to pursue retroactive permitting or price and disclose accordingly, before an appraiser or a buyer's inspector finds it for you.

If you're weighing whether to permit an existing unit before you list, or you want a clear read on how a Campbell ADU will actually price out at resale, Diane Bucher Real Estate Group can walk through the disclosure and valuation picture with you before the sign goes in the yard. Request Your Personalized Home Valuation and bring the permit questions with you.