San Diego ADU ROI calculator 2026

Simple payback on a $250,000 ADU is about 8.9 years. Fully borrowed at today's rates, a HELOC about breaks even each month, and a home equity loan falls short.

That uses HUD's one-bedroom rent of $2,335, less 25% for vacancy and upkeep and about $235 a month of property tax on the new unit. An interest-only HELOC at 7.28% leaves you about $1 short. Repayment starts in year 11 at about $1,980 a month. Even with rent rising 2.2% a year, that leaves you about $92 short. A 15-year home equity loan at 8.46% leaves you about $940 short from the first month.

• Local entity or code: HUD FY2027 Fair Market Rents, San Diego metro; San Diego County Auditor and Controller tax rates, FY 2025-26; Fannie Mae Selling Guide B3-3.8-02
• The core takeaway: Fully borrowed at today's rates, a one-bedroom ADU's rent does not reliably cover a second mortgage once the new property tax is counted. Try a cash share in the calculator to see what changes.

Not necessarily, and nobody can promise it.

No public study measures how much of an ADU's cost comes back in a San Diego appraisal or sale. Fannie Mae has the appraiser adjust for the ADU on its own line of the comparable-sales grid. It rejects appraisals that rest only on cost or only on income. FHFA reports a 2023 median appraised value of $1,064,000 for California homes with ADUs and $715,000 for homes without. Those are different homes, so the gap is not what an ADU adds.

• Local entity or code: Fannie Mae Selling Guide B4-1.3-05 and B4-1.3-10; Freddie Mac ADU fact sheet
• The core takeaway: What an ADU adds comes from local sales of similar homes.

Yes, as a condominium, but not by splitting the lot.

The City of San Diego has allowed it outside the Coastal Zone since August 22, 2025. The unincorporated County has since April 4, 2026. Every lender with a loan on the property must consent. You also need a subdivision map, a condominium plan, CC&Rs, and notice to the utilities. No public data yet shows what these units sell for.

• Local entity or code: SDMC 141.0302(f); County Ordinance No. 10986; Government Code 66342 (AB 1033)
• The core takeaway: Plan on a condominium subdivision with lender sign-off. It takes time.

Nobody has measured how a backyard tenant changes your own home's resale value.

The closest evidence is a 2025 Los Angeles study in Regional Science and Urban Economics. Nearby home prices dipped about 3% within 300 meters (about 1,000 feet) where ADU density rose half a percentage point. That study looked at neighbors, not the lot with the ADU. Privacy and marketability are yours to judge. Fannie Mae has the appraiser account for any effect the ADU has on the home.

• Local entity or code: Fannie Mae Selling Guide B4-1.3-05
• The core takeaway: Each appraiser judges the effect on your home from local sales.

Public numbers can't settle it yet.

No public dataset reports what traveling nurses pay for housing in San Diego, or how long a furnished ADU sits empty between contracts. The reference points are HUD's FY2027 one-bedroom rent of $2,335 a month and GSA's FY2027 lodging cap of $208 a night ($249 in June and July). The GSA figure is a limit on what federal travelers are repaid, and it is not a rent. In the City of San Diego, any ADU lease must run at least 31 days.

• Local entity or code: City of San Diego Information Bulletin 400; GSA FY2027 per diem, San Diego County
• The core takeaway: A furnished lease of 31 days or more is allowed. How it compares turns on how often the unit is rented, and no public source tracks that.

It depends on your all-in cost, rent, loan, and how long you hold it. No public study measures ADU returns in San Diego.

The only statewide owner survey covers ADUs permitted or finished in 2018 and 2019, with owners surveyed in 2020. It found a median cost of $150,000 and a median rent of $2,000. Today's inputs are higher. HUD's FY2027 one-bedroom rent for the San Diego metro is $2,335, and the 30-year mortgage average was 7.03% on September 24, 2026.

• Local entity or code: HUD FY2027 Fair Market Rents, San Diego metro; UC Berkeley Center for Community Innovation 2021 ADU owner survey
• The core takeaway: The survey is dated. Your own cost and rent decide it, and that is what the calculator runs.

Yes, by about 1.02% to 1.29% of the ADU's assessed value each year. Your house keeps its current assessment.

That rate is the 1% base plus local bonds, and it depends on your tax rate area. The San Diego County Assessor adds the market value of the new construction, which it says is usually the full construction cost. After that, the value can rise no more than 2% a year until a sale or more construction.

• Local entity or code: San Diego County Assessor; County Auditor and Controller tax rates, FY 2025-26; California Constitution article XIII A
• The core takeaway: Only the ADU is added, at a value the Assessor sets.

Not a new one in the City of San Diego.

There, an ADU may not be leased for less than 31 consecutive days. Only companion units permitted before October 15, 2017 may be short-term rentals. State law lets cities require ADU rentals of 30 days or longer, and it requires JADU rentals to run longer than 30 days. The unincorporated County sets a 31-day minimum for both. In another city, check that city's rules.

• Local entity or code: City of San Diego Information Bulletin 400 and SDMC 141.0302; County Ordinance No. 10986; Government Code 66315 and 66333
• The core takeaway: Plan on a lease of 31 days or more.

Ask your insurer, in writing, before you rent it out.

No California agency publishes landlord insurance prices for ADUs. The Department of Insurance's homeowners guide puts detached structures under Coverage B, which is normally limited to 10% of the dwelling limit. The same guide says a landlord's policy does not cover a tenant's belongings.

• Local entity or code: California Department of Insurance, Residential Insurance: Homeowners and Renters
• The core takeaway: How a rented ADU is covered depends on your policy's wording.

HUD's FY2027 Fair Market Rent for the San Diego metro is $2,192 for a studio, $2,335 for one bedroom, and $2,823 for two bedrooms.

Those include utilities and take effect October 1, 2026. HUD also publishes them by ZIP code, which is what the calculator uses. They are the 40th percentile of recent rents. No agency publishes rents for ADUs alone.

• Local entity or code: HUD FY2027 Fair Market Rents and Small Area Fair Market Rents, San Diego-Chula Vista-Carlsbad metro
• The core takeaway: HUD's figure is a government benchmark. A particular unit can rent for more or less.

The IRS treats a rental ADU as residential rental property, depreciated over 27.5 years.

If you rent out part of your property, you split shared expenses between the rental and personal parts. Rental income and expenses go on Schedule E, and land cannot be depreciated. The calculator leaves income tax out. Ask a tax professional how these rules apply to you.

• Local entity or code: IRS Publication 527, Residential Rental Property
• The core takeaway: The federal rules are in IRS Publication 527. The calculator does not model them.

There is no official answer for ADUs.

The two public yardsticks are the 10-year Treasury, at 5.18% on September 24, 2026, and San Diego apartment cap rates of about 4.7% to 5%. A cap rate is a year of net rent over the price or cost, so it leaves the loan out. Apartment cap rates price large, managed buildings. An ADU is one unit behind your own house.

• Local entity or code: Federal Reserve H.15; CBRE U.S. Cap Rate Survey H2 2025, San Diego; Kidder Mathews San Diego Multifamily Q2 2026
• The core takeaway: The calculator shows your cap rate next to both yardsticks.