San Diego is the most ADU-friendly city in California, and ADUs now account for 30-45% of new housing permits in the city.1 Build costs typically run $200K-$450K depending on size, location, and finishes. Rents on completed ADUs run $1,900-$3,500/month — meaningful rental income that can offset financing costs entirely. The financing question is the bottleneck for most homeowners. Here are the five most common paths, with realistic 2026 numbers, qualification mechanics, and the Prop 13 implications that make ADUs unusually tax-efficient compared to typical real estate investments.
The five primary financing paths
- HELOC (home equity line of credit)
- Cash-out refinance
- Construction loan / renovation loan (Fannie Mae HomeStyle or FHA 203k)
- SDHC ADU Finance Program (San Diego Housing Commission)
- After-renovation value (ARV) loans
Each suits different homeowner profiles. The right one depends on your existing mortgage rate, equity position, income, project size, and willingness to accept rental restrictions.
Path 1: HELOC
The most common path: 56% of mortgage-based ADU financing comes from HELOCs or home equity loans.2 Mechanics:
- Lender opens a revolving credit line secured by your home's equity
- You draw funds as needed during construction (interest-only on amount drawn)
- 10-year draw period typical, then 20-year amortizing repayment
- LTV typically capped at 80%-85% combined (first mortgage + HELOC)
HELOC math example
Home value: $1,100,000. Existing mortgage: $500,000 at 3.25% (locked in 2021). HELOC math:
- Maximum combined LTV (80%): $880,000
- Available HELOC capacity: $880,000 − $500,000 = $380,000
- Sufficient for typical $250K-$350K San Diego ADU build
Why HELOC dominates: it preserves your existing low-rate first mortgage. Refinancing a $500K loan at 3.25% to combine cash-out into a single new loan would mean re-pricing the entire balance at 6.23% — a $10,750+ annual interest cost increase that wipes out years of ADU rental income. Full cash-out vs. HELOC comparison.
Path 2: Cash-out refinance
The right choice when your existing mortgage is already at current market rates (post-2022 originations). Mechanics:
- Replace existing mortgage with a larger loan; receive the difference in cash
- Single payment, fixed rate (typically)
- LTV capped at 80% on conventional cash-out (75% on jumbo)
- Interest is tax-deductible if proceeds are used for "buy, build, or substantially improve" — which an ADU qualifies as3
Cash-out makes sense when:
- Your existing rate is already 6%+ (no benefit to keeping it)
- You need a fixed payment rather than variable HELOC rate
- You want a single loan rather than two
- Construction is complete or near-complete (cash-out usually disburses lump sum at closing)
2026 San Diego County conforming loan limit is $1,104,100 — meaning cash-out refinances stay within conforming pricing for most properties.4
Path 3: Construction loan / renovation loan
The right path when you don't have enough equity to fund the build through HELOC or cash-out alone. Two common products:
Fannie Mae HomeStyle Renovation
- Loan based on the home's after-renovation value (ARV) — important for projects where the ADU adds substantial value
- Up to 97% LTV on primary residence, 85% on investment property
- Loan amount caps at 75% of ARV
- Funds disburse in draws as construction progresses
- Converts to standard mortgage after completion
FHA 203(k) Renovation Loan
- FHA's renovation product — lower down payment requirements (3.5%)
- Allows projected ADU rental income to be counted in qualifying — a unique feature
- FHA loan limits and mortgage insurance apply
- More restrictive on contractor selection and approval
Construction/renovation loans are critical when ARV calculations are needed. Example: home worth $900K today, $300K ADU build will produce ARV of $1.3M. A traditional HELOC bases borrowing on the current $900K value; a HomeStyle loan bases borrowing on the projected $1.3M ARV — significantly more capacity.
Path 4: SDHC ADU Finance Program
The San Diego Housing Commission offers a unique product specifically for City of San Diego homeowners (zip codes 921XX): up to $250,000 in construction financing, 1% interest during construction, then 4% fixed for 15 years on the permanent loan.5
Key terms:
- Income limit: homeowner income up to 150% of San Diego AMI (roughly $231,000 for a household of 4 in 2026)
- Affordability covenant: the completed ADU must rent to households earning 80% AMI or less for at least 7 years
- Geographic limit: City of San Diego only (not unincorporated county, not other cities)
- Property type: single-family residence with owner occupying either the main home or the ADU
The catch: 7 years of below-market rents. Market rent on a 1-bedroom San Diego ADU is roughly $2,500-$2,900/month. The 80% AMI restricted rent is closer to $1,800-$2,100/month — a $700-$800/month "subsidy" the homeowner is effectively giving the tenant in exchange for the program's below-market financing.
The math works for some homeowners (especially those with limited equity who couldn't otherwise afford the build), but the rental income trade-off is substantial over 7 years. A $700/month rent reduction × 84 months = $58,800 in foregone rental income.
Path 5: ARV loans (newer product)
Several California credit unions and ADU-focused lenders now offer products specifically structured for ADU builds. These tend to:
- Underwrite to after-renovation value rather than current value
- Offer terms up to 20 years
- Not require refinancing the existing first mortgage
- Have less complex draw schedules than traditional construction loans
- Sometimes count projected ADU rental income in qualifying
Pricing varies considerably. Rates often run 1-2% higher than HELOC rates but compete favorably with construction loans. For homeowners caught between HELOC capacity limits and the complexity of construction loans, ARV products fill a real gap.
The Prop 13 advantage: only the ADU is reassessed
One of the most important — and most underappreciated — features of ADU economics: only the new construction triggers reassessment. Your existing home retains its current factored base year value (FBYV), and only the ADU's added value gets reassessed at current market rates.6
Worked example
Home purchased in 2008 for $450,000. 2026 FBYV: roughly $642,711. Current market value: $1,100,000.
Without ADU: Annual property tax = $642,711 × 1.18% = $7,584
With $250,000 ADU added in 2026:
- Existing home FBYV: $642,711 (unchanged)
- ADU added value: $250,000 (new construction, assessed at full value)
- New total assessed value: $892,711
- New annual property tax: $10,534
- Tax increase from ADU: $2,950/year, or $246/month
Compare this to selling and buying a larger home. Sell at $1,100,000, buy a $1,350,000 home with an ADU:
- New home assessed value: $1,350,000
- Annual property tax: $15,930
- Tax increase vs. current: $8,346/year
The ADU strategy preserves $5,396/year in tax savings simply by avoiding full reassessment. More on Prop 13.
The rental income question
San Diego ADU rents in 2026 by size:1
| ADU size | Typical monthly rent | Annual gross rent |
|---|---|---|
| Studio | $1,900-$2,400 | $22,800-$28,800 |
| 1 bedroom | $2,300-$2,900 | $27,600-$34,800 |
| 2 bedroom | $2,800-$3,500 | $33,600-$42,000 |
The break-even calculation depends on financing structure:
$300,000 ADU financed via HELOC at 8%, interest-only during 10-year draw: monthly cost roughly $2,000. A 1-bedroom ADU renting at $2,600/month produces $600/month net cash flow plus tax benefits and equity build. Break-even on construction cost typically 7-12 years depending on financing structure and rent levels.
The qualification trap: rental income usually doesn't count
One of the most common ADU financing frustrations: most lenders will not count projected ADU rental income in qualification math.7 They underwrite based on your current income only — meaning a homeowner whose ADU will produce $30,000+ annual rent has to qualify for the loan as if the ADU didn't exist.
Three exceptions:
- FHA 203(k) renovation loans allow some rental income inclusion
- Some California credit unions and specialty ADU lenders count projected rents on a case-by-case basis
- Once the ADU is built and producing documented rental income (typically 12+ months of leases), refinancing into a product that counts the income becomes easier
For most San Diego homeowners, this means qualifying for the build based on current income, then capturing the rental income upside post-completion.
Permits, fees, and the AB 2533 path
Two important regulatory points:
- Impact fees waived under 750 sq ft: California law waives certain impact fees for ADUs under 750 square feet, materially reducing total project cost.
- School fees for larger units: ADUs over 500 sq ft may face school district fees; verify with your specific district.
- AB 2533 (legalizing existing unpermitted units): California's path to retroactively permit existing illegal ADUs, often unlocking tens of thousands in borrowing capacity once permitted.
Common mistakes
- Underestimating cost. San Diego ADU costs routinely run $375-$600/sq ft for stick-built; budgeting based on $200/sq ft national averages produces shock at lender appraisal time.
- Refinancing a low-rate first mortgage to fund the ADU. If your first mortgage is below 4%, almost always use HELOC, construction loan, or ARV product instead. Cash-out refinance pricing makes the ADU dramatically more expensive over the life of the loan.
- Not budgeting for utility hookups, site work, and permits. These can run $30,000-$80,000 above construction costs, particularly on detached units requiring new sewer laterals or panel upgrades.
- Skipping the SDHC program when eligible. The 1%/4% pricing is dramatically below market. The 7-year affordability covenant trade-off is worth modeling carefully.
Run an ADU financing scenario at current rates.
Open the calculator →The honest read
San Diego ADU financing has matured considerably from 5 years ago — HELOCs, construction loans, ARV products, and the SDHC program now offer multiple paths for homeowners with different equity and income situations. The Prop 13 advantage is genuinely substantial: building an ADU adds roughly 25-30% in property value while only triggering reassessment on the ADU portion. Combined with rental income that often covers debt service plus all carrying costs, ADUs are one of the most tax-efficient real estate investments available in California. The biggest pitfalls are underestimating costs and refinancing low-rate first mortgages unnecessarily. Talk to a San Diego ADU specialist before committing to any financing structure — the right product can save $50,000+ over the project lifetime.
ADU regulations and financing programs change frequently. Always verify current rules with the City of San Diego, San Diego County, and your lender. Educational content only — not legal, tax, or financial advice.
References
- Streamline Design Group. (2026, January). ADU financing in San Diego: What options work in 2026. Retrieved April 28, 2026, from https://streamlinedesigngroup.com/blog/guide-to-adu-financing-san-diego
- Better Place Design Build. (2026, January). Using a HELOC to fund an ADU in San Diego. Retrieved April 28, 2026, from https://betterplacedesignbuild.com/blog/adu-heloc-financing/
- Internal Revenue Service. (2024). Publication 936: Home mortgage interest deduction. Retrieved April 28, 2026, from https://www.irs.gov/publications/p936
- Federal Housing Finance Agency. (2025, November). Conforming loan limit values for 2026. Retrieved April 28, 2026, from https://www.fhfa.gov/data/conforming-loan-limit-cll-values
- San Diego Housing Commission. (n.d.). ADU finance program. Retrieved April 28, 2026, from https://adu.sdhc.org/
- California State Board of Equalization. (n.d.). How property is assessed for property taxation. Retrieved April 28, 2026, from https://boe.ca.gov/pdf/pub800-10.pdf
- Better Place Design Build. (2026, February). Best ADU loan programs in California: San Diego guide. Retrieved April 28, 2026, from https://betterplacedesignbuild.com/blog/adu-loan-programs/