This calculator provides estimates for educational purposes only. It is not affiliated with any bank, lender, or financial institution. Results are not a loan offer or guarantee of terms. Consult a licensed mortgage professional for advice specific to your situation.
HELOC for ADU Construction
Accessory dwelling units (ADUs) are increasingly funded with HELOCs because the draw-period flexibility aligns with the contractor-invoice schedule across a 9 to 18 month construction timeline. Detached ADUs cost $150,000 to $500,000 in 2026 depending on size and market. The rental-income math often makes the financed build self-amortizing over 10 to 15 years.
ADU cost ranges and sizing
ADU construction costs vary dramatically by size, finish level, and local market. The clearest reference for 2026 California costs (where ADU activity is heaviest) is the California HCD ADU Handbook, which reports typical all-in detached ADU costs of $300 to $500 per square foot for new construction in California markets. Outside California, the per-square-foot costs typically run $200 to $400. A 600 sqft detached ADU at the California midpoint of $400 per sqft costs $240,000; outside California at $300 per sqft the same ADU costs $180,000.
The cost components break down approximately as: foundation and site work 15% to 20%, structural shell (framing, sheathing, roof) 25%, mechanical (plumbing, electrical, HVAC) 20%, interior finishes 20% (more for upgraded finishes), permits and design 5% to 10%, contractor overhead and profit 15%. The site-work component varies most by site conditions: a flat lot with easy utility access runs at the low end of the range; a sloped lot requiring retaining walls and a long utility trench runs at the high end or beyond. Pre-construction soil testing and utility-access verification is essential to avoid mid-project budget surprises.
Permit and design costs deserve specific attention. California's 2023 ADU streamlining legislation (AB 2221 and AB 671) reduced permit timelines and lowered impact fees in many jurisdictions, but local implementation varies. Some California cities now permit ADUs in 60 to 90 days with $5,000 to $15,000 in permit fees; other cities still take 6 to 12 months with $20,000 to $50,000 in fees. Outside California, the patchwork of local ADU rules makes permit timing even more variable. Build the permit timeline and cost into your HELOC sizing and project schedule.
HELOC sizing and draw alignment
ADU construction is one of the cleanest HELOC use cases because the contractor-invoice schedule naturally aligns with the draw-period flexibility. The standard ADU contractor draw schedule pays roughly 5% to 10% at contract signing, 15% to 20% at foundation completion, 20% to 25% at framing completion, 15% to 20% at mechanical rough-in, 15% to 20% at drywall and interior finishes, and 10% at substantial completion. The borrower draws the HELOC at the same pace as the contractor invoices, which keeps the HELOC balance growing in step with actual spend rather than carrying interest on undeployed funds.
For a $300,000 ADU build with a 12-month construction timeline, the HELOC carrying-cost math looks like: month 1 draw $30,000, month 3 draw $50,000, month 5 draw $60,000, month 7 draw $50,000, month 9 draw $50,000, month 12 draw $30,000 plus retainage release. Average balance across the 12 months is roughly $150,000 to $175,000, generating $11,000 to $13,000 in draw-period interest at 7.02%. By comparison, drawing the full $300,000 on day one would generate roughly $21,000 in interest across the same 12 months, an extra $8,000 to $10,000 in cost for no benefit.
HELOC sizing should include a meaningful contingency. ADU construction surprises are common: unexpected utility relocation costs, code-required upgrades to the existing home's electrical service to support the ADU, soil conditions requiring engineered foundations, and permit-driven design changes. A 20% to 30% contingency on the base contractor bid is realistic. For a $300,000 base contract, sizing the HELOC at $375,000 to $400,000 gives headroom without over-borrowing. Any unused capacity at project completion can remain available for future use or simply remain undrawn (carrying no interest).
ADU rental income payback math
| ADU size | Build cost | Monthly rent (mid-market) | Gross annual rent | Net annual (after 25% costs) | Years to recoup build |
|---|---|---|---|---|---|
| 400 sqft | $200,000 | $1,600 | $19,200 | $14,400 | ~13.9 years |
| 600 sqft | $280,000 | $2,200 | $26,400 | $19,800 | ~14.1 years |
| 800 sqft | $350,000 | $2,800 | $33,600 | $25,200 | ~13.9 years |
| 1,000 sqft (CA) | $450,000 | $3,500 | $42,000 | $31,500 | ~14.3 years |
Build cost figures are mid-market national averages. California costs typically run 30% to 50% higher. Rent estimates assume long-term rental in a moderate-rent market; vacation-rental yields can be substantially higher in tourist markets but with higher operating cost and more variable income. Recoup years exclude any home-value appreciation from the ADU addition, which is typically positive.
Tax treatment of ADU HELOC interest
ADU construction on the same parcel as the primary residence qualifies as substantial improvement of the home that secures the HELOC, making the interest deductible under IRS Publication 936 (subject to itemization and the $750,000 combined-debt cap). The deduction applies regardless of whether the ADU is occupied by the borrower's family, used as long-term rental, or used as short-term vacation rental, as long as the construction qualifies as substantial improvement to the property.
If the ADU is rented, a parallel set of tax mechanics applies on the rental side. Rental income is reported on Schedule E. Operating expenses (insurance, repairs, maintenance, property management, utilities if paid by landlord) are deductible against rental income. The ADU's allocated portion of property tax and depreciation is also deductible. The depreciation is calculated by allocating the building basis between the ADU and the existing primary residence based on square footage or appraisal, then depreciating the ADU's allocated share over 27.5 years for residential rental property. This depreciation deduction often makes the rental income tax-favoured in the early years of operation, partially offsetting the HELOC interest expense even before considering the home-improvement deduction.
For a borrower in the 24% federal tax bracket with a $300,000 HELOC at 7.02% used entirely for ADU construction, the annual interest at full draw is $21,060. If fully deductible (itemizing, $750k cap satisfied), the after-tax interest cost is $16,006. The 24% deduction effectively reduces the rate from 7.02% to 5.34%. Combined with the rental-income-side tax benefits (depreciation, operating expenses), the all-in tax-adjusted cost of borrowing for an ADU can be meaningfully lower than the pre-tax interest rate suggests. Consult a CPA to model your specific situation; the interaction of personal and rental-side tax rules can be complex.
Frequently asked questions
What does an ADU cost to build in 2026?
A 400-600 sqft detached ADU runs $150,000 to $250,000 in most US markets. A 600-800 sqft ADU runs $200,000 to $350,000. A 800-1,200 sqft ADU runs $300,000 to $500,000. Costs are 30% to 60% higher in California, Hawaii, and high-cost metros. The figures include site preparation, foundation, utility connections, permits, structure, and standard interior finishes.
Is HELOC the right product for ADU construction?
Often yes, because the draw-period flexibility aligns with contractor invoices over a 9 to 18 month construction timeline. Alternatives include a construction-to-permanent loan (stricter inspections, fixed rate, sometimes lower rate than HELOC), a renovation loan (FHA 203k or Fannie HomeStyle, complex paperwork), or cash. The HELOC wins on flexibility and speed for borrowers with enough equity to cover the build; the construction loan wins for borrowers who need higher LTV than HELOCs allow.
What HELOC size do I need for an ADU?
Take the contractor's estimated all-in cost (including site prep, utilities, permits), add a 20% to 30% contingency, then subtract any cash you intend to contribute. For a $300,000 ADU build with $50,000 cash, the HELOC sizing is roughly ($300,000 plus $75,000 contingency minus $50,000 cash) equals $325,000 HELOC. Round up to the nearest standard product size at your chosen lender.
Is ADU construction interest tax-deductible via HELOC?
Yes if the ADU is constructed on the same parcel as the primary residence, and the borrower itemizes deductions, and the combined first-mortgage plus HELOC balance stays under $750,000. The IRS treats ADU construction on the primary parcel as substantial improvement of the home that secures the HELOC. Documentation through contractor invoices is essential to support the deduction.
Will an ADU pay back its cost?
Depends on whether you rent it. A long-term rental of a 600-800 sqft ADU in a moderate-rent market ($1,800 to $3,000 per month) produces $20,000 to $35,000 of annual gross rent, which against a $300,000 build cost gives a 7% to 12% gross yield. After expenses (insurance, maintenance, vacancy, management) the net yield typically runs 4% to 8%. Over 10 to 15 years, a rented ADU often more than recoups its build cost.
What is the construction-draw schedule for an ADU?
Standard ADU contractor draws typically follow: 5% to 10% at contract signing, 15% to 20% at foundation completion, 20% to 25% at framing completion, 15% to 20% at mechanical rough-in (plumbing, electrical, HVAC), 15% to 20% at drywall and interior finishes, and 10% at substantial completion. Drawing the HELOC at the same pace aligns interest costs with actual spend.