Accessory dwelling units (ADUs) — granny flats, in-law suites, garage conversions, backyard cottages — are one of the fastest-growing segments in residential real estate. California alone permitted roughly 25,000 ADUs annually in recent years. Investors and homeowners are building them for rental income, multigenerational living, and property-value gains.
What is an ADU?
An ADU is a secondary dwelling on the same lot as a primary residence. Types include:
| ADU type | Description | Typical cost |
|---|---|---|
| Detached backyard cottage | Stand-alone structure behind main house | $150K–$350K+ |
| Garage conversion | Existing garage converted to living space | $80K–$180K |
| Basement / attic conversion | Interior conversion within primary structure | $60K–$150K |
| Attached addition | New square footage attached to primary home | $120K–$280K |
| Prefab / modular ADU | Factory-built unit delivered and installed | $100K–$250K |
The American Planning Association ADU guide tracks state and local policy trends driving adoption nationwide.
Why ADU demand is surging
Housing shortage. The U.S. underbuilt housing for a decade after 2008. ADUs add density without new land — a politically palatable way cities increase supply.
Rental income. A permitted ADU in a high-rent market can generate $1,200–$3,500+/month depending on size and location. That income supports DSCR financing at 5.75%–10.5% on the combined property once stabilized.
Multigenerational living. Aging parents, adult children, and caregivers occupy ADUs while maintaining privacy — a demographic trend the Pew Research Center documents in multigenerational household growth.
State preemption laws. California, Oregon, Washington, and other states passed laws limiting local ADU bans. California’s ADU statutes effectively require most cities to allow at least one ADU per single-family lot.
Property value increase. Appraisers often credit permitted ADU square footage at 50–80% of primary-home value per square foot — a meaningful ARV bump on refinance.
ADU economics for investors
Investors use ADUs in three strategies:
House hack. Owner-occupant lives in the primary unit; ADU rent offsets the mortgage. Conventional owner-occupied financing may apply to the primary — ADU construction often uses renovation or construction loans.
BRRRR on SFR + ADU. Acquire a property with ADU potential, permit and build the unit, stabilize combined rent, refinance on total ARV. Bridge the construction phase on hard money at 8.99%–13.5%; exit to DSCR once both units are leased.
STR / mid-term rental. Some markets allow ADU short-term rental at premium nightly rates. Check local STR ordinances before modeling income — see short-term rental laws for investors.
Worked example: Los Angeles detached ADU
| Line item | Amount |
|---|---|
| Existing SFR value (as-is) | $850,000 |
| ADU construction cost | $220,000 |
| All-in basis | $1,070,000 |
| Post-ADU appraised value | $1,280,000 |
| Primary rent | $3,200/mo |
| ADU rent | $2,400/mo |
| Combined rent | $5,600/mo |
| DSCR at 75% LTV, 7.25% | ~1.15 on combined PITIA |
City-specific guides: top 5 cities to build ADUs · ADU zoning basics Chicago · worried about ADU zoning in Miami
Financing ADU construction
Most ADUs require construction or renovation capital before they generate income:
| Phase | Typical product | Rate band |
|---|---|---|
| Acquisition + ADU build | Hard money / fix-and-flip | 8.99%–13.5% |
| Stabilized hold (both units rented) | DSCR | 5.75%–10.5% |
| Owner-occupied house hack | Conventional / FHA (ADU income may count) | Market rates |
Jaken Finance Group funds non-owner-occupied investment property ADU projects where ARV and exit support the file. See rehab loans for investment property and fix-and-flip loan requirements.
Zoning and permit checklist
Before breaking ground:
- Confirm ADU allowance on your zoning district — city planning department or online GIS
- Review setback, height, and size limits — many cities cap ADUs at 800–1,200 sq ft
- Obtain building permits — unpermitted ADUs fail appraisal and DSCR refi
- Check HOA restrictions — CC&Rs may ban ADUs even where city allows them
- Plan utility connections — separate meter vs. sub-meter affects lease structure
State rules that changed the ADU math
California’s housing department updated its ADU handbook in March 2026. On March 25, 2024, Senate Bill 477 renumbered the Government Code sections that cover state ADU and junior ADU law. Local ADU ordinances are optional. The department reviews the ones cities choose to adopt. A local write-up is not a building permit. An unpermitted unit still fails a refinance appraisal. Source: California HCD, Accessory Dwelling Units.
Washington limits how far a city or county can restrict a second unit. RCW 36.70A.681 says the owner cannot be forced to live on the lot. Impact fees on the ADU cannot exceed 50 percent of the fees on the main house. In urban growth areas that allow single-family homes, the jurisdiction must allow at least two ADUs. The allowed pairs are one attached and one detached, two attached, or two detached. A city or county may not set a maximum floor area below 1,000 square feet. Roof limits may not sit under 24 feet unless the main house is itself capped lower. Within a half-mile walk of a major transit stop, off-street parking may not be required. Smaller lots are capped at one space. Larger lots are capped at two. A safety study or a busy-airport radius can reopen those parking limits. Read the section before you model a Seattle or Spokane lot.
Those two states are not a national permit. Miami, Chicago, and a rural county can still shrink or ban the unit. Confirm the zoning district before you price the lot. See Miami ADU zoning and Chicago ADU basics.
What construction prices did from August 2025 to August 2026
The producer price index for construction materials was 375.908 in August 2026. It was 341.458 in August 2025. That is 10.1 percent higher. The series is not seasonally adjusted. Source: FRED WPUSI012011.
A bid from last summer is stale. Ask the contractor to reprice materials before you lock draws. Keep the bid’s own contingency. Do not treat the index as a line-item quote for lumber or concrete.
House prices rose more slowly than that materials index. The FHFA purchase-only index was 443.52 in July 2026, up 2.6 percent from 432.40 in July 2025. January 1991 equals 100. The series is seasonally adjusted. Source: FRED HPIPONM226S.
When materials outrun resale prices, the after-repair value cap binds sooner. On a qualified investment file, Jaken Finance Group can fund up to 100 percent of cost. The loan still cannot exceed 75 percent of after-repair value. Funding stops at the lower number.
The average 30-year fixed mortgage was 7.28 percent in the week of October 1, 2026. It was 7.03 percent in the week of September 24. Source: FRED MORTGAGE30US. That survey is an owner-occupied benchmark. It is not the construction rate. Investment hard money on an ADU is 8.99%–13.5% interest-only, for 6–12 months.
Illustration: a garage conversion where the value cap binds
This is an example. It is not a quote and not a closed loan.
| Line | Amount |
|---|---|
| Existing rental, as-is value | $610,000 |
| Garage conversion budget | $140,000 |
| All-in cost if the purchase is financed too | $750,000 |
| Example value after the ADU is legal | $920,000 |
| 75 percent of that value | $690,000 |
| Example loan (the lower figure) | $690,000 |
| Cash still needed to cover cost | $60,000 |
Interest-only at 11 percent, inside the 8.99%–13.5% band, is $6,325 a month on $690,000. Eight months of full-balance carry is $50,600. Staged draws cost less because the balance starts smaller. A complete hard-money file often closes in 7–10 business days. A ground-up backyard cottage, rather than a conversion of an existing garage, is usually 10–14 business days.
Primary rent in the example is $2,800. ADU rent is $1,650. Combined gross is $4,450. Taxes, insurance, and the new payment still have to clear the ratio. This example stops before that full DSCR. After both units are leased, the exit rate band is 5.75%–10.5%. Purchase leverage can reach 85 percent. Cash-out can reach 80 percent. A rate-and-term refinance can reach 85 percent. Those caps are for select markets and qualified borrowers. Plan on about 14 business days for the refinance. Do not reuse the construction close for the permanent loan.
Permits, crews, and the file that keeps draws moving
The Census Bureau counted 118,333 new private housing units authorized in August 2026. August 2025 was 113,896. The counts are not seasonally adjusted, and they cover every state and every building type. They are not an ADU tally. Source: Building Permits Survey, state monthly.
Busy crews are the practical point. Book inspection windows when you write the scope. A national permit total does not tell you whether your lot allows a second kitchen.
Bring these items before the first draw:
- A zoning letter, or the code section that covers that parcel.
- Stamped plans, not a sketch.
- A contractor bid with materials priced in the current quarter.
- A utility plan that says who pays the meter.
- An insurance quote for the finished two-unit use.
- Three comps that include a legal second unit.
- An exit sheet with either a sale price or a DSCR payment, plus an interest reserve.
Jaken Finance Group finances non-owner-occupied investment property. A house hack you live in is a different product. Call (833) 264-7776 or pre-qualify with the bid and the zoning note. If the garage slab will not carry the new unit, compare the scope with a ground-up loan versus a flip loan. The leased exit is covered in construction to DSCR.
Is an ADU right for your property?
ADUs work best where rents justify construction cost, zoning permits the unit, and your exit (hold or sell) captures the ARV premium. Markets with restrictive STR rules may still support long-term ADU leases at strong DSCR.
Questions about financing an ADU project? Start with ADU construction loans · Pre-qualify · submit a flip scenario · (833) 264-7776
ADU popularity drivers — financing each strategy
| Strategy | Product | Rate band |
|---|---|---|
| Garage conversion ADU | Hard money / rehab | 8.99%–13.5% IO |
| Detached new build ADU | Ground-up or rehab | 8.99%–13.5% IO |
| ADU + hold primary | HELOC / personal | Not investor product |
| ADU on rental — add unit | BRRRR then DSCR | 5.75%–10.5% permanent |
Zoning is local — ADU construction loans · Chicago ADU basics · Miami ADU zoning · Ohio ADU guide · rehab loans.
Rates, terms and conditions offered only to qualified borrowers and are subject to change at any time without notice. Closing times are in business days and commence upon satisfaction of borrower conditions. All loans are subject to full underwriting for loan approvals. Jaken Finance Group only finances non-owner occupied investment properties.
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