Estimate the true cost of an accessory dwelling unit — detached, attached, garage conversion, above-garage, prefab or JADU. Get a line-item breakdown of foundation, framing, MEP, finishes, permits and site work, compare state-by-state costs, calculate rental ROI and payback period, and see the property value premium an ADU adds at resale. Includes 2025–2026 cost data, financing options from HELOC to FHA 203(k), and the California 750 sq ft impact fee waiver explained.
- 6 ADU types
- State cost multipliers
- Line-item breakdown
- ROI & payback
- Financing options
- Permit fee guide
- Property value lift
- Downloadable tables
Detailed Cost mode. Enter your ADU type, size, state, site difficulty and finish level. The calculator builds a line-item estimate covering foundation, framing, MEP, finishes, site work, permits and contingency.
Most expensive, most flexibility
Typical detached ADU: 500–800 sq ft
Drives the regional cost multiplier
Affects grading, trenching, foundation
Materials, fixtures, appliances
Used for ROI and payback calculation
Results are at rest
Choose your ADU type, size, state and options, then press Calculate ADU Cost.
Inputs changed. Press Calculate ADU Cost to refresh.
Cost breakdown
ROI and payback analysis
Real-world context
How to Use the ADU Cost Calculator
This calculator has three modes covering the three questions every ADU project asks. Pick the mode that matches what you need, fill the fields, and the tool returns a line-item estimate, a quick range, or a full ROI analysis.
Detailed Cost mode
Enter ADU type, size, state, site difficulty and finish level. The calculator builds a line-item estimate covering site work, foundation, framing, MEP rough-in and trim, insulation, drywall, interior finishes, permits and contingency. This is the most accurate mode.
Quick Estimate mode
Same inputs but returns a simple per-square-foot range with a midpoint. Use this for a fast sanity check before you talk to builders or request bids.
ROI & Payback mode
Add your expected monthly rent. The calculator returns gross yield, net yield after vacancy and operating costs, cash-on-cash return, payback period, and the property value lift at resale based on published ADU premium data.
Set precision
The Decimals selector controls whether the result rounds to whole dollars or shows cents. Use 0 for planning and 2 for a formal budget.
Press Calculate ADU Cost
The result panel stays at rest until you press the button. After calculation you get the primary total, the detail rows, the full cost breakdown, the ROI analysis (if you entered rent) and the real-world context — how your cost per square foot compares to the national range for that ADU type.
Copy and compare
The copy button grabs the full result set. Run the calculator twice with different ADU types or finish levels to compare, then use the downloadable tables below to verify the numbers against published cost data.
Need to convert measurements for your ADU plans? BuildersMeasures has a Length Converter and a Square Footage Calculator. For loan payment math, use the Interest Rate Calculator.
Where the Money Goes — Visual Breakdown
Every ADU project divides into the same cost categories. The percentages shift with type, size, site conditions and finish level, but the structure is universal. This diagram shows the typical allocation for a mid-market detached ADU.
Diagram 1 — ADU cost allocation (typical detached, mid-market)
The two line items people underestimate. Site work and contingency. Site work can double if your lot is sloped or the garage is detached and needs a trenched utility run. Contingency covers the surprises — an undersized electrical panel, a sewer line that needs replacing, or a plan revision the city requires. Budget 15–20% and do not spend it on finishes.
ADU Cost Tables by Type and Size
These tables give the national cost ranges by ADU type, size and finish level. Use them to sanity-check the calculator's output or to compare two ADU strategies before committing to a design.
Cost by ADU type — national ranges (2025–2026)
| ADU type | Typical size | Cost range | Cost per sq ft | Notes |
|---|---|---|---|---|
| Detached new build | 500–800 sq ft | $220K–$400K | $250–$500+ | Highest flexibility, highest site cost |
| Attached (in-law suite) | 400–800 sq ft | $190K–$330K | $200–$450 | Shares a wall, moderates exterior costs |
| Garage conversion | 350–600 sq ft | $150K–$260K | $125–$200 | Structure exists, utility upgrades common |
| Above-garage ADU | 400–700 sq ft | $180K–$320K | $220–$450 | Structural reinforcement needed |
| Prefab / modular | 400–800 sq ft | $180K–$320K | $250–$400 | Speed and price certainty, crane access needed |
| Junior ADU (JADU) | 250–500 sq ft | $40K–$150K | $100–$200 | Inside existing walls, no new foundation |
| Tiny home on foundation | 200–400 sq ft | $30K–$100K | $150–$300 | May not meet ADU code in all states |
Cost by size — detached and conversion comparison
| Size | Detached total | Garage conversion total | Attached total | JADU total |
|---|---|---|---|---|
| 300 sq ft | $120K–$200K | $60K–$95K | $90K–$150K | $40K–$75K |
| 400 sq ft | $150K–$240K | $75K–$115K | $110K–$180K | $50K–$90K |
| 500 sq ft | $180K–$290K | $90K–$140K | $130K–$220K | $60K–$110K |
| 600 sq ft | $210K–$340K | $105K–$165K | $150K–$260K | $70K–$130K |
| 700 sq ft | $240K–$390K | $120K–$190K | $170K–$300K | $80K–$150K |
| 800 sq ft | $270K–$440K | $135K–$215K | $190K–$340K | — |
| 1,000 sq ft | $330K–$550K | $160K–$260K | $230K–$420K | — |
| 1,200 sq ft (CA max) | $390K–$650K | $185K–$300K | $270K–$500K | — |
Finish level multiplier
| Finish level | Multiplier | 600 sq ft cost | What it includes |
|---|---|---|---|
| Basic (rental grade) | 0.80× | $140K–$220K | Vinyl flooring, laminate counters, standard appliances, builder-grade fixtures |
| Standard (mid-market) | 1.00× | $175K–$275K | Engineered hardwood, quartz counters, stainless appliances, mid-range fixtures |
| Premium (custom) | 1.30× | $228K–$358K | Hardwood, natural stone, panel-ready appliances, designer fixtures, custom cabinetry |
How to read the tables. Ranges assume standard site conditions and national average labor. California, New York, Massachusetts and Washington sit at the top of each range; Texas, Georgia and the Midwest sit at the bottom. The calculator applies a state multiplier to the midpoint. Use the tables for a mental check — if your calculator result is far outside the range for your type and size, something in your inputs needs a second look.
ADU Costs by State — Zoning, Timelines and Multipliers
Where you build matters as much as what you build. By-right states have streamlined permitting and, in several cases, removed owner-occupancy requirements entirely. High-cost states push the per-square-foot number up even when the rules are friendly. The table below combines both dimensions.
| State | By-right? | Max size | Owner-occ. required? | Permit days | $/sq ft range | Cost multiplier |
|---|---|---|---|---|---|---|
| California | Yes | 1,200 sq ft | No | 60 | $200–$450 | 1.35× |
| New York | No (local) | Local | Local | 180 | $250–$500 | 1.40× |
| Massachusetts | Yes | 900 sq ft | No | 60 | $220–$480 | 1.30× |
| Washington | Yes | 1,000 sq ft | No | 120 | $180–$400 | 1.20× |
| Oregon | Yes | 800 sq ft | No | 90 | $175–$380 | 1.15× |
| Colorado | Yes | 1,000 sq ft | Local | 75 | $180–$380 | 1.10× |
| Montana | Yes | 750 sq ft | No | 60 | $160–$320 | 0.95× |
| Arizona | No (local) | Local | Local | 150 | $160–$320 | 0.95× |
| Texas | No (local) | Local | Local | 120 | $140–$280 | 0.85× |
| Florida | No (local) | Local | Local | 90 | $150–$300 | 0.90× |
| Illinois | No (local) | Local | Local | 180 | $180–$360 | 1.05× |
| Georgia | No (local) | Local | Local | 150 | $140–$280 | 0.85× |
| North Carolina | No (local) | Local | Local | 150 | $150–$300 | 0.90× |
| Virginia | No (local) | Local | Local | 180 | $170–$350 | 1.00× |
| Pennsylvania | No (local) | Local | Local | 180 | $170–$350 | 1.00× |
The California advantage — and cost. California has the most permissive ADU law in the country (AB 2221 forces 60-day approval, SB 9 allows lot splits, owner-occupancy is no longer required for standard ADUs) but the highest construction costs. A 600 sq ft detached ADU that costs $210K in Texas runs $280K–$320K in California before you add impact fees. The 750 sq ft impact fee waiver saves $8K–$15K on a qualifying unit. States with no statewide law leave it to each city — check your local zoning before you design anything.
How to Finance an ADU — Every Option Explained
ADU construction is a capital project, and most homeowners do not have $200K sitting in cash. The right financing option depends on how much equity you have, whether you will live on the property, and whether you need the ADU's future rental income to qualify.
| Option | Amount available | Rate type | Best for | Key constraint |
|---|---|---|---|---|
| HELOC | Up to 85% of home equity | Variable, typically prime + 0–2% | Homeowners with strong equity who want flexibility | Rate can rise; lender may freeze the line |
| Cash-out refinance | Up to 80% of home value | Fixed, near conventional mortgage rates | Replacing an existing mortgage while extracting equity | Resets your entire mortgage rate |
| FHA 203(k) | Based on after-completion value | Fixed, FHA rates | Limited equity, will live in the primary home | 3.5% down minimum; FHA loan limits apply |
| Fannie Mae HomeStyle | Based on after-completion value | Fixed, conventional rates | Owner-occupants who want to borrow against the ADU's future value | 5% down minimum; owner-occupancy required |
| Freddie Mac CHOICERenovation | Based on after-completion value | Fixed, conventional rates | Similar to HomeStyle; compare rates | Lender availability varies |
| Construction-to-permanent | Up to 80% of completed value | Variable during build, fixed after | Major builds where future ADU value exceeds current equity | Interest paid during construction; stricter qualifications |
| ADU-specific loan | Factors in future rental income | Varies | Investors who want the ADU's rent counted in underwriting | Fewer lenders; higher rates |
| CalHFA ADU grant | $40K grant + $25K loan | Below-market | Income-eligible California homeowners | Income limits; must complete program requirements |
The Fannie Mae change that matters. Fannie Mae now purchases loans on two-to-three unit properties that include ADUs — up to four units per property. It also allows ADU rental income to count toward loan qualification, capped at 30% of total qualifying income and using 75% of the projected rent. If you add three ADUs renting at $1,200 each, that is $43,200 annually hitting your bank account even though only one ADU's income counts toward the loan. That is the cash-flow play that has investors paying attention.
ADU Rental ROI and Payback Period
An ADU is both a home and an investment. The rental income it generates can cover the financing, and the property value lift at resale adds equity. The question is whether the numbers work in your market.
| ADU type | Typical cost | Monthly rent | Gross yield | Net yield | Payback |
|---|---|---|---|---|---|
| Garage conversion | $90K–$140K | $1,800–$3,200 | 15–22% | 10–16% | 5–8 yrs |
| Attached ADU | $130K–$220K | $1,500–$2,800 | 8–14% | 6–10% | 8–12 yrs |
| Detached ADU | $210K–$340K | $2,000–$3,500 | 7–12% | 5–9% | 10–15 yrs |
| Prefab / modular | $180K–$320K | $1,800–$3,000 | 8–13% | 6–10% | 9–14 yrs |
| Above-garage ADU | $180K–$320K | $1,600–$2,800 | 8–12% | 6–9% | 9–14 yrs |
| JADU / basement | $40K–$150K | $1,200–$2,200 | 12–20% | 8–15% | 5–9 yrs |
The payback trap. Payback period on cash flow alone often exceeds 15 years for a detached ADU in a moderate rent market. The investment case works because of two things the payback number ignores: the property value lift at resale (20–35% in strong ADU markets) and rent appreciation over time. A 15-year payback on year-one rent might be a 10-year payback by year five if rents grow 4% annually. Run both numbers before you decide.
How Much Does an ADU Increase Property Value?
An ADU adds value two ways: the appraised value of the structure itself, and the market premium buyers pay for a property that can generate rental income. The second effect is larger than most homeowners expect.
| Study / source | Value increase | Notes |
|---|---|---|
| FHFA (California, 2023) | Median appraised value $1,064,000 with ADU vs $715,000 without | 49% higher median, though the comparison is not matched for location or home quality |
| National Association of REALTORS | About 35% higher sale price on average | ADU-equipped homes sell for a measurable premium |
| Zillow data | Up to 30% more | Homes with ADUs sell faster and at higher prices |
| Remodeling Cost vs. Value Report | 20–35% value increase | Range depends on market, ADU size and finish level |
| Porch | 35% higher than comparable properties | Consistent with NAR estimate |
What this means for your project. On a $500,000 home, a 20–35% value lift is $100,000–$175,000. A $250,000 detached ADU that adds $150,000 in value plus $2,000/month in rent is a different financial decision than a $90,000 garage conversion that adds $80,000 in value and $1,800/month in rent. The conversion almost always has the better ROI on paper. The detached unit almost always has the better rental income and flexibility. Run both.
Detached ADU: $250–$500+ per sq ft. Attached: $200–$450. Garage conversion: $125–$200. JADU: $100–$200. Site work and contingency are the two line items people underestimate. Budget 15–20% contingency and do not spend it on finishes. Garage conversions have the highest ROI because the structure exists. Detached ADUs have the highest rental income and flexibility. California is the most permissive state but also the most expensive. A 600 sq ft detached ADU costs $210K–$340K nationally and $280K–$450K in California. The 750 sq ft impact fee waiver in California saves $8K–$15K. Fannie Mae now counts ADU rental income toward loan qualification. Properties with ADUs sell for 20–35% more than comparable homes without them.
People Also Ask
How much does an ADU cost to build?
Most ADUs land between $180,000 and $400,000 depending on type and site work. A garage conversion can run $150K–$260K, an attached ADU $190K–$330K, and a detached new build $220K–$400K. Junior ADUs (inside the existing walls) run $40K–$150K. Use the calculator above for a line-item estimate specific to your state and site.
What is the cheapest type of ADU to build?
The garage conversion is the cheapest because the structure, foundation and roof already exist. A garage conversion costs $125–$200 per square foot versus $250–$500+ for a detached new build. A Junior ADU (JADU) inside the existing home is even cheaper — $40K–$150K total — but is capped at 500 square feet and must share some facilities with the main house in most jurisdictions.
How much does a 600 square foot ADU cost?
A 600 sq ft detached ADU costs $210,000–$340,000 nationally. In California that rises to $280,000–$450,000. A 600 sq ft garage conversion runs $105,000–$165,000. The calculator above adjusts for your state, finish level and site difficulty.
Does an ADU increase property value?
Yes, significantly. Published data shows ADU-equipped properties sell for 20–35% more than comparable homes without them. FHFA data from California found a median appraised value of $1,064,000 for ADU properties versus $715,000 for comparable properties without them. The premium comes from both the appraised value of the structure and the rental income potential that buyers value.
Can I finance an ADU with no money down?
Not quite. The lowest-down options are FHA 203(k) at 3.5% down and Fannie Mae HomeStyle at 5% down. Both require owner-occupancy. A HELOC or cash-out refinance lets you borrow against existing equity — if you have enough of it, the effective cash outlay can be low, but you are still borrowing against your home. Some California programs offer grants up to $40,000 for income-eligible homeowners.
What is the ROI on an ADU rental?
Gross yields range from 7% to 22% depending on ADU type. Garage conversions have the highest ROI (15–22% gross) because the build cost is lowest. Detached ADUs have the lowest ROI on a percentage basis (7–12%) but the highest absolute rental income. Net yields after vacancy and operating costs run 5–16%. Payback periods are typically 5–15 years on cash flow alone.
What is the 750 sq ft impact fee waiver in California?
Under California law, ADUs with 750 square feet of interior livable space or less are exempt from all local impact fees — park fees, traffic fees, school fees and similar charges. This saves $8,000–$15,000 on a typical project. ADUs 750 sq ft or larger can be charged impact fees, but only proportionally based on the square footage above 750. The waiver applies statewide and is one of the reasons a 749 sq ft ADU is a common design target.
Do I need to live on the property to build an ADU?
It depends on the state. California, Oregon, Washington, Massachusetts and Montana no longer require owner-occupancy for standard ADUs — you can build and rent out both the main house and the ADU. New York, Texas, Florida and most other states leave it to local ordinance. Junior ADUs (JADUs) still require owner-occupancy in California. Check your city's rules before you design.
How long does it take to build an ADU?
Permitting takes 60 days in California (mandated by AB 2221) and 90–180 days in local-control states. Construction takes 6–12 months for a detached new build and 3–6 months for a garage conversion. Prefab and modular units shorten the on-site construction phase but still require foundation work and utility connections. Total timeline from design to move-in is typically 12–18 months.
Is this calculator free and does it store my data?
Yes and no. Every calculation runs entirely in your browser. Nothing is sent to a server, nothing is stored. Your inputs are not saved anywhere. The only thing remembered is the rating counter, which uses your browser's local storage and never leaves your device.
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References
- ADU Planet, "ADU Costs in 2025: What to Expect & How to Budget." aduplanet.com/costs
- Calk-USA, "ADU Cost Calculator." calk-usa.com/calculator/adu-cost
- Neighbor, "How Much Does It Cost to Build an ADU? (Types of ADUs Ranked from Cheapest to Most Expensive)." neighbor.com
- EffectiveAgents, "Can I Build an ADU on My Property? Costs, Laws & ROI." effectiveagents.com
- HammerIO, "ADU Cost + Zoning by State 2026." hammerio.com
- The ADU Pro, "ADU Permits Orange County: 2025 Guide." theadupro.com
- American Apartment Owners Association, "Fannie Mae Just Made ADUs a Cash Flow Machine." american-apartment-owners-association.org
- California Government Code §66311.5(c)(1) and §65852.2(f)(3) — impact fee exemptions for ADUs 750 sq ft or less.
- California Assembly Bill 2221 (2024) — 60-day approval requirement for ADU permits.
- California Senate Bill 9 (2021) — lot split provisions enabling ADU development.
- Federal Housing Finance Agency (FHFA), property valuation data for ADU-equipped properties in California, 2023.
- National Association of REALTORS, ADU value premium data.
- Remodeling Magazine, Cost vs. Value Report 2025, Accessory Dwelling Unit.
- Zillow, ADU property value analysis.
Confirm ADU costs against local builder bids and your jurisdiction's current fee schedule before committing to a design or applying for permits. Cost ranges in this calculator are planning estimates derived from published 2025–2026 industry data and regional cost multipliers. Actual costs depend on your site, your design, your contractor and your local market. Verify zoning, setback and owner-occupancy rules with your city planning department.