ADU Cost Summary — 2026
ADU construction costs in the United States range from $42,000 for a basic garage conversion in a lower-cost market to $420,000+ for a large detached new build in California or New York. The wide range is not imprecision — it reflects real variation in local labor markets, material costs, permit fee schedules, foundation requirements, and regulatory environments across 50 different states.
The most important number for any homeowner is not the national average. It's the cost in your specific city, for your specific ADU type, on your specific lot. Use this guide to understand the framework, then drill into your state guide for market-specific data.
| ADU Type | National Low | National High | Typical Mid-Market |
|---|---|---|---|
| Detached ADU (New Construction) | $80,000 | $420,000+ | $150,000 – $250,000 |
| Attached ADU (Addition) | $78,000 | $315,000 | $130,000 – $220,000 |
| Garage Conversion | $42,000 | $185,000 | $75,000 – $130,000 |
| Basement Conversion | $50,000 | $200,000 | $80,000 – $145,000 |
| Prefab / Modular ADU | $65,000 | $280,000 | $110,000 – $195,000 |
| Junior ADU (JADU) | $25,000 | $90,000 | $40,000 – $70,000 |
* Ranges reflect completed projects including design, permits, and construction. Site-specific conditions (lot access, soil, utility distances) can move costs outside these ranges. See methodology.
These are planning estimates, not contractor bids. Use them to understand the likely budget before requesting quotes. Always obtain a minimum of three contractor bids before committing to any project budget. Contractors price based on your specific site, soil, utility access, and finish selections — conditions we cannot assess remotely.
Which ADU Type Fits Your Budget?
The ADU type is the single biggest variable in your project cost. A garage conversion can cost less than one-third of a comparable detached new build. The right choice depends on your lot, your existing structures, your budget, and your local zoning rules.
Cost Per Square Foot by ADU Type
| ADU Type | Low End ($/sq ft) | Mid-Market ($/sq ft) | High End ($/sq ft) |
|---|---|---|---|
| Detached (new construction) | $145 | $220 | $380+ |
| Attached (addition) | $135 | $200 | $340+ |
| Garage conversion | $90 | $145 | $245 |
| Basement conversion | $95 | $155 | $260 |
| Prefab / modular | $125 | $185 | $300 |
* Cost per sq ft decreases as size increases — fixed costs (design, permits, utility connections) are spread over more square footage. Conversions are cheaper per sq ft because existing structure is retained.
ADU Cost by U.S. Region
Labor costs alone vary 2–3× across U.S. markets. A detached ADU in coastal California or New York costs 2.5–3× more than the same structure in rural Tennessee or Ohio. Regional variation is the most underestimated factor in ADU budgeting.
For market-specific data, select your state from the complete state guide directory.
ADU Cost by Size
As ADU size increases, cost per square foot decreases — fixed costs like design fees, permit fees, and utility connections don't scale linearly with size. A 600 sq ft ADU costs significantly less per square foot than a 300 sq ft studio, even though the absolute price is higher.
| Size | Typical Type | Statewide Low | Mid-Market | High-Cost Market |
|---|---|---|---|---|
| Under 300 sq ft | Studio / JADU | $25,000 | $65,000 – $120,000 | $140,000 – $200,000 |
| 300–500 sq ft | Studio | $55,000 | $90,000 – $155,000 | $165,000 – $260,000 |
| 500–700 sq ft | 1-Bedroom | $72,000 | $120,000 – $195,000 | $200,000 – $320,000 |
| 700–900 sq ft | 1–2 Bedroom | $90,000 | $148,000 – $240,000 | $245,000 – $390,000 |
| 900–1,200 sq ft | 2 Bedroom | $108,000 | $175,000 – $285,000 | $290,000 – $420,000+ |
* Ranges span all ADU types. Detached new construction skews toward the high end. Conversions skew toward the low end of each size range.
Most municipalities cap ADUs at 800–1,200 sq ft. California allows up to 1,200 sq ft under AB 2221. Many Midwest and Northeast cities cap at 1,000 sq ft. Some jurisdictions tie the maximum to a percentage of the primary home's square footage. Always verify your local ADU size limits before finalizing design — it determines your maximum rental income potential.
The Six Variables That Drive Every ADU Cost
Understanding why ADU costs vary so dramatically is the most valuable thing you can do before talking to a contractor. These six factors explain why an identical ADU can cost $140,000 in Columbus and $380,000 in San Jose.
Permit Fees and Soft Costs to Budget
Permit and soft costs are frequently underestimated in early ADU budgets. In high-cost markets, these "non-construction" costs can add $30,000–$60,000 to a project. In smaller cities they may be under $5,000 total. Always budget for these before finalizing your project scope.
| Cost Item | Low Market | Mid Market | High Market (CA/NY) |
|---|---|---|---|
| Architecture / design fees | $3,500 | $8,000 – $18,000 | $18,000 – $40,000 |
| Building permit fees | $400 | $1,500 – $5,000 | $5,000 – $16,000 |
| Impact / development fees | $0 | $1,000 – $6,000 | $6,000 – $25,000 |
| Structural engineering | $1,200 | $2,000 – $5,000 | $5,000 – $12,000 |
| Geotechnical / soils report | $800 | $1,500 – $3,500 | $3,500 – $8,000 |
| Survey | $500 | $700 – $2,000 | $2,000 – $5,000 |
| Utility connections (all) | $3,000 | $6,000 – $18,000 | $18,000 – $45,000 |
For state-by-state permit requirements, fees, and review timelines, see the ADU Permits Guide.
ADU ROI and Rental Income
An ADU is not just a construction project — it is a real estate investment with a measurable return. The combination of monthly rental income and property value appreciation makes ADUs one of the most capital-efficient residential investments available to homeowners.
| U.S. Region | Avg Monthly Rent (1-bed) | Est. Payback Period | Property Value Impact |
|---|---|---|---|
| Pacific Coast (CA, WA, OR) | $1,600 – $3,200 | 8 – 14 years | +18% – +28% |
| Northeast (NY, MA, MD) | $1,400 – $2,800 | 9 – 15 years | +15% – +25% |
| Mountain West (CO, AZ, NV) | $1,100 – $2,200 | 9 – 14 years | +14% – +22% |
| Southeast (FL, GA, NC, VA) | $950 – $1,800 | 8 – 13 years | +12% – +20% |
| Midwest (IL, MI, OH, MN, WI) | $900 – $1,700 | 9 – 15 years | +10% – +18% |
| South Central (TX, TN) | $850 – $1,600 | 8 – 13 years | +10% – +18% |
Use the free ROI Calculator to model payback period, monthly rental income, and property value impact for your specific market, ADU type, and budget. → Open ROI Calculator
How These Estimates Are Built
Every cost range in this guide is built from the same regional modeling framework used across all 50 of our state cost guides. Understanding how estimates are produced helps you evaluate whether they're applicable to your specific situation.
What goes into each estimate
Regional base cost: A baseline cost per square foot is established from local construction labor data, regional material pricing, and published construction cost reports for each U.S. market.
ADU type multiplier: Each ADU type (conversion vs. new construction, attached vs. detached) carries a multiplier applied to the regional base cost. Garage and basement conversions are 0.5–0.65× the cost of equivalent detached new construction because existing structure is retained.
Permit and soft cost estimate: A market-specific permit cost estimate is added based on municipal permit fee schedules, published impact fee schedules, and typical soft cost ratios for each market.
±15% range output: A ±15% confidence range is applied to the midpoint estimate to produce the low and high ranges shown in this guide. This range reflects the variability in contractor pricing, site conditions, and specification levels that cannot be assessed remotely.
What these estimates do not include
Site-specific conditions that require on-site assessment: slope and grading costs beyond typical, unusual soil conditions, tree removal, exceptionally long utility runs, structural complications in conversion projects, and finish specification premiums above standard grade.
These are educational planning estimates, not contractor bids. Always obtain a minimum of three licensed contractor bids before committing to any project budget. This guide is designed to help you understand the market before those conversations happen — not to replace them.