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Cost Estimates by ADU Type
| ADU Type | Low Estimate | Mid Estimate | High Estimate | Per Sq Ft |
|---|---|---|---|---|
| Detached ADU (800 sq ft) | $150,000 | $225,000 | $350,000 | $190-$440 |
| Attached ADU (600 sq ft) | $100,000 | $165,000 | $250,000 | $165-$415 |
| Garage Conversion (400 sq ft) | $60,000 | $110,000 | $180,000 | $150-$450 |
| Basement ADU (500 sq ft) | $50,000 | $90,000 | $150,000 | $100-$300 |
| JADU (400 sq ft) | $30,000 | $50,000 | $80,000 | $75-$200 |
| Prefab ADU (500 sq ft) | $80,000 | $150,000 | $250,000 | $160-$500 |
Cost Breakdown
Here is where your ADU budget typically goes:
| Category | % of Total | Typical Range |
|---|---|---|
| Construction / Labor | 50-60% | $75,000-$210,000 |
| Materials | 20-30% | $30,000-$105,000 |
| Design & Architecture | 8-12% | $8,000-$35,000 |
| Permits & Fees | 5-10% | $5,000-$30,000 |
| Utility Connections | 3-8% | $5,000-$25,000 |
| Contingency | 10-15% | $15,000-$50,000 |
Hidden Costs to Watch For
- Sewer/water connection fees (can be $5,000-$15,000+)
- School impact fees (waived 750 sq ft or less of interior livable space in CA)
- Tree removal or site grading
- Upgraded electrical panel for main house
- Landscaping restoration after construction
Finance Your ADU Project
Most ADU projects are funded through HELOCs, construction loans, or cash-out refinancing. Compare rates from top lenders.
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Costs by City
Construction costs vary significantly by location. Here are average ADU build costs for popular California cities:
| City | Avg Build Cost | Cost/Sq Ft | Permit Fees |
|---|---|---|---|
| Birmingham | $90,000-$180,000 | $120-$200 | Confirm with Dept. of Planning, Engineering & Permits |
| Huntsville | $95,000-$185,000 | $125-$200 | Confirm with Huntsville Development Services |
| Mobile | $90,000-$175,000 | $120-$195 | Per the City of Mobile permit fee schedule — confirm current amounts with Build Mobile Permitting & Inspections Division |
| Chandler | $125,000-$250,000 | $140-$280 | Valuation-based per Chandler fee schedule using ICC Building Valuation Data (minimum permit fee $60); third-party guides estimate roughly 1.5-2% of construction value plus plan review — confirm exact amounts with Chandler Development Services |
| Gilbert | $130,000-$260,000 | $150-$280 | Building permit and plan review fees based on valuation — Confirm with Gilbert Development Services |
| Mesa | $120,000-$240,000 | $135-$270 | Valuation-based per Mesa's Schedule of Fees & Charges; no flat ADU fee published — confirm with Mesa Development Services |
| Peoria | $120,000-$245,000 | $145-$270 | Building permit and plan review fees apply — Confirm current amounts with Peoria Development Services |
| Phoenix | $120,000-$240,000 | $135-$270 | $2,500-$4,200 |
| Scottsdale | $140,000-$280,000 | $200-$400 | $5,000-$8,000 |
| Tempe | $120,000-$250,000 | $140-$280 | Building permit and plan review fees based on valuation — Confirm with Tempe Community Development |
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