ADU Cost Calculator
An Accessory Dwelling Unit (ADU) is a small, independent residential structure built on the same lot as a single-family home. Also known as a granny flat, backyard cottage, or in-law unit, an ADU can provide rental income, housing for relatives, or flexible living space.
🧾 What Goes Into ADU Costs?
The cost of building an ADU varies based on location, size, design, and site-specific factors. Here’s a breakdown of common components:
| Component | Typical Range |
|---|---|
| Construction Cost (Per Sq Ft) | $150 – $350 |
| Permit & Design Fees | $5,000 – $25,000 |
| Utility Connections | $3,000 – $15,000 |
| Land Prep (Excavation, Grading) | $2,000 – $20,000 |
| Total Cost Range | $80,000 – $400,000+ |
🧮 How to Use the ADU Cost Calculator
- ADU Size (sq ft): Enter the planned size of your ADU (e.g., 400–1200 sq ft).
- Cost per Sq Ft: Estimate based on your location or contractor quote.
- Permit Fees: Add architectural, engineering, and city permit fees.
- Utility Hookups: Include sewer, water, gas, and electricity costs.
- Land Prep Cost: Estimate cost for clearing, grading, or excavation.
Click Calculate to instantly see:
- Construction Cost
- Total Estimated ADU Cost
🛠️ Example
Let’s say you’re building a 600 sq ft ADU:
- Cost per sq ft: $250
- Permit Fees: $10,000
- Utility Hookups: $8,000
- Land Prep: $5,000
→ Construction = 600 × $250 = $150,000
→ Total = $150,000 + $10,000 + $8,000 + $5,000 = $173,000
🎯 What Affects ADU Costs?
- Size & Layout: Larger units with bathrooms and kitchens cost more.
- Design Type: Detached ADUs cost more than garage conversions.
- Materials: High-end finishes or appliances raise per sq ft cost.
- Location: Urban areas may have higher permitting and labor costs.
- Foundation Type: Slab, raised, or basement foundations affect total cost.
🏗️ Types of ADUs
| Type | Description | Cost Impact |
|---|---|---|
| Detached ADU | Standalone unit | Most expensive |
| Attached ADU | Shares wall with main home | Moderate |
| Garage Conversion | Converts existing garage | Most cost-effective |
| Above-Garage ADU | Built on top of garage | High cost due to structure |
💸 Can You Finance an ADU?
Yes! ADU financing options include:
- Home equity loans or HELOC
- Cash-out refinancing
- Construction loans
- State or city ADU incentives/grants (depending on region)
📈 ROI of Building an ADU
Adding an ADU can:
- Increase your property value
- Generate monthly rental income
- Serve as a home office or guest space
- Support multi-generational living
❓ FAQs About ADU Costs
1. What’s the average cost of an ADU?
Most ADUs cost between $100,000–$300,000 depending on size and location.
2. Are prefab ADUs cheaper?
Yes, they can reduce labor and timeline costs but may still need foundation and utility work.
3. Do I need permits for an ADU?
Yes, all ADUs require permits and often architectural plans.
4. How long does it take to build an ADU?
Typically 6–12 months, including design and permit approvals.
5. What size ADU can I build?
Laws vary by state and city. Many areas allow up to 800–1200 sq ft.
6. Will building an ADU raise my property taxes?
It may slightly increase your assessment depending on local regulations.
7. Is it cheaper to convert a garage or build new?
Garage conversions are usually cheaper due to existing structure.
8. Are ADUs legal in my area?
Many U.S. states like California, Oregon, and Washington now allow ADUs, but check local zoning laws.
🧠 Final Thoughts
Building an ADU is a smart long-term investment for homeowners looking to expand living space, earn passive income, or add value to their property. Use this ADU Cost Calculator to plan your budget and get a realistic view of your project costs.