Free ADU Cost Calculator Online
Adding an accessory dwelling unit (ADU) to your property is a great way to expand living space, increase property value, or generate rental income—but construction costs can vary widely. A ADU Cost Calculator helps estimate your total project cost by factoring in size, layout, materials, labor, permits, and location. With tools like Construction Calculators, homeowners and developers can receive instant, personalized ADU cost estimates to plan budgets with greater accuracy.
What Is an ADU Cost Calculator?
An ADU (Accessory Dwelling Unit) Cost Calculator is a digital tool that estimates the complete cost of building a secondary housing unit on a residential property. It accounts for:
- ADU size (square footage)
- Construction type (detached, garage conversion, attached)
- Foundation and structural needs
- Interior finishes (kitchen, bath, flooring)
- Site work and utility connections
- Design and permit fees
It gives a breakdown of both hard costs (construction, materials) and soft costs (design, permits).
Types of ADUs
| ADU Type | Description | Cost Range per sq ft |
|---|---|---|
| Detached | Free-standing unit | $300 – $500 |
| Garage Conversion | Converts existing garage to living space | $150 – $300 |
| Attached | Adds to main home (shares wall) | $250 – $400 |
| Basement Conversion | Finishes an existing basement | $150 – $250 |
| Above-Garage ADU | New construction over garage | $250 – $450 |
Each type affects both cost and construction time.
Example:
- 600 sq ft detached ADU
- $350/sq ft base cost
- Permit and site fees: $20,000
- Total: (600 × 350) + 20,000 = $230,000
Average Cost Breakdown
| Category | Estimated Cost Range |
|---|---|
| Construction (hard costs) | $120,000 – $250,000 |
| Design & permits | $8,000 – $25,000 |
| Utility upgrades | $5,000 – $30,000 |
| Site prep & foundation | $10,000 – $40,000 |
| Total ADU Cost | $150,000 – $350,000+ |
The calculator uses these variables to fine-tune your estimate.
Key Factors That Influence ADU Costs
1. Size and Layout
Larger ADUs cost more, but cost per square foot may go down due to economies of scale.
2. Construction Method
Garage conversions are cheaper than new detached ADUs, which require full foundations.
3. Location and Regulations
Cities like Los Angeles and San Diego have streamlined ADU rules, but permit fees still vary.
4. Site Conditions
Grading, tree removal, and sewer tie-ins all affect cost.
5. Finish Quality
Standard vs. high-end finishes (flooring, tile, cabinets) can change total cost significantly.
Detached ADU Example
- Size: 800 sq ft
- $400/sq ft build cost
- Permits: $15,000
- Site utilities: $25,000
- Estimated Total: $360,000
The calculator lets you toggle these values based on your site and preferences.
Garage Conversion Example
- Size: 400 sq ft
- $225/sq ft build cost
- Minimal site work
- Estimated Total: $90,000 – $110,000
A popular option for budget-conscious homeowners.
Utilities & Site Work
| Item | Cost Range |
|---|---|
| Sewer/septic connection | $5,000 – $20,000 |
| Electrical upgrades | $3,000 – $10,000 |
| Water/gas lines | $2,500 – $8,000 |
| Foundation work | $10,000 – $35,000 |
Some ADUs require trenching, new meters, or panels.
Financing an ADU
ADU calculators can also project total project costs to:
- Determine loan amounts
- Compare HELOC vs. renovation loans
- Support grant or subsidy applications
- Inform equity and ROI projections
They help set realistic goals before approaching lenders.
Common ADU Features That Add Cost
| Feature | Extra Cost Estimate |
|---|---|
| Full kitchen | $10,000 – $25,000 |
| Full bathroom | $8,000 – $20,000 |
| HVAC system | $5,000 – $15,000 |
| Solar panels | $10,000 – $20,000 |
| ADA compliance | $5,000 – $15,000+ |
Use the calculator to include or exclude these optional items.
ROI and Rental Income Potential
ADUs can generate strong returns:
- Rent: $1,500 – $3,000/month depending on location
- Payback period: 5–10 years
- Increased home value: $100,000 – $300,000+
The calculator can help compare build cost vs. long-term income.
FAQs
How accurate is an ADU cost calculator?
It provides a close estimate based on your inputs. Final costs may vary depending on contractor bids and city requirements.
Can I use it for multi-unit ADUs?
Yes—just adjust the square footage and features accordingly.
Does the calculator include permits and fees?
Yes, you can input or select estimated permit costs based on your area.
What’s the cheapest way to build an ADU?
Garage or basement conversions are most affordable. Keep the footprint small and finishes basic.
Is an ADU a good investment?
In many cities, yes. It can increase property value and create passive income.
Final Thoughts
Building an ADU is a smart way to expand your space, generate income, or create multi-generational housing. But knowing the true cost upfront is essential. With the ADU Cost Calculator, you can plan your project efficiently, avoid budget overruns, and build with confidence.