What is an ADU and Why Calculate the Cost?
Planning to build an Accessory Dwelling Unit? Understanding the potential cost upfront is crucial for budgeting and decision-making. An ADU—whether it's a backyard cottage, basement apartment, or garage conversion—can add valuable living space to your property while potentially generating rental income or housing family members.
This calculator helps you estimate your ADU construction costs based on the size of your planned unit and local building costs per square foot. The formula accounts for the efficiency factor that makes ADUs typically more cost-effective per square foot than building a traditional home.
The ADU Cost Formula
The calculator uses a straightforward formula to estimate your ADU construction cost:
$$\text{ADU Cost} = \text{Total Area} \times \text{Cost per sqft} \times 0.85$$
Where:
- Total Area is the square footage of your planned ADU
- Cost per sqft is the local construction cost per square foot
- 0.85 is the efficiency factor accounting for ADU-specific cost savings
Calculation Example
Let's walk through a typical ADU cost calculation:
Given:
- ADU Size: 400 square feet
- Cost per square foot: $250
Calculation:
$$\text{ADU Cost} = 400 \times 250 \times 0.85$$
$$\text{ADU Cost} = 100,000 \times 0.85$$
$$\text{ADU Cost} = $85,000$$
So a 400 square foot ADU at $250 per square foot would cost approximately $85,000.
Understanding Construction Costs by Region
Construction costs vary significantly by location. Here's a general reference:
| Region | Cost per sqft Range | 400 sqft ADU Est. |
|---|---|---|
| Rural Areas | $150 - $200 | $51,000 - $68,000 |
| Suburban | $200 - $300 | $68,000 - $102,000 |
| Urban | $300 - $450 | $102,000 - $153,000 |
| High-Cost Metro | $400 - $600+ | $136,000 - $204,000+ |
Factors That Influence ADU Costs
Several elements can push your actual costs above or below this estimate:
Cost Increasers:
- Detached units (vs. attached or conversions)
- Complex architectural designs
- Premium finishes and appliances
- Challenging site conditions
- Extensive utility work
- Permit and impact fees
Cost Reducers:
- Garage or basement conversions
- Simple, efficient designs
- Standard finishes
- Existing utility connections
- Prefab or modular construction
Making the Most of Your ADU Investment
When planning your ADU budget, add 10-20% contingency for unexpected costs. Factor in soft costs like permits, architectural plans, and engineering studies, which typically add 15-25% to construction costs. Getting multiple contractor bids and understanding local ADU regulations will help you arrive at the most accurate budget for your specific project.