ADU Cost Estimator
This tool provides a very rough, preliminary estimate of potential costs for building an Accessory Dwelling Unit (ADU). Costs vary significantly based on location, ADU type, size, site conditions, and chosen finishes.
Enter the approximate size, select the type, and choose a general cost region to get a basic cost range estimate. This estimate does NOT include site-specific challenges, high-end finishes, or unforeseen costs.
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Understanding ADU Construction Costs
Factors Influencing ADU Costs:
The final cost of an ADU is influenced by numerous factors beyond just size and type:
- Location: Labor, material, and permit costs vary dramatically by region, state, and even city.
- Type: Converting existing space (garage, basement) is generally less expensive than new construction (attached or detached) because it leverages existing foundations, walls, and utility connections.
- Size: While cost per square foot can decrease slightly with larger units, the total cost increases with size.
- Site Conditions: Difficult access, slopes, poor soil, or required utility trenching add significant costs.
- Finishes & Features: The quality of materials used for flooring, cabinets, fixtures, appliances, and HVAC systems has a major impact. Custom features increase cost.
- Permitting & Fees: Local regulations, zoning, impact fees, and the complexity of obtaining permits can add substantial time and expense.
- Design & Engineering: Costs for architects, designers, engineers, and surveys are additional.
This calculator uses highly generalized cost ranges for different types and locations. It provides a starting point, but a detailed estimate requires specific project planning.
ADU Cost Examples (Illustrative)
These examples illustrate typical scenarios and potential cost ranges, which can vary greatly in reality. The numbers below are *not* calculated by the simple tool above but show plausible outcomes for detailed planning.
Example 1: Small Garage Conversion (Low Cost Area)
Scenario: Convert a 300 sq ft detached garage in a low-cost area (e.g., Oklahoma). Simple finishes, existing foundation is good, utilities nearby.
Inputs: Size ≈ 300 sq ft, Type = Garage Conversion, Location = Low Cost
Plausible Estimate Range: $50,000 - $85,000
Notes: Conversions are often the cheapest route, but structural issues or needing significant utility runs can increase cost.
Example 2: Medium New Detached ADU (Medium Cost Area)
Scenario: Build a new 600 sq ft detached ADU on a flat lot with good access in a medium-cost area (e.g., Atlanta, GA). Standard finishes.
Inputs: Size ≈ 600 sq ft, Type = New Detached, Location = Medium Cost
Plausible Estimate Range: $180,000 - $280,000
Notes: New construction involves full foundation, framing, roofing, etc., which is more expensive than conversions.
Example 3: Small Basement Conversion (High Cost Area)
Scenario: Convert 400 sq ft of unfinished basement space in a high-cost area (e.g., Seattle, WA). Requires adding egress windows and upgrading plumbing.
Inputs: Size ≈ 400 sq ft, Type = Basement Conversion, Location = High Cost
Plausible Estimate Range: $100,000 - $160,000
Notes: Basement conversions can be complex due to waterproofing, head clearance, and egress requirements.
Example 4: Large New Attached ADU (High Cost Area)
Scenario: Build a new 800 sq ft ADU attached to an existing home in a high-cost area (e.g., Los Angeles, CA). Standard to slightly upgraded finishes.
Inputs: Size ≈ 800 sq ft, Type = New Attached, Location = High Cost
Plausible Estimate Range: $350,000 - $550,000+
Notes: Attached can sometimes be slightly cheaper than detached due to shared walls/utilities, but large units in high-cost areas are significant projects.
Example 5: Garage Conversion with Structural Issues (Medium Cost Area)
Scenario: Convert a 450 sq ft garage in a medium-cost area (e.g., Denver, CO), but the foundation needs significant repair/reinforcement.
Inputs: Size ≈ 450 sq ft, Type = Garage Conversion, Location = Medium Cost
Plausible Estimate Range: $90,000 - $150,000
Notes: Unexpected site conditions or necessary structural upgrades can push conversion costs significantly higher than the base estimate.
Example 6: New Detached ADU on Sloped Lot (High Cost Area)
Scenario: Build a 500 sq ft new detached ADU on a lot with a moderate slope in a high-cost area (e.g., Portland, OR). Requires retaining walls and complex foundation.
Inputs: Size ≈ 500 sq ft, Type = New Detached, Location = High Cost
Plausible Estimate Range: $250,000 - $400,000+
Notes: Site challenges like slopes dramatically increase excavation, foundation, and drainage costs.
Example 7: Basement Conversion (Low Cost Area)
Scenario: Convert 700 sq ft of a large, easily accessible basement in a low-cost area (e.g., Kansas City, MO). Minimal structural changes needed.
Inputs: Size ≈ 700 sq ft, Type = Basement Conversion, Location = Low Cost
Plausible Estimate Range: $80,000 - $130,000
Notes: Larger basement conversions can be cost-effective per square foot if the existing space is suitable.
Example 8: Tiny Detached ADU (Medium Cost Area)
Scenario: Build a small 250 sq ft detached "tiny house" style ADU in a medium-cost area (e.g., Phoenix, AZ). Simple design.
Inputs: Size ≈ 250 sq ft, Type = New Detached, Location = Medium Cost
Plausible Estimate Range: $80,000 - $130,000
Notes: Smaller units can have a higher cost per square foot due to minimum costs for kitchens, bathrooms, and utility connections, but lower total cost.
Example 9: New Attached ADU with High-End Finishes (Medium Cost Area)
Scenario: Build a 500 sq ft attached ADU in a medium-cost area (e.g., Austin, TX) with high-end kitchen, bath, and flooring finishes.
Inputs: Size ≈ 500 sq ft, Type = New Attached, Location = Medium Cost
Plausible Estimate Range: $200,000 - $300,000+
Notes: Finish choices can add 20-50% or more to the base construction cost.
Example 10: Garage Conversion Requiring Full Utility Trenching (High Cost Area)
Scenario: Convert a 400 sq ft detached garage in a high-cost area (e.g., Boston, MA) located far from main house utilities, requiring extensive trenching for sewer, water, and electric.
Inputs: Size ≈ 400 sq ft, Type = Garage Conversion, Location = High Cost
Plausible Estimate Range: $120,000 - $200,000+
Notes: Utility connections and trenching, especially over long distances or difficult terrain, are major cost drivers.
Frequently Asked Questions about ADU Costs
1. How accurate is this ADU cost calculator?
This calculator provides a very rough, preliminary estimate. It is based on generalized cost ranges for different types and locations. Actual costs can vary significantly based on specific design choices, site conditions, local regulations, and the contractor you choose.
2. What factors does this calculator consider?
It considers the approximate size in square feet, the type of ADU (new detached, attached, or conversion), and a general factor for your location's construction cost level (high, medium, or low).
3. What costs are NOT included in this estimate?
This estimate typically does NOT include costs for architectural design, engineering, permits and impact fees (which can be substantial), site-specific challenges (difficult access, slopes, soil issues), utility upgrades beyond basic connection, landscaping, or high-end custom finishes.
4. Why do ADU costs vary so much by location?
Location significantly impacts labor rates, material costs, local building codes, and permit/impact fees, all of which contribute to the overall project expense.
5. Is converting a garage or basement always cheaper than new construction?
Generally, yes, because conversions utilize existing structures and foundations. However, if the existing structure requires significant upgrades, structural repairs, or extensive utility work, the cost difference might narrow.
6. How much do permits and fees add to the cost?
Permit and impact fees vary wildly by jurisdiction. They can range from a few thousand dollars to tens of thousands of dollars or more, depending on local regulations and the scope of the project.
7. How does the size of the ADU affect the cost per square foot?
Smaller ADUs often have a higher cost per square foot because certain fixed costs (like connecting utilities, a kitchen, and a bathroom) are spread over fewer square feet. Larger units might have a lower per-square-foot cost but a higher total cost.
8. Does the quality of interior finishes matter for the cost?
Absolutely. Choosing standard, builder-grade finishes is much less expensive than selecting high-end custom cabinetry, countertops, flooring, or fixtures. Finishes can significantly impact the final budget.
9. How can I get a more accurate estimate for my specific project?
For a precise estimate, you need to have detailed architectural plans, potentially structural or civil engineering reports (especially for challenging sites), and then get bids from multiple qualified contractors who have visited the site.
10. What are typical cost ranges I might expect?
Based on industry averages, basic garage conversions might start from $50,000-$100,000+. New construction (detached or attached) typically ranges from $150,000 to $400,000+ or even much higher in expensive markets or for large/complex builds. These are very rough figures.
