Estimating the Cost
On average, building a new ADU in Metro Phoenix can range from $100,000 to over $300,000. This wide range is influenced by the type of unit, its size, and the quality of finishes.
For a more granular view, costs are often broken down by square footage. Homeowners can expect to budget anywhere from $250 to $550 per square foot for new construction. Conversions of existing spaces like garages tend to fall on the lower end of the cost spectrum.
Here’s a snapshot of what to expect for different types of ADUs:
| ADU Type | Typical Size Range (Sq. Ft.) | Estimated Total Cost |
| Garage Conversion | 400 – 600 | $40,000 – $125,000 |
| Attached ADU | 600 – 1,000 | $120,000 – $220,000 |
| Detached New Build | 600 – 1,200 | $160,000 – $300,000+ |
Key Factors Influencing Your ADU Budget
Several key variables will determine where your project falls within these cost ranges.
1. Type of Construction:
- Detached New Construction: Offers the most privacy and design flexibility but is the most expensive due to the need for a new foundation, structure, and separate utility hookups.
- Attached ADU: By sharing a wall and potentially extending existing utilities from the main house, attached units can offer some cost savings compared to detached structures.
- Garage or Interior Conversion: This is often the most cost-effective route as the primary structure already exists. However, costs can increase if significant upgrades are needed for plumbing, electrical systems, insulation, and foundation to meet residential building codes.
2. Size and Layout: The larger the ADU, the higher the overall cost. However, the cost per square foot often decreases as the size increases due to efficiencies of scale. Complex layouts with multiple rooms and corners will be more expensive to frame and finish than a simple, open-plan studio.
3. Site Conditions and Preparation: The state of your property can significantly impact the budget.
- Utilities: The distance and difficulty of connecting to existing sewer, water, and electrical lines are major cost factors. A lengthy trench across extensive landscaping will cost more than a short, simple connection.
- Topography: A sloped lot may require more extensive foundation work and grading, adding thousands to the site preparation costs.
- Access: Difficult site access for construction vehicles and equipment can increase labor costs.
4. Quality of Finishes and Materials: Your design choices will heavily influence the final price tag.
- Standard Finishes: Using builder-grade materials for flooring, cabinets, countertops, and fixtures can keep costs down.
- High-End Finishes: Opting for custom cabinetry, stone countertops, premium appliances, and designer tile will place your project at the higher end of the cost spectrum.
5. Labor and Professional Fees: Labor typically accounts for a significant portion of the total budget. This includes a general contractor, who may charge 10-20% of the project cost, as well as subcontractors like electricians, plumbers, and framers.
Breaking Down the Costs: A Sample Budget Estimate
To create a realistic budget, it’s helpful to break down the expenses into major categories. The following is an estimated cost breakdown for a hypothetical 600-square-foot detached ADU in Phoenix using the figures you provided.
| Category | Estimated Cost Range | Description |
| Soft Costs (Pre-Construction) | $7,500 – $15,000 | Includes architectural and engineering plans, soil reports, and energy calculations. |
| Permits & Fees | $1,251 – $2,651 | Varies significantly by municipality. The City of Phoenix uses a valuation-based fee structure for building permits and plan reviews. Development impact fees can add a substantial amount, potentially over $10,000 depending on the neighborhood. |
| Site Preparation & Foundation | $12,000 – $22,000 | Includes excavation, grading, and pouring a concrete slab foundation. Costs increase for sloped lots or difficult soil conditions. |
| Framing & Exterior | $20,000 – $35,000 | Lumber, framing labor, sheathing, roofing, windows, and exterior doors. |
| Utilities (Rough-In) | $15,000 – $30,000 | Trenching and installation of plumbing, electrical, and HVAC systems before drywall is installed. |
| Interior Finishes | $30,000 – $60,000 | Insulation, drywall, flooring, paint, cabinets, countertops, and tile. |
| Finishing Touches | $10,000 – $25,000 | Installation of appliances, plumbing fixtures, light fixtures, and final HVAC components. |
| Contingency Fund | 10-15% of Total Cost | It is crucial to set aside a contingency fund to cover unexpected costs that inevitably arise during construction. |
| Estimated Total | $95,751 – $189,651 | (Before Contingency) |
How to Get a More Accurate Estimate
- Consult with Professionals: Engage with architects, designers, and experienced ADU builders in the Phoenix area. They can provide valuable insights into current material and labor costs.
- Get Multiple Bids: Once you have a set of plans, solicit bids from at least three reputable general contractors. This will give you a competitive and realistic view of the market rate for your project.
- Check with Your Local Municipality: Contact the planning and development department of your specific city (e.g., Phoenix, Scottsdale, Tempe, Mesa) to get a detailed fee schedule. Be sure to inquire about both permit fees and potential impact fees.
CONTACT US TO DISCUSS BUILDING YOUR ADU PROJECT
We offer free consultation and estimation for your ADU Project
(602-345-0779)
UNITED SPECIALTY CONTRACTING
3101 N. Central Ave – Suite 784 – Phoenix, AZ 85012
AZ ROC# 332505
