ADU Cost Calculator
ADU Cost by Type and Finish Level (600 sqft, Medium-Cost Area)
Estimated construction cost ranges excluding permit fees. Add 50-100% for high-cost areas.
| ADU Type | Basic | Mid-Range | High-End | Timeline |
|---|---|---|---|---|
| Garage Conversion | $60,000 | $84,000 | $120,000 | 3-5 months |
| Basement Conversion | $48,000 | $72,000 | $108,000 | 2-4 months |
| Above-Garage | $78,000 | $114,000 | $168,000 | 5-8 months |
| Detached New Build | $90,000 | $138,000 | $210,000 | 6-12 months |
How We Calculate This
This adu cost calculator uses established formulas and industry-standard data to provide accurate estimates.
- Enter your specific values into the calculator fields above
- Our algorithm applies the relevant formulas using your inputs
- Results are calculated instantly in your browser — nothing is sent to a server
- Review the detailed breakdown to understand how each factor affects your result
These calculations are estimates based on standard formulas. For critical decisions, always consult a qualified professional.
How to Convert Oven Recipes to Air Fryer
This calculator estimates ADU construction costs based on the type of build, square footage, location, and finish level. It also projects potential rental income and return on investment.
The basic rule:
- Garage conversion: $100–$200/sqft (existing structure, needs insulation, plumbing, finishes)
- Detached new build: $150–$350/sqft (full construction from foundation up)
- Basement conversion: $80–$180/sqft (existing structure, may need egress, waterproofing)
- Above-garage: $130–$280/sqft (structural reinforcement + full build-out)
ADU costs vary significantly by region. California and New York can be 50-100% more expensive than national averages. Permit costs and timelines also vary — some cities have streamlined ADU permitting while others require lengthy review processes. Always get multiple contractor quotes.
When Would You Use This Calculator?
This adu cost calculator is designed for anyone who needs quick, reliable estimates without complex spreadsheets or professional consultations.
- When you need a quick estimate before committing to a purchase or project
- When comparing different options or scenarios side by side
- When planning a budget and need to understand potential costs
- When you want to verify a quote or estimate you've received from a professional
- When teaching or learning about the concepts behind these calculations
Frequently Asked Questions
How much does it cost to build an ADU?
ADU costs typically range from $80,000 to $250,000+. Garage conversions run $60,000-$120,000, basement conversions $50,000-$100,000, and detached new builds $100,000-$300,000+. Costs vary enormously by location — a detached ADU in Los Angeles averages $200,000-$350,000, while in a mid-cost area it might be $120,000-$200,000.
What is the cheapest type of ADU to build?
Basement conversions are typically cheapest ($80-$180/sqft) since the structure already exists. Garage conversions are next ($100-$200/sqft). Both leverage existing foundations, walls, and roofs. Detached new builds are most expensive but offer the most design flexibility and often command higher rents.
How long does it take to build an ADU?
Typical timelines: garage conversions take 3-5 months, basement conversions 2-4 months, and detached new builds 6-12 months. Add 2-6 months for permitting and design before construction begins. Prefab/modular ADUs can reduce build time to 2-4 months after site prep.
Do ADUs increase property value?
Yes. ADUs typically increase property value by 20-35% of their construction cost, and often more in high-demand rental markets. A $150,000 ADU might add $100,000-$200,000 in property value, plus generate $1,500-$2,500/month in rental income. The income potential often makes the ROI very attractive.
What permits do I need for an ADU?
Requirements vary by jurisdiction but typically include: building permit, planning/zoning approval, utility connections (sewer, water, electric), and sometimes a separate address. California has streamlined ADU permitting statewide. Permit fees range from $2,000-$15,000 depending on location and ADU type.
Can I build an ADU on my property?
In most US cities, yes — especially after recent zoning reforms. California, Oregon, and Washington have statewide ADU legalization. Check your local zoning for: minimum lot size, setback requirements, maximum ADU size (often 800-1,200 sqft), height limits, and owner-occupancy requirements. Many cities have relaxed these rules significantly since 2020.