What Is the ADU Cost Calculator?
The ADU Cost Calculator is a free online tool designed for users who need quick, accurate calculations in the practical calculation space. By entering your adu type, size, location cost tier, you get instant results including estimated cost range, permit fees, estimated timeline. No formulas to memorize, no spreadsheets to build — just enter your numbers and get the answer in seconds. Whether you're a beginner or experienced professional, this calculator saves you time and eliminates guesswork.
Why This Calculation Matters
Getting estimated cost range right can make the difference between success and costly mistakes. In practical calculation, small errors compound quickly. Manual calculations are error-prone and time-consuming, especially under pressure. This calculator applies proven formulas used by users worldwide, giving you confidence that your numbers are correct. Use it to get accurate results with precision and avoid common pitfalls that trip up beginners.
When Should You Use This Calculator?
This tool is most useful when you know your adu type and need to find the right estimated cost range. It's also great for quick estimates before committing to a decision, and to double-check manual calculations or professional quotes, and when comparing different scenarios side by side. Bookmark this page and come back whenever you need a fast, reliable answer — the calculator is always free and requires no signup.
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 to Use This Calculator
- Enter Your ADU Type: Start by entering your adu type — this is the primary input for the calculation.
- Fill In Additional Details: Complete the remaining fields: size, location cost tier, finish level, expected monthly rent. Each value refines the calculation for greater accuracy.
- Click Calculate: Hit the Calculate button to run the numbers. Results appear instantly below.
- Review Your Results: Check your estimated cost range, permit fees, estimated timeline. Use these figures to inform your next decision or compare against alternative scenarios.
How It Works
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.
Tips & Considerations
- Double-check your adu type before calculating — even small input errors can significantly change your results.
- Run the calculator with different values to compare scenarios and find the optimal approach for your situation.
- Pay attention to both estimated cost range and permit fees — they work together to give you the full picture.
- Bookmark this page for quick access next time you need to get accurate results.
- If you're unsure about your expected monthly rent, start with a conservative estimate and adjust from there.
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.