The short answer: For a California detached ADU, the ADU design cost with an architect is typically $10,000 to $25,000 for a standard design or $18,000 to $40,000 or more for a custom or complex one, commonly framed as 5% to 15% of construction cost. Sources report $2,500 to $8,000 for a non-licensed building designer, and a city’s pre-approved plan can bring design fees down to about $2,000 to $5,000. The cheapest route is a pre-approved plan, but only if it fits your lot and your needs.
Last verified: October 5, 2026 ยท Written by the ADU Scope editorial team
Attention: These fee ranges come from a single ADU-industry guide (The ADU Pro, published January 2025 and updated June 2026), which is a commercial service, not a professional association. Fees vary widely by firm, region and scope. Ask each professional for a written scope and fee.
What ADU Design Cost Pays For
First, the drawings are the legal basis for the permit and the instructions for the builder. A complete set normally covers the site plan, floor plans, elevations, structural and systems details and the energy-compliance documents California requires. In contrast, a thin set of plans tends to produce correction rounds at the city and change orders on site, which is why the lowest ADU design cost is not always the cheapest outcome.
Three ways to structure ADU design cost
- Percentage of construction cost: the guide puts it at 5% to 15%.
- Flat fee: a fixed price for a defined scope.
- Hourly: the guide puts California ADU architects at $100 to $250 per hour, and general residential architects in lower-cost markets at $75 to $150.
A percentage fee ties the designer’s income to the project’s cost, which can work against you if you want to bring costs down. By contrast, a flat fee gives you a predictable number. Therefore, ask which model each professional uses.
Key fact: ADU design cost is a one-time expense, but its effect is permanent. A design that fits the lot, shortens utility runs and avoids unnecessary complexity can save more than it costs. Our guide to utility hookup costs shows how much distance alone can add.
ADU Design Cost by Unit Type
| Unit type | Reported flat-fee range |
|---|---|
| Junior ADU (JADU) | $3,000 to $8,000 |
| Garage conversion | $5,000 to $12,000 |
| Attached ADU | $8,000 to $18,000 |
| Standard detached ADU | $10,000 to $25,000 |
| Custom or complex detached ADU | $18,000 to $40,000 or more |
In addition, the same guide says most California homeowners pay $8,000 to $15,000 for a straightforward design. In a project budget these fees sit in the design block, and they overlap with the “soft costs” that some builders quote as a share of project value; see our ADU budget breakdown for how to avoid counting them twice.
Lower ADU Design Cost Alternatives
| Option | Reported cost | Trade-off |
|---|---|---|
| City pre-approved plan | $2,000 to $5,000 in design fees | Limited customization; may not fit site conditions |
| Non-licensed building designer or draftsperson | $2,500 to $8,000 | Not a licensed architect; scope and liability differ |
| Design-build firm | Bundled, with design folded into one price | Design and construction come from one company |
Also, what a non-licensed designer may prepare for a permit depends on the jurisdiction and the type of work, and we did not verify the rules city by city. Therefore, ask your building department before you hire.
Pre-approved plans in practice
For example, several large cities run programs of pre-reviewed designs, among them Los Angeles, San Jose, San Diego and Seattle. Our city guides cover how each program works in Los Angeles, San Jose and San Diego.
Meanwhile, Seattle publishes the most detail on the trade-offs. The city says its ADUniverse designs can clear permitting in as little as two to six weeks on most sites, and that permit fees drop because the city needs less review, though it gives no dollar figure for the reduction. It also notes that each designer sets a licensing fee for their plan, up to $1,000 in the current gallery. A pre-approved plan is therefore not always free. The city’s submission guide for the next round of designs schedules the new gallery for November 2026, so check which catalog is live when you apply.
Whatever the city, pre-approved plans share the same limits. They may not fit an unusual lot, they do not remove site-specific requirements such as grading, drainage or utility connections, and they leave little room to customize. Finally, read the plan’s conditions and ask the city what it covers.
Tip: Before hiring an architect, check your city’s pre-approved list and ask a builder whether any plan fits your lot. If one does, you can reserve the architect budget for site-specific work such as grading, utilities and landscaping.
How to Choose Within Your ADU Design Cost Range
The choice of designer depends on your lot and priorities, not only on ADU design cost. This is our view, not a rule:
- Simple, flat lot and a standard layout: a pre-approved plan is often the best value.
- Unusual lot, slope, trees or tight setbacks: an architect or an experienced designer is worth the fee.
- Garage conversion: a designer or architect for the conversion scope, since someone must assess the existing structure.
- Wanting one point of responsibility: consider a design-build firm, and compare its all-in price with separate design and construction.
What to Get in Writing Before You Hire
Whichever route you choose, a written proposal protects both sides. Note that this checklist is our suggestion, not a legal requirement:
- Scope: which drawings and documents are included, and whether structural and energy-compliance work is in the fee or the designer bills it separately.
- Fee model: percentage, flat fee or hourly, and what triggers extra charges.
- Revisions: how many rounds of changes are included, and how the designer handles city correction comments.
- Permit support: whether the designer submits to the city and responds to plan-check comments.
- Site work: whether a survey, soils report or utility sketch is included or you must order it.
- Construction support: whether the designer answers builder questions during construction and for how long.
In fact, a fee that looks low but leaves out structural drawings, energy compliance or correction rounds can end up costing more than a higher flat fee that includes them. Then compare proposals line by line, the same way you would compare construction bids; our ADU budget breakdown shows how.
Frequently Asked Questions
How much does an ADU architect cost?
According to one industry guide, a standard detached ADU costs $10,000 to $25,000, or 5% to 15% of construction cost.
Can I use a designer instead of an architect?
One guide puts non-licensed designers at $2,500 to $8,000. However, check what your building department accepts for permits.
Do pre-approved plans cost anything?
Not always. For instance, a guide puts design fees at $2,000 to $5,000 with a pre-approved plan, and Seattle lets designers charge a licensing fee of up to $1,000.
Do pre-approved plans guarantee approval?
No, because you still need a site plan and compliance with site conditions.
Next Steps
- The Cheapest Way to Build an ADU Without Cutting Corners
- ADU Budget Breakdown: Three Sample Budgets
- How Much Does an ADU Cost in California?
- Seattle ADU Fees: Permit, Sewer Capacity and Utility Charges
General information, not financial advice. Fees vary, so request written proposals.
Sources: The ADU Pro: cost of hiring an architect ยท City of Seattle: ADUniverse gallery ยท City of Seattle: ADUniverse 2.0 submission guide