The ADU Process: From First Idea to Final Inspection

Building an ADU can feel complicated at the start. There are design decisions, city requirements, utilities, budget questions, inspections, and construction details to think through all at once. For many homeowners in the Bay Area and Sacramento, the hardest part isn’t the construction—it’s not knowing what happens next.

The good news is that the process becomes far easier to understand when it’s broken into clear steps. Below is a practical walkthrough of what typically happens from the first ADU idea to final inspection, along with the current California rules and regional realities that shape each stage.

1. Start with the reason for the ADU

Before thinking about plans or finishes, start with the goal. The “why” drives almost every decision that follows.

People build ADUs for all kinds of reasons:

• Aging parents or adult children
• Rental income
• Guest space or a home office
• Long-term flexibility
• A place to live during a future main-house remodel

Each reason points the design in a different direction. An ADU for aging parents prioritizes single-level access and a curbless shower; a rental leans toward durability and a private entrance; a home office needs less plumbing but more light and sound separation. Getting clear on the “why” early is the easiest way to avoid expensive changes later.

2. Understand what may be possible on the property

The next step is a basic feasibility review. The goal here isn’t to finalize anything—it’s to understand whether the idea is realistic and what issues should be checked early.

A feasibility review usually looks at:

• Where the ADU could go, and what type might fit
• Backyard or garage access
• Slope, drainage, trees, and existing structures
• Where utilities run
• Local city requirements

In older Bay Area neighborhoods, tight lots and limited side-yard access often steer the design. In parts of Sacramento with larger lots, there’s frequently more room for a detached unit—but more grading and drainage to plan for.

California has made feasibility friendlier than it used to be. Statewide law now protects an ADU of at least 800 square feet and 16 feet in height with four-foot side and rear setbacks, and cities can’t reject those on the basis of local zoning. That baseline gives most properties a realistic starting point even when the lot feels small.

3. Choose the right ADU type

There are four common options, each with its own tradeoffs:

• Detached backyard ADU — the most privacy and resale flexibility, but requires a new foundation and its own utilities, so it tends to cost the most per square foot.
• Garage conversion — reuses existing structure and is often the most economical path.
• Attached ADU — can share some systems with the main home; makes sense depending on the existing layout.
• Junior ADU (JADU) — carved out of the existing home and capped at 500 square feet under current California law. The smallest and often least expensive, with its own rules about shared facilities.

The right choice depends on the property, the budget, and how the space will be used. It’s rarely one obvious answer, which is why the feasibility review matters before this step is locked.

4. Develop the design

Once the direction is clear, the design process begins. This is where the unit takes shape and where small decisions have a big impact on daily comfort.

Design usually covers:

• Size and layout
• Entry location and privacy
• Windows and natural light
• Kitchen, bathroom, storage, and laundry
• Exterior style and outdoor space
• Accessibility

It’s also the time to think through the details that don’t show up on a floor plan but matter every day: where the morning light lands, whether the entry feels private, and how sound travels between the ADU and the main home. Keeping building code and California’s 2025 Title 24 energy standards in view from the start is far cheaper than reworking plans after a plan-check comment.

A few people typically shape the design:

• You, the homeowner — the calls on layout, finishes, and how the space will be used
• A designer or architect — turning your goals into buildable, code-compliant plans
• A structural engineer — foundation and framing details, especially on detached units
• A Title 24 energy consultant — confirming the plans meet California’s energy standards
• Your builder — keeping the design realistic for the budget and site

With that many hands involved, someone has to keep them coordinated so the design, structural details, energy compliance, and budget all line up before plans go to the city. On many projects the builder plays this connecting role, acting as the through-line for the whole team.

There’s flexibility in how it comes together. If you already have a designer or architect you trust, a good builder can work alongside them. If you’re starting from scratch, an experienced builder will have established relationships with reputable design and engineering partners they’ve worked with before—which tends to make the process smoother, since the team already knows how to collaborate. Either way, the goal is the same: design choices and cost stay aligned early, instead of colliding at permit time.

5. Review budget and scope

Before moving too far, homeowners should understand exactly what’s included in the project and what could move the cost.

A few things move the budget more than others:

• Site access (can equipment reach the backyard?)
• Utility work and electrical panel upgrades
• Foundation and structural needs
• Finish level
• Permits, drainage, and slope conditions
• Any changes to the existing home

Often a backyard with no side access, or a panel that needs upgrading, shifts a budget more than the finishes do.

Here’s roughly what detached ADUs run in 2025–2026:

• Bay Area — about $300 to $600 per square foot for construction, with design, engineering, and permits on top.
• Sacramento — lower, often around $250 to $450 per square foot. Larger, flatter lots, less complex terrain, and lower permit and impact fees all help. (Smaller units tend toward the higher end per square foot, since fixed costs like the kitchen and bath are spread over fewer feet.)

The Bay Area’s higher costs aren’t about quality—they reflect tighter urban lots with limited access and more demanding site conditions. These are planning ranges, not quotes; the point of this step is a clear, written scope so there are no surprises later.

6. Submit for permits

After the design and technical details are ready, plans go to the city. California has worked hard to make this stage more predictable than it once was.

The city reviews items such as:

• Site plan and building plans
• Structural details
• Energy compliance
• Fire and safety requirements
• Utility connections
• Local ADU standards

The encouraging part: ADU applications are reviewed ministerially, which means a complete application that meets the standards must be approved without a public hearing or discretionary review—and the city generally has 60 days to approve or deny it. As of January 1, 2026, SB 543 also requires agencies to issue a completeness determination within 15 days and to provide a written list of any deficiencies, so you know exactly what needs fixing and when.

In practice, cities still issue comments that need to be addressed before permits are granted. A well-prepared submittal is the single biggest factor in moving through plan check quickly.

7. Prepare for construction

Before work begins, the project needs to be organized so the build starts cleanly.

This usually includes:

• Finalizing the schedule
• Ordering materials with lead times in mind
• Coordinating trades
• Preparing and protecting the site
• Confirming access
• Walking the homeowner through what to expect

In occupied homes—which is most ADU projects—setting expectations about noise, dust, parking, and access matters as much as the schedule itself, and helps the whole project run more smoothly.

8. Build the ADU

Construction usually happens in clear phases:

• Site preparation
• Foundation
• Framing
• Roofing and exterior work
• Rough plumbing, electrical, and mechanical
• Insulation and drywall
• Interior finishes
• Cabinets, fixtures, and appliances
• Exterior details and cleanup

Inspections happen at key points throughout—after foundation, framing, rough mechanical, insulation, and so on—and work can’t move to the next phase until each one passes. These checkpoints are a normal part of the process, and a good builder schedules around them so the project keeps momentum rather than stalling between stages.

9. Complete final inspections

Near the end of the project, the city completes final inspections to confirm the ADU meets the approved plans and applicable code.

This is when final details are checked, any corrections are completed, and the project moves toward approval for occupancy. A clean final inspection usually reflects the quality of everything that came before it—the earlier inspections, the plan-check work, and the care taken during the build.

10. Final handoff

Once inspections are complete, the homeowner receives the finished space.

A good handoff should include:

• A walkthrough of the completed ADU
• A review of key systems and how they work
• Warranty and maintenance information
• Final cleanup
• Any remaining closeout items

This is the moment the project shifts from a construction site back to part of the home—and a thorough handoff is what makes the space feel finished, not just built.

The bottom line

The ADU process has many steps, but it doesn’t have to feel mysterious. The most important thing is to understand the property, plan carefully, and make practical decisions early. California’s rules have made permitting more predictable than ever, and a clear scope keeps the budget honest.

A successful ADU isn’t just about getting through permits or adding square footage. It’s about creating a space that works well for the property, the homeowner, and the people who will use it.

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Thinking about building an ADU on your Bay Area or Sacramento property?

Burton Builders can help you understand the process, evaluate the practical details, and plan the right next steps—from first idea through final inspection. Reach out for a feasibility conversation about your property.