How Long Does It Really Take to Build an ADU in Southern California?
- kayla5523
- Dec 10, 2025
- 8 min read
If you’re a homeowner in Southern California, chances are you’ve thought about adding more space, more flexibility, or more income to your property. And if so, you’ve very likely explored the idea of building an Accessory Dwelling Unit, a backyard home that can serve your family, your future, or your finances. But even before the conversation turns to rental income, design, or layout options, almost everyone asks the same question of how long does it actually take to build an ADU?
It’s a fair question, and a wise one. An ADU is more than a construction project. It’s an investment in your property’s long-term value, your family’s comfort, and your financial future. Having the right timeline helps you plan with confidence, avoid surprises, and understand exactly what to expect before the first shovel hits the ground.
But if you’ve searched online for answers, you’ve probably seen a variety of timelines with three months here, sixteen months there, or the vague “it depends.” The truth is that ADU timelines vary widely, and most of the articles you’ll find online fail to explain why they vary or how you can avoid the delays that slow many homeowners down.
At Wealth Nest, we believe clarity is part of good construction. So we’re going to walk you through the complete ADU timeline, from the very first conversation to move-in day, in a way that’s honest, realistic, and based on years of experience navigating Southern California regulations.
Even more importantly, most homeowners don’t know that the timeline is dramatically different depending on whether you choose a custom, site-built ADU or a prefabricated HUD-coded ADU like the ones we offer. That single decision shapes everything from the permitting to the construction, the cost, and yes, the timeline.
Let’s begin with the thing that matters most.
Prefab vs. Site-Built: The Timeline You Choose Depends on the Path You Choose
This is the part that almost no ADU article online explains clearly. There is not one ADU timeline, but two. And they are not even remotely close.
A traditional site-built ADU requires a full architectural design phase with drawings, engineering, multiple rounds of revisions, structural changes, plan check corrections, and iterative updates. Homeowners are often surprised to learn that this design stage alone can last five to eight months before their city ever even sees the plans for approval. And that’s if things go smoothly.
Once the plans are submitted, permitting takes additional months, and construction must happen entirely on the property, step by step, trade by trade, weather-dependent and fully exposed to scheduling delays. It’s a long, complicated, and sometimes unpredictable path, one that many homeowners underestimate.
A prefabricated HUD-coded ADU, on the other hand, works very differently. Instead of beginning with months of architectural design, the core home is already engineered, approved, and built in a factory. You don’t wait for your ADU to be designed from scratch; you begin with a model that already meets federal and state standards. Your site still needs its own tailored plans with things like grading, foundation, utilities, and local requirements, but the structure itself does not require a long design process.
This means a prefab HUD-coded ADU avoids the single biggest cause of delays in ADU building with iterative design. While site-built projects wait months for architectural drawings, engineering revisions, and plan corrections, prefab homeowners move straight into permitting preparations and site planning.
That difference alone can remove half a year from your timeline.
It’s the reason Wealth Nest homeowners often complete their ADU journey, from the very first conversation to the day the keys are in hand, in five to eight months, while traditional ADUs often stretch well beyond a year. Understanding that difference is essential before you begin, because your choice of ADU type determines everything that follows.
The Wealth Nest Process: A Clear, Predictable Path from Start to Finish
We structure your ADU journey in three simple phases.
Define the Vision
Plan the Vision
Build the Vision
Each step has a purpose, a rhythm, and a clear set of outcomes so you always know where you are in the process and what comes next. Because we build HUD-coded prefab homes, much of the uncertainty and many of the long delays common to traditional ADUs simply aren’t part of your experience.
Let’s explore each phase in detail so you can understand the full journey from start to completion.
Phase One: Define the Vision (1–2 Weeks)
Every successful project begins with clarity. During this first stage, we focus on understanding your goals, studying your property, and identifying which of our models fit your lifestyle, your lot, and your long-term plans.
This is when we visit your property, either virtually or in person, and evaluate every factor that determines what can be built. Setbacks, slopes, access points, utility locations, privacy concerns, and overall layout all matter. We also discuss the purpose of your ADU, whether it will serve as a home for a parent, a guest space, a rental unit, or a future downsizing plan. These conversations guide everything that follows.
By the end of this phase, you’ll know exactly what is possible on your property, which models are the best fit, what your investment looks like, and what your expected timeline will be based on your jurisdiction. It’s a short stage, but it’s foundational. It gives you a clear picture before the real work begins.
This phase typically takes one to two weeks.
Phase Two: Plan the Vision (3–6 Weeks)
Once your model is selected and feasibility is confirmed, we shift into planning with the stage where your project becomes a set of detailed, permit-ready documents. And this is where the value of prefab becomes most obvious.
For custom, site-built ADUs, this is the stage that stretches for months. Architects begin drawing, engineers revise, plans are redrawn, corrections are made, and homeowners wait while the design slowly takes shape. It’s a long, iterative process that regularly reaches five, six, or even eight months.
For Wealth Nest homeowners, the structure is already engineered and approved. Instead of designing the ADU, we design your site with the foundation, grading, utility connections, and everything needed for city review. This eliminates months of back-and-forth and allows your project to move forward at a pace that simply isn’t possible with traditional builds.
During this period, we study your lot in detail. We determine where utilities will run, how the unit will be set, how the foundation will be constructed, and how the ADU will integrate with your property. We assemble the required documentation specific to your city or county, prepare Title 24 energy compliance, and ensure every local requirement is met. Some cities require drainage plans; others require fire access details; others require specific property diagrams. Because we’ve worked across Southern California for decades, we know what each agency expects and how to prepare your documents accurately the first time.
If a correction or clarification is needed, we handle it quickly. Corrections for prefab ADUs tend to be minor and administrative, not structural or design-based, which helps keep your project moving smoothly.
This planning stage generally takes three to six weeks.
Phase Three: Build the Vision (4–6 Months)
This final stage includes everything from submitting your plans to the city to preparing your site, setting your ADU, completing the finishing touches, and receiving your Certificate of Occupancy. While it is the longest stage, it is also the most rewarding because you can see your ADU come to life step by step.
Permitting and City Review
Once your plans are submitted, the timeline becomes largely dependent on your city or county’s internal process. Some jurisdictions move quickly and return comments in about six weeks. Others may take ten to twelve weeks, especially if they are experiencing backlogs or staffing shortages. Throughout this period, Wealth Nest manages every interaction with the city. We follow up on your behalf, respond to corrections, answer reviewer questions, and ensure your project stays on track.
Because your ADU itself is already engineered and approved, the city is primarily reviewing the site-specific details, not redesigning the structure. This is a major advantage for prefab homeowners. Instead of months of structural corrections, the city review tends to be more straightforward.
Contract Finalization and Pre-Construction
After your permit is approved, we finalize your construction schedule, payment structure, and timeline. This step typically takes about a week and ensures that everything is aligned before work begins.
Site Preparation and Construction
Once pre-construction is complete, your property is prepared for the ADU. During this phase, our crews clear and grade the site, excavate trenches, install water and sewer lines, run electrical conduit, prepare the foundation, pour footings, set block, complete inspections, and establish everything the ADU needs to be safely and properly supported.
While all of this is happening on your property, your ADU is being built in a controlled factory environment. It is constructed indoors according to federal HUD standards, inspected throughout the process, protected from weather delays, and completed with exceptional precision. This is another key benefit of prefab: work happens in parallel. Instead of waiting for site construction to finish before the ADU can begin, the ADU is ready shortly after your site is prepared.
This stage of site work generally lasts eight to sixteen weeks, depending on your property’s conditions and your city’s inspection schedule.
Delivery, Set, and Final Touches
Once the foundation and utilities are ready, your ADU is delivered and set on the property. If a crane is required, we coordinate the logistics, traffic control, and placement. After the unit is positioned, our finishing crews complete all final details: connecting utilities, refining interior finishes, completing exterior touches, and preparing the ADU for inspection.
A final city inspection is scheduled, and once the Certificate of Occupancy is issued, your backyard home is officially ready for use, whether that means welcoming family or generating rental income.
This final stage usually lasts one to two weeks.
So How Long Does It All Take?
When you combine all three phases, defining, planning, permitting, site work, factory construction, delivery, and inspection, the typical timeline for a Wealth Nest prefab HUD-coded ADU lands between five and eight months. This includes city review, site preparation, and every major milestone along the way.
This timeline assumes that you are choosing a prefab unit whose design is already complete. If you were building a custom, site-built ADU, you would need to add five to eight months of architectural and engineering design before any of this could begin, which is why traditional ADUs often exceed a year or more from first consultation to completion.
This difference is the heart of why so many homeowners in Southern California choose prefab. It’s fast, efficient, predictable, and designed to avoid the delays that derail traditional construction.
Why Your Timeline Matters More Than You Think
A predictable timeline isn’t just a convenience, it allows you to make confident decisions. It helps you plan rental income, prepare for family transitions, schedule your future move-in, and avoid overlapping costs. A clear timeline turns an ADU project into a manageable, organized experience rather than a long, uncertain waiting game.
Prefab units offer that predictability. They take the guesswork and delay out of the earliest stages of the project and replace them with clarity and momentum. With Wealth Nest, you’re not waiting on months of drawings or weeks of back-and-forth corrections. You’re making progress from the moment we begin.
From Dream to Done
In less than a year, often in just five to eight months, you can transform your backyard into something meaningful. More space. More income. More connection. More value. More options. And more freedom.
Whether you’re building to support family, planning for aging parents, preparing for the future, or creating new financial opportunities, an ADU unlocks possibilities without uprooting your life or leaving the home you love.
At Wealth Nest, our approach is simple. We handle everything, communicate clearly, and deliver an ADU that brings your vision to life with speed, transparency, and care.
If you’re ready to explore what an ADU could look like on your property, and how soon you could begin, schedule your free consultation at www.wealthnestadu.com.
Your ADU starts with a conversation. Your future starts in your own backyard.
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