How Long Does it Take to Build an ADU? Here's the Total Rundown From Start to Finish

The answer to the question “How long does it take to build an ADU?” depends primarily on your ADU type. The two primary types you’ll encounter are stick-built and prefab ADUs, and the latter will make your life much easier. Stick-built ADUs come together piece by piece, whereas prefab ADUs – we like to call their process “offsite construction” – are built to preset floor plans. These units go from the factory to your lot in one piece that’s complete on both the inside and outside.

While there are theoretically infinite possibilities with traditional stick-built ADUs, Spacial’s offsite construction process results in a much shorter project timeline. Below, learn all the steps involved in building an ADU – and how offsite construction gets your ADU ready in four to nine months.

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What are all the steps to building an offsite-constructed ADU?

Assuming you work with an ADU design company, below are the steps that your ADU must go through before it’s ready for move-in.

1. Signing a contract

Before you sign a contract with an ADU agency, you should look at the company’s floor plans to see how many square feet and bedrooms are available. You should also check for accessibility features which means the ADU can grow with you as your needs change. If you see both, you’ve chosen a reliable builder. You’ll also know you’re in good hands if your contract fully clarifies how all the responsibilities are divided between you and the ADU company. And then, you can sit back and let the ADU firm handle all the hard work.

2. Site measurements and construction planning 

Before your company starts building your ADU, the team will measure your site and decide which construction methods make sense. This process involves assessing whether the ADU site is flat or sloped (the latter can add some complications). 

The process also encompasses determining where and how to connect your water, electrical, and sewer lines. Your team will also determine whether to remove old garages, patches of concrete, and trees (which can sometimes be trimmed instead). Then, your team will figure out where to place the crane – and, just as importantly, where the crane will reach.

3. City and county code research

Your ADU team will include a contractor and an architect. Both these roles are key to the third step of building your ADU: Researching city and county codes and planning your ADU accordingly. They’ll then put together an ADU plan based on what they see. You’ll get the final sign-off on this plan – this is your chance to ask for changes – before it goes to the city for review.

4. Permit approval and offsite construction

Your ADU designer will combine all your plans into a package called a permit set. Your city and county regulatory bodies will review this permit set and request changes. For ADU projects in California, you must meet additional energy-efficiency requirements based on the state’s Title 24 legislation.

Your ADU company will entirely handle any required changes – you can take a step back and trust that the process will go according to plan. Once all is in order, you can obtain your official construction permits. Although you’ll likely need to go through both city and county bodies to get proper licensure, your ADU design firm can make that easy.

That’s especially true if you choose Spacial since we deal with all authorities having jurisdiction (AHJ) on your behalf. We’ll submit your plans, answer your city’s questions and respond to their comments, and pick up your permit. We’ll also work with your regional utility providers to receive any required permission. 

Doing all this work can feel like a part-time job, and that’s why we include it in our services. And as we make any necessary changes, we’ll start the offsite construction process too. This process happens without any commotion or noise in your backyard and shaves months off your timeline. We’ll tell you all about it when you contact us to share your vision for your ADU.

5. Permit issuance and contractor scheduling

Once your ADU company makes all the necessary changes, the city will issue a permit. That means your team can start building – that’s a huge step toward your ADU coming to life. Your team will then schedule the contractor and get ready to bring the ADU from its offsite location into your backyard.

6. Site and foundation preparation

Site and foundation preparation is the final big step before your company builds your ADU. Site prep involves clearing obstructions and debris, demolishing any structures that are no longer necessary, and running the utility lines. After that, your ADU builder will excavate your site to receive the foundation. And then, after that, they’ll pour concrete to install your foundation. Around the same time, your water, electricity, and sewer lines get put into place. Think of this step as putting all the puzzle pieces in place.

7. ADU installation

With your site and foundation set and your offsite construction complete, everything is ready to come together on-site. Some ADU companies can make this happen in just one day. This way, your backyard is noisy and chaotic for as short a time as possible. 

Spacial is a great example. Where other companies might assemble your ADU from individual panels, Spacial’s volumetric modular units ship as a singular unit. The crane literally drops them right into place – no need to connect a bunch of modular panels. That’s how we get your ADU into place in literally one day. And when you walk inside, you’ll see complete walls, flooring, and cabinets – there’s only a bit left to do.

8. Last-minute contractor touch-ups and exterior finishes

For the most part, the interior and exterior of your ADU are complete after your company cranes it into place. That said, your contractor will come in and make some last-minute touch-ups to take your ADU from excellent to perfect. These touch-ups may include removing the sticky coating from your windows and taking the floor covering up. They can also include placing a skirt around the bottom of the unit, adding steps to the unit, and furnishing the unit.

Simultaneously, you may want to make some exterior renovations to truly make your ADU feel like a home. Driveway renovation, patio and sidewalk installation, new landscaping, and more are all on the table. 

9. Certificate of occupancy issuance and owner furnishing

The final step of ADU building comprises two parts. First, the contractor will obtain a temporary certificate of occupancy (TCO) and give you the keys to your brand-new ADU. After that, your ADU company can furnish your home for you. At Spacial, we do exactly this through our West Elm partnership. 

Not long afterward, you’ll receive a final certificate of occupancy (COO). If you’re thinking of renting your ADU, you’ll need to wait until this step to start searching for a tenant. Or if you’re making space for family or friends, you can invite them to start moving in. Everything will be ready to go and feel just like home.

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10 differences between traditional stick-built and prefab ADUs

Many ADU companies will present you with a selection of prefab ADUs. These units differ from traditional stick-built ADUs. Below is a more in-depth explanation of each ADU type’s differences.

Traditional stick-built ADU

A traditional stick-built ADU:

• Is fully customizable. If you have a distinct vision for your ADU and you won’t settle for less, you should choose a traditional stick-built ADU. The big downside with this customization is that you have to make literally hundreds of decisions, which can be a big distraction from the rest of your life. It’s like all the stress of renovating your bathroom or kitchen, but for an entire ADU.
• Can be done anywhere. Since traditional stick-built ADUs are fully customizable, you can build them anywhere on your lot. That means traditional stick-built ADUs can go hundreds of feet behind a house, whereas that’s infeasible with prefab units.
• Requires a linear timeline. With traditional stick-built ADUs, all steps must happen in sequence. The customized, piece-by-piece building approach means you can’t prepare a site and foundation while, at the same time, constructing your ADU offsite. That’s because traditional stick-built ADUs can introduce construction guesswork that’s only solvable if you go through each step in order.
• Is more prone to weather delays. Since traditional stick-built ADU timelines are linear, all manufacturing work must happen on-site right in your backyard. That’s why bad weather is such a problem for stick-builds – a day of pouring rain puts you a day behind schedule. Offsite construction ADUs are ready to go, and they’re built in factories, so they have four walls protecting them from the weather. 
• Are more expensive. Given all the above, traditional stick-built ADUs are almost always more expensive than hiring an ADU company for offsite construction. You’ll have to cover the costs of designing something from scratch and building new, individual ADU pieces. That means your price tag can quickly balloon. When you choose offsite construction instead, you’ll be buying a fixed-cost unit. 
• Can take longer to complete. The customizable nature of traditional stick-built ADUs generally leads to longer project timelines. Offsite construction ADUs, on the other hand, are based on a reliable step-by-step process that leads to a faster timeline.

Prefab ADU

A prefab ADU:

• Lets you choose among several models. When you go the prefab route, you’ll have several options from which you can choose. These choices eliminate the burden of having to design your own home from scratch. That’s important if you’re looking for a quick build rather than your absolute dream home.
• Offloads planning and approval to an ADU company. Choosing offsite construction means you barely have to be involved in planning and approval. Instead, an ADU company intimately familiar with the process from start to finish will handle everything for you. That includes site planning, site preparation, installation, and your final certificate of occupancy – and everything in between.
• Allows for a condensed timeline. Prefab ADUs are constructed offsite while your ADU team preps your site and foundation. This way, these two phases wrap up at the same time rather than one after the other. The result is a shorter timeline – sometimes half what it might be with traditional stick-built ADUs.
• Costs less than a traditional stick-built ADU. There are no customization costs involved in prefab ADUs. Instead, you’ll work from a fixed-cost package. That’s a big reason why prefab ADUs are generally less expensive than traditional stick-built models. Another reason: Your ADU company can divide (and thus lessen) the cost of an offsite construction’s architectural design across several of its units. Were you building your own unit, you’d bear the full burden of that cost.
• Takes four to nine months to complete. The result of all the above is that prefab ADUs can be ready to go in as few as four months. At most, you’ll wait nine months. Were you to choose a traditional stick-built ADU, you might need as long as a year and a half.

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What can slow down an ADU project?

Any ADU project – even one as tried and true as an offsite construction unit – can experience delays. The following factors can extend your project timeline.

• A bad inspection or a design that city planners reject. If your permit set gets shot down or your ADU can’t pass an inspection, you’ll need to change your ADU accordingly. These changes can set you back by weeks or months.
• Supply chain slowdowns. All kinds of materials go into ADUs, ranging from pipes to roofing. If the supply chains for any of these materials fall into disarray, these items could become inaccessible. Your ADU project would sit in limbo until these materials are newly available. 
• Bad weather. Rain on a day designated for on-site work can delay your ADU’s completion. That’s especially true if you choose a traditional stick-built ADU since everything happens right in your backyard. Offsite construction projects are more weatherproof since they come together inside a factory.
• Planning indecision. In some ways, you only get one shot to build a home. That notion can make it understandable to fret over every little decision. But doing so is unwise, as spending too much time on decisions can delay you from acting. Choosing offsite construction can eliminate this obstacle. The materials and design can’t be changed.
• Lot attributes. If your lot is on a hill, you’ll need to add extra time for a soils engineering report. You may also need to account for water under your lot. Of course, these aren’t quite things you can do yourself. Your ADU builder can determine which of your lot attributes might pose problems and then take care of them. The result is a faster process all-around.

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Build an ADU better—and faster—with Spacial

With the right ADU partner on your side, your unit can be ready in well under a year – sometimes as quickly as four months. Spacial can be that partner if you’re building an ADU in the San Francisco Bay Area. We build studio and one-bedroom ADUs and do all the heavy lifting – including the craning – to get them in place. Plus, our offsite construction means you’ll have far less commotion in your backyard as your ADU comes to life. Contact us today to learn how we can make your dream ADU a reality.