Key Takeaways:
- ADU Timeline: The average time to build an ADU ranges from 6 to 18 months depending on the type, size, and local rules.
- Key Steps: The process involves planning, design and permitting, construction, and final inspections.
- Factors Affecting Timeline: Size, complexity, type of ADU, local permitting, availability of contractors, and weather all influence the timeline.
- Speeding Up the Process: Hiring experienced builders, preparing documents early, considering prefab ADUs, and staying in regular contact with your builder can help speed up the project.
- Value: An ADU can add living space, rental income, and increase property value.
Have you ever heard of an ADU? It stands for Accessory Dwelling Unit. It’s like a small home built on the same property as a bigger house. Some people call it a backyard cottage, a granny flat, or even an in-law suite.
People build ADUs for lots of good reasons. Maybe they need extra space for family to stay. Some want to rent it out and earn more money. Others just want to make their home worth more.
But one big question always comes up: How long does it take to build an ADU?
In this guide, we’ll walk you through the steps, the timeline, and what you can expect if you decide to build one.
The Average Timeline to Build an ADU
If you’re thinking about building an ADU, one of the first things you probably want to know is how long it’s going to take. On average, building an ADU can take anywhere from 6 to 18 months from the very first step to move-in day.
The reason the timeline can vary so much is that not all ADUs are the same. Some projects are quicker and easier, while others are bigger and more complex. For example, converting an existing garage into a living space is usually faster than building a brand-new cottage from the ground up.
Here’s a quick look at the typical timelines for different types of ADUs:
| ADU Type | Average Time to Build |
| Garage Conversion | 6–12 months |
| New Detached ADU | 10–18 months |
| Prefab ADU | As little as 1 month |
As you can see, a garage conversion can often be done in about 6 to 12 months because much of the structure is already there. A new detached ADU, like a small house in the backyard, usually takes longer — closer to 10 to 18 months — because you’re starting from scratch.
Prefab ADUs are the fastest option. Since these units are built in a factory and then brought to your property, they can sometimes be installed in as little as one month once the site is ready. However, don’t forget that you’ll still need time for permits and site preparation even with a prefab unit.
Key Steps in the ADU Building Process
Building an ADU is a big project, and it helps to know what to expect at each stage. Here are the key steps you’ll go through, along with how long each part usually takes:
Planning and Feasibility (1–2 months)
The first step is all about getting the basics right. You’ll need to check if building an ADU is even possible on your property. This means looking into local zoning rules — does your city allow ADUs? Is there enough space in your yard?
You should also set a realistic budget at this stage. Think about how much you’re willing to spend, and make sure you have enough to cover everything from permits to construction. It’s also time to start looking for a builder or architect who can help bring your vision to life. Take the time to find someone experienced with ADUs to ensure everything goes smoothly.
Design and Permitting (2–5 months)
Once the planning phase is done, it’s time to get into the design. This is when you’ll work with a designer or architect to create the plans for your ADU. They’ll help you figure out what’s possible within your space, and help design a unit that fits your needs and budget. Expect some back-and-forth during this stage to make sure everything looks perfect.
Once the plans are ready, you’ll submit them to the city for permits. The permitting process can take time, sometimes several months, depending on where you live. Your city may need to review your plans, suggest changes, or even schedule site visits. Be patient and stay in contact with your designer and local officials to ensure everything is moving forward.
Construction (4–9 months)
This is the exciting part — where your ADU starts to take shape! Construction is typically the longest phase, and it can take anywhere from 4 to 9 months, depending on the complexity of the design and the size of the ADU.
The process starts with site preparation: clearing the land, digging for the foundation, and setting up utilities like water and electricity. Then comes the foundation and framing — this is when the bones of your ADU are built. After that, the construction crew will work on exterior work, such as roofing and siding, and start bringing in the utilities like plumbing and electrical systems.
As construction continues, you’ll see things like insulation, drywall, flooring, and the final interior finishes come together. Keep in mind, the timeline can vary due to weather (storms can delay work), availability of materials, and how complicated your design is.
Final Inspections and Move-In (1–4 weeks)
Once the construction is complete, the final step is getting the city inspections and ensuring everything is up to code. This is when city officials will inspect things like plumbing, electrical, and structural integrity to make sure your ADU is safe to live in.
During this phase, you’ll also have utility hookups (water, electricity, and gas) finalized. If everything passes, you’ll receive your certificate of occupancy, which means the ADU is ready for you to move in.
Factors That Affect the Timeline
Building an ADU can take different amounts of time depending on a few things. While we’ve given you an idea of how long the process usually takes, there are many factors that can make it faster or slower. Let’s take a look at what those factors are:
Size and Complexity of the ADU
The size and complexity of your ADU are really important. If you’re converting a small garage into a living space, it might only take a few months. But if you’re building a big, separate house in your yard, that will take a lot longer. The more rooms or features you want, like a kitchen or extra bathroom, the longer it will take to build.
Type of ADU: Garage Conversion, Detached, Attached, or Prefab
Different types of ADUs take different amounts of time to build. For example:
- Garage conversions are usually the fastest to build, taking about 6 to 12 months.
- Detached ADUs (like a small house in your backyard) take longer, around 10 to 18 months.
- Prefab ADUs (made in a factory and brought to your site) can be ready in as little as 1 month, but you’ll still need time for permits and setting things up.
Local Permitting Process and City Workload
In Los Angeles, the ADU permitting process typically takes four to six months, though this can vary depending on your project’s complexity and the city’s workload. After submitting your plans, the city’s planning department reviews them, followed by a safety check. If your property is in a Historic Preservation Overlay Zone or if major changes are planned, the process may take longer. To avoid delays, contact your local building department early, ensure your plans are complete, and stay in regular contact with city officials.
Availability of Materials and Contractors
The materials and workers you need can also affect how long your ADU takes to build. If the materials are hard to find or if your contractor is busy, it could take longer to start or finish the project. It’s a good idea to plan ahead and make sure you have everything you need before starting.
Weather and Site Conditions
The weather can slow down your ADU project. If it rains or snows a lot, it can delay construction. The ground where you want to build also matters. If your site has a lot of rocks or trees that need to be removed, that could take extra time. Try to plan for some weather delays, especially if you live in an area with bad weather.
Changes to Plans After the Project Starts
If you change your mind about something after building starts, it can delay the project. For example, if you decide you want more space or a new feature, your contractor might need to get new permits, which takes time. To keep things on track, try to stick with the original plans and avoid making changes once construction begins.
Ways to Speed Up the Process
To make your ADU project faster and more efficient, consider these strategies:
Hire Experienced Builders
Choosing builders or design-build firms with experience in ADU construction can help save valuable time. They are familiar with the process, know how to avoid common pitfalls, and have a network of trusted subcontractors. This expertise ensures the project stays on track and helps avoid costly delays.
Prepare Documents Early
Having all your plans, drawings, and required documents ready before you apply for permits can speed up the approval process. If you submit incomplete or incorrect documents, it can delay your project as the city may send them back for revisions. The more prepared you are, the quicker the approval process will go.
Consider Prefab ADUs
Prefab ADUs are a faster option because they are built off-site in a factory, reducing construction time significantly. Once the unit is ready, it’s delivered and assembled on your property, sometimes in as little as a month. If speed is a priority, prefab ADUs are worth considering.
Stay in Touch
Maintaining regular communication with both your builder and local city officials is key. By staying involved and checking in regularly, you can quickly address any issues, approve changes, and ensure inspections are completed on time. This proactive approach helps prevent any delays in construction or permitting.
Conclusion
Building an ADU is a big project, but knowing the steps and timeline makes it easier to plan. Most projects take 6 to 18 months to complete, depending on factors like location, size, and permitting rules.
Careful planning is key. By setting a budget, preparing documents ahead of time, and hiring experienced professionals, you can avoid delays and keep things on track. The right team, whether it’s a builder or architect, can speed up the process and reduce mistakes.
While building an ADU takes time, it can add valuable living space, rental income, and increased property value. Proper planning and a skilled team ensure the project runs smoothly and efficiently.
FAQ
Can I live in my house during construction?
Yes, you can live in your house during ADU construction. However, construction might create noise and disrupt certain areas of your home. It’s a good idea to plan for temporary adjustments, like using a different entry or avoiding the work zone.
Does an ADU increase home value?
Yes, an ADU can increase your home’s value. It adds extra living space, which is appealing to potential buyers, especially if they need a rental unit or additional family space. The increase in value depends on the ADU’s quality, location, and local market demand.
What if there are delays?
Delays can happen due to weather, permit issues, or material shortages. To minimize these, stay in contact with your builder and city officials, and plan for possible setbacks in your timeline. Having a buffer in your schedule can help.
Is a prefab ADU right for me?
Prefab ADUs are a great option if you want a faster build. They’re built off-site and installed quickly, often in as little as one month. If speed and cost are your main priorities, a prefab ADU might be the right choice, but make sure the design suits your needs.
