How to Build an ADU: 8 Things You Must Consider

May 23, 2022

To quote the University of California, Berkeley’s Center for Community Innovation: In California, the ADU revolution has begun. Getting in on this fast-growing housing market requires building an accessory dwelling unit of your own. But the process is not as simple as hiring a contractor and calling it a day. You’ll need to manage every step of the process, from permits to construction to installation to furnishings, unless you hire an ADU company to handle the process for you. Whichever avenue you take, understanding how to build an ADU is key to a successful project. 

What is an accessory dwelling unit? 

An accessory dwelling unit (ADU) is an independent living space on the same lot as another home. A small home in a backyard is a common example, but they come in many sizes and forms, including conversion of an existing structure like a garage or building an attachment onto your existing home. ADUs are fully functioning homes with all utilities and facilities, including a kitchen. In that way, it stands apart from guest houses, which do not have kitchens.

ADUs are especially popular in California, where the housing market is quite competitive and rents there are among the highest in the United States. As a result, there’s a demand for a fresh supply of affordable housing. An increasing number of Bay Area homeowners have built ADUs as an option for these tenants, from whom they can earn substantial rental income. 

Both within and outside California, ADUs may also function as “granny flats” where homeowners’ aging parents can live. This approach to using ADUs is basically a reversal of moving back to your parents’ area as they grow older to help care for them. Instead, they can live on your property but in a separate space. With this arrangement, you can care for your parents without uprooting your own home.

Types of ADUs

As you plan your ADU project, you’ll encounter several types of ADUs. Below are the four most common types you’re likely to see. 

Architect at work
Image by Daniel McCullough on Unsplash
  1. Detached ADU. A detached ADU (DADU) is fully separated from your existing structures. They can be built new from the ground-up, or they can be a converted shed, garage, pool house, or other existing structure that shares no walls with your house. It’s the best option if you’re looking for the most valuable ADU to add both square footage and dollar value to your property.
  2. Attached ADU. An attached ADU typically shares a wall with your home. In some cases, such as an ADU you build atop an attached garage, your ADU’s floor may also be attached to your home. Note that, in California, you can only increase the size of your existing home by at most 50 percent. That means attached ADUs have built-in size limits, whereas only your lot size and local code restricts a detached ADU’s square footage.
  3. Garage conversion ADU. A garage conversion is typically the least expensive way to design and build an ADU. After all, your garage is already on your property – you just have to make it a fully independent, livable space. That said, if you’re looking into ADUs as investments you can sell for a profit later, losing a garage might let you down, as the separate unit can command a higher value at sale.  Additionally, if a garage is not in good shape, a garage conversion could be substantially more expensive than a detached ADU.
  4. Interior conversion ADU. Sometimes, an ADU can exist inside your home. If there’s a part of your home you barely use – say, the basement – you can convert it to a livable space. You’ll also need to create a separate entryway, outfit it with the usual household appliances, and ensure utilities are up to par. Going this route removes the potentially lengthy steps of site and foundation preparation from the ADU construction process. However, it limits your ADU size to your basement’s existing footprint.

How to prepare to build an ADU

Before you start breaking ground on an ADU, you’ll need to be sure you can actually do so. There are two key steps to ensure you’ve cleared all legal hurdles: Following the regulations of your municipality for residential dwellings, and securing the proper permits. Here’s what you need to know about each.

1. Check your state and local ADU regulations.

Woman working on a computer
Photo by Annie Spratt on Unsplash

California law sets out guidelines for the allowed square footage and height of your proposed ADU, among other requirements. Additionally, your plans must meet California’s Title 24 energy-efficiency requirements in newly constructed buildings.

Local ordinances may also affect what you can and can’t do while building your ADU. For example, under San Francisco law, you can only add one ADU to a lot with four or fewer residential units. Assuming your house is on its own lot, that means you can only build one ADU on your property. Additional restrictions may pertain to fire safety, parking options, and your ADU’s entryway requirements, which may need to be built for accessibility.

An ADU design and installation firm can help you navigate your area’s unique ADU requirements. These firms routinely go through the approval and permitting process as part of their design and installation work. During this phase of building, the firm will combine your regulations with its designs into ADU plans that the government reviews. That takes you to the next step.

2. Secure the proper zoning and building permits

All your ADU plans go into something called a “permit set” that your city and county regulatory agencies review. You’ll absolutely need to hire a designer, architect, or turnkey ADU services provider to create your permit set. Many permit sets are 100 to 200 pages – not the kind of thing you can properly put together without expert help. Plus, if you’re looking for the fastest, most hassle-free ADU process possible, do you really want to do all that hard, tedious work yourself?

Once your city and county regulatory agencies review your permit set, they may approve it as is or return it with requests for changes. Don’t let the term “requests” fool you, though: These changes aren’t optional. Once their requested changes are implemented and all is approved, the city will issue a permit for your project. And then, the ADU building process can begin.

How to build an ADU

With your permits in place, you (or your ADU design firm, if you’ve hired one) are ready to start building your ADU. Here’s how the process goes.

1. Site and foundation preparation

Site and foundation preparation is basically the process of getting your backyard ready to accommodate an ADU. To prepare your site for your ADU, debris and obstructions will be cleared from your lot and the site where you plan to build. Demolition may also be a part of the process if certain structures interfere with the ADU building process. Once that’s all taken care of, it’s time to run the utility lines.

After all that hard work, the site will be excavated so that it can properly receive your foundation. The next step is pouring concrete into this excavated site so it hardens up and forms your foundation. At this point, your lot is in prime condition for your ADU to be put into place (or, in Spacial’s case, craned into place).

2. Construction

Construction worker
Photo by Josh Olalde on Unsplash

The construction process typically begins once the foundation is poured and set. However, select ADU firms complete construction off-site, which offers its own advantages over on-site construction. We’ll dive into the details of off-site construction here. 

While the site and foundation are prepared, the ADU firm that conducts off-site construction will start that process. That’s an important detail – when offsite construction happens at the same time as site and foundation preparation, you can shave months off your ADU construction timeline. Plus, your daily life won’t be interrupted by loud and intrusive construction work around your home.

Before your ADU firm starts assembling your unit offsite, you’ll choose your design from a selection of pre-established ADU models and floor plans. That means you won’t need to design your own ADU, either. The only choice you’ll have to make is exterior color. Otherwise, the entire process is in the hands of an ADU construction expert.

The result of offsite construction is a shorter, less costly process. After all, preset designs prevent you from going overboard with unnecessary design bells and whistles. Furthermore, your ADU can divide and lessen its architectural design costs across several units. That’s just not possible if you were to build your ADU from scratch, which might take a year and a half. Offsite construction caps the process at nine months – quite literally half as long as the typical construction timeline.

3. ADU installation when constructed off-site

Once your site is ready, your foundation is poured, and your unit is fully constructed offsite, it’s time for your ADU to make its way into its permanent home. Although this part of the ADU phase is often associated with your backyard becoming unavailable and chaotic for days on end, some ADU firms can circumvent that whole concern. In fact, offsite construction means it’ll only take a day or so of backyard commotion to get your ADU into place.

Or, at least that’s the case if you choose Spacial as your ADU design and installation firm. That’s because, unlike other ADU firms that assemble ADUs on-site from individual panels,  Spacial installs single-unit, ready-to-go volumetric modular units. In other words, that means we can literally drop your ADU into place after our site and foundation prep – doing so takes well under 24 hours. 

4. Last-minute touch-ups and finishes

Before your ADU is move-in ready, you’ll need a contractor to come in for some final touch-ups. These touch-ups can take on a few forms: Some can be small tasks like peeling protective coating off your windows and floors, while others may be larger projects, like adding steps or installing a skirt around your ADU. On the outside of your ADU, your contractor can wrap things up with sidewalk and patio installation, driveway renovation, new landscaping, and other finishes. 

While none of these last-moment additions are requirements for completing your ADU, they’re all highly recommended. They can make your ADU more usable for those living in them and add extra value and beauty to your property.

5. Certificate of occupancy issuance, furnishing, and move-in

Insurance certificate
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With all the above checked off your list, you’ll need to obtain a temporary certificate of occupancy (TCO) for your ADU. You’ll also get the keys – and that means your project is nearly done. With that TCO in hand, you can begin to furnish your ADU. Spacial handles that for you through our partnership with West Elm. 

After just a bit longer, your ADU firm will present you with your final certificate of occupancy (CO). You need this permit before anybody can move into your ADU. It might seem like a long wait, but if you’ve chosen Spacial, all the work will already have been done by move-in time. Whomever you want to move in can do that pretty much right away once you have your CO in hand. And then, your ADU project is officially complete.

7 things to consider when building an ADU

Building an ADU is quite a process from start to finish, and an especially daunting task to handle on your own. But with the help of a knowledgeable and helpful ADU firm, you’ll have expert assistance while making some big decisions about your ADU build. Among the decisions you’ll need to make are:

1. The purpose of the ADU you want to build

mini figurine of The Thinker
Photo by Tingey Injury Law Firm on Unsplash

An ADU can serve many purposes – housing older family members, generating rental income, giving guests a comfy place to stay, or creating your own private area for work or artistic pursuits that need space, like sculpting or painting. But an ADU that works well for one of these purposes might not work as well as for others. 

A DADU is best for renters or family members who need their own separate space, while a conversion or an attached ADU may be acceptable for adding office space to your home. Similarly, in cases when you want someone to move into your home but with their own individual living space – perhaps a parent for whom you’re caring – an interior conversion can be a great choice.

2. The cost of contractors (and the time you’ll spend finding them)

Unless you’ve renovated your home several times, you’re probably not used to the process of finding, screening, and hiring contractors. You’ll need to look up and down for reputable contractors for each part of the ADU building process. You’ll need to ask each contractor a whole lot of questions to ensure they’re a fit. But what if you could avoid all this hassle?

With Spacial, you can do exactly that. We’re your all-in-one solution for your studio or one-bedroom ADU project in the San Francisco Bay Area. We’ll handle your permitting, site preparation, foundation pouring, offsite construction, ADU installation, final touch-ups, and occupancy certificates all under one roof. You’ll primarily pay a fixed cost for your unit and these services – there are far fewer variables with Spacial.

3. How you’ll insure your ADU

Just as you insure your home, you’ll likely want to insure your ADU. Many homeowners insurance companies will gladly insure your ADU as well, and you should contact your insurance carrier to learn more about their options. You may also be required by law to purchase certain insurance coverage types.

This consideration is especially important for any ADU that isn’t an interior conversion. These ADUs include exteriors exposed to the environment, meaning that they may require additional coverage. Without this additional coverage, your current plan might not cover damage to your ADU. 

4. The steps you’ll need to take for your foundation

Home foundation
Photo by Scott Blake on Unsplash

Depending on where you’re building your ADU, you may need to take certain steps before pouring your foundation. For example, some city or county regulatory bodies will require you to test the soil (with a geotechnical report) on your lot before you pour your foundation. These steps can extend your ADU project’s timeline. That said, if you choose an all-in-one ADU firm to build your unit, these steps – and the time they take – are already factored into the process. 

5. Any changes you’ll need to make for utilities

An ADU needs its own sewer, water, electric, and cable, and all the appliances that come along with it. In some cases, your existing utility lines and other infrastructure can accommodate your ADU. But for some properties, you may need to install more infrastructure – say, another electrical connection – to accommodate your ADU. That needs to be considered as part of the construction process before you embark on an ADU project.

6. How you’ll go about interior design

Living Room of Spacials One Bedroom ADU
Photo by Spacial

Often, building an ADU on your property also means furnishing it. This key step can add weeks more to your project, not to mention additional tens of thousands of dollars. It can also introduce some more abstract stressors such as, say, deciding whether your interior design ideas will please your occupants. But it doesn’t always have to be this way.

With Spacial, the notion of an all-in-one solution extends to furnishing your ADU. We’re partners with West Elm, so we can furnish your ADU on your behalf. This partnership means we’ll have this furniture ready to go the moment you get your TCO, eliminating all that needless extra furnishing time. It’s a one-stop shop for your ADU’s interior design. 

7. Your timeline

At the end of the day, any ADU project – no matter how many steps you can streamline – will take many months. And no, you don’t want to cut corners – the time it takes to build a high-quality, well-built ADU is well worth the time spent. What you do want to do is choose an ADU firm with a proven track record of maintaining this quality on a timeframe both realistic and condensed. Spacial’s process, which takes well under a year, is a great example: It’s enough time to get everything working perfectly, yet short enough that move-in day will arrive before you know it. 

Building an ADU the easy way with Spacial

Exterior of Spacials One Bedroom ADU
Photo by Spacial

With all the complicated moving parts involved in building an ADU, it’s not advisable to go it alone. With Spacial, a Bay Area ADU specialist handles the whole process for you from start to finish. The details of each ADU are carefully considered, a high quality build inside and out. Available in studio and one-bedroom layouts, our detached ADUs are fully assembled off-site to respect your property and your time. And we work closely with the proper authorities and handle the permitting process so you don’t have to. By partnering with Spacial, you can rest assured that your new ADU is in the right hands.