When your loved one needs a place to stay, what do you do? You give them a place to rest. And when they need a place to live, you do everything in your power to ensure they have a home.

That’s why so many families in California are now turning to accessory dwelling units as an affordable, realistic housing option to help their loved ones thrive.

If you’re thinking of building an accessory dwelling unit, it pays to be prepared. Here’s the ultimate guide to ADUs, walking you through all the important points you need to consider and all the major steps of the building process.

What is an Accessory Dwelling Unit?

An accessory dwelling unit, or ADU, is a legal and regulatory term for a secondary dwelling unit sharing the building lot of a larger, primary residence such as a house.

You may have also heard of them as:

  • Attic apartment
  • Backyard cottage
  • Backyard house
  • Basement apartment
  • Carriage house
  • Garage conversion
  • Garden cottage
  • Granny flat
  • Mother-in-law suite
  • Mother-in-law unit
  • Secondary dwelling unit
  • Tiny house (but not quite the same)

However, it doesn’t matter if you know it by any other name. An ADU is a smaller secondary residence that always has a few features that qualify it as an ADU:

  • It has its own water hookup
  • It has its own utility hookup (electricity, gas, etc.)
  • It is a fully functional living space (kitchen, bedroom, bathroom, living room, etc.)
  • It has a foundation (its own foundation or foundation shared with the primary residence)

Think of it as an apartment, but without a full apartment building. Keep in mind that tiny houses may not always qualify as ADUs if they do not have utility hookups and foundations, and mobile homes or trailers are not ADUs--ADUs must be permanent residences.

Types of ADUs

There are four types of ADUs:

  1. Detached ADU
  2. Attached ADU
  3. Garage conversion
  4. Interior conversion

Think of a detached ADU like a cottage, carriage house, or little apartment in your backyard. It is a freestanding, fully functional independent living space with its own utilities and its own entrance to the property. Someone could live there with the same level of independence from the main house as if you were next-door neighbors, if they were so inclined.

An attached ADU is slightly closer to home (literally). Attached ADUs share one wall with the primary residence, though there is no internal entrance--you have to exit the primary residence to enter the ADU. Attached ADUs may have their own utility hookups or share it with the main residence, but they must be fully functional living spaces.

Garage conversions may be attached or detached, depending on your garage. The idea is exactly what it sounds like on the tin: a garage converted into a complete apartment space. Thanks to new reduced parking requirements, you may not need to replace the lost parking.

Finally, there are interior conversions. This isn’t a spare room, but a fully functional apartment, often in the basement or attic. It may not be visible from the outside, but the unit could function completely independently from the main residence.

Before You Get Started

ADUs have seen a boom in the last several years--in fact, a raft of 2019 housing laws were geared toward creating a backyard building boom.

That’s largely to the credit of California’s affordable housing crisis, which has resulted in skyrocketing homelessness, a consistent insufficiency of new construction, and the worst poverty rate in the nation. California is home to four of the country’s most expensive real estate markets, with median home prices more than twice the national average.

And for individuals already in crisis, the economic fallout of the coronavirus may push many over the edge into untenable housing situations.

Of course, as a family member, you’re not thinking of the pandemic or the poverty rate in California. You’re just thinking of finding a home your loved one can afford. And that’s where ADUs can help.

But before you start building, it helps to keep a few things in mind.

Think About Your Reasons for Building

Before you look into the cost of building an ADU, the materials you might need, or the permits you’ll have to acquire, you need to ask yourself one question: why do you want to build an ADU?

It sounds like a simple question, but it opens up a whole realm of possibilities--all of which could radically alter the outcome for your ADU.

For example, are you building for:

Each of these individuals will need a slightly different space. An elderly loved one, for example, may need an ADU that is suitable for limited mobility and is built to account for potentially reduced mobility as they grow older.

A loved one who needs health support may benefit from an ADU with greater access to the main residence, while a loved one who needs just a bit of familial support but not 24/7 assistance may thrive in a more independent space.

A young adult recently returned from college want independent space that feels like their own while they get their feet under them. Older boomerang kids may have their own spouses or young families and need space for more than one person. Similarly, a multigenerational living solution may need to account for multiple family units.

Consider the Type of ADU

With this in mind, think about the type of ADU that may be best suited to your loved one.

If you have a loved one with limited mobility, an ADU with stairs or an attic or basement conversion may be inaccessible for them. On the other hand, if your loved one has their own family, an attic or basement conversion might be enough to drive everyone up the walls.

On the other hand, if you have a loved one who needs in-home care, you may want a unit that’s closer to home so that it’s easier for you to access your loved one (and conversely, for your loved one to access you).

Look at Your Property

On the other hand, the type of ADU available to you may depend on your property.

First, it depends on whether you live in a single-family residence, a multi-family residence, or a duplex. Single-family residences are allowed one ADU and one JADU (junior ADU) with restrictions, while multi-family homes and duplexes may allow two detached ADUs and one attached ADU (subject to restrictions).

This will also depend on the space you have to work with.

For example, if you want to convert your attic but there’s not enough space in the attic for a fully-functional living space, an attic conversion may not be in the cards. If you want a backyard detached ADU but don’t have enough space on your lot to meet zoning requirements, you may need to explore other options. In fact, attic ADUs are notoriously tricky projects and most builders avoid them.

The key is to find the balancing point between what your property allows and what your loved one needs.

Consider Any Special Requirements

With that in mind, think about any special requirements that may apply to your loved one.

Limited mobility is a significant consideration in this case, especially if your loved one relies on a wheelchair. While your loved one’s limited mobility may be the reason they’re not able to live in your house with you, your ADU must meet the accessibility requirements lacking in your house or it won’t do you any good.

The key is to think about universal design (i.e. design that is accessible for all). For a loved one in a wheelchair, for example, that means more than flat living spaces--it means wider doors, doors that slide into the wall instead of opening out, storage set low enough to reach.

Also, keep in mind that accessibility applies to far more than just mobility.

NOMA Gallaudet University in Washington DC, for example, is a pioneering design space for the deaf community--think 45-degree corners instead of 90 degrees so that people who are signing can continue a conversation without running into each other, spaces lit to limit eye strain, circular study spaces for people who need to see each other to communicate.

When in doubt, talk to your loved one! Ask them what they wish they could see in a living space that would improve their quality of life, then see how you can add those features into your ADU. After all, if you’re designing a living space for your loved one, you want it to be a true haven where they can thrive.

Think Long-Term

With that in mind, think about your loved one’s long-term needs as well. For a senior loved one, for example, you have to think about what your loved one may require as time goes on.

Let’s say that your loved one can live independently and doesn’t need home support but doesn’t have the income to afford independent housing. They may not need accessible space now, but their needs may change in the future, and you don’t want to build a home for them that won’t serve their needs for years to come.

Similarly, if a loved one is largely mobile now but may reasonably expect to have mobility issues in the future, planning ahead for such concerns will make the transition considerably easier as the years go on.

Remember, if you’re building a home for your loved one, it should be a space that they can use for many years to come, not just the next few years.

Again, talk with your loved one about their needs and wants, and talk about what you expect this new living situation to be like. Are you looking into it so that your parents can help provide extra support with the kids? Are you aiming for an affordable living solution that retains their independence? These situations call for two rather different units.

Look Into Cost

From there, you can start to think about the cost of building your ADU.

Like any construction project, it will take some investment on your part. You have a few financing options available, but how much you need and what type of financing you opt for will depend on the unique features of your project.

What Contributes to ADU Building Costs

The exact cost of an ADU depends on the project in question, so it’s difficult to give an exact number without knowing the details of your project. That said, most ADU projects have a few cost features in common, though the exact pricing of each will vary.

Common price factors include:

  • The type of ADU (attached, detached, conversion)
  • Whether you’re building the ADU from scratch
  • Location (city, county, even neighborhood)
  • Whether you need new utility hookups or you’re retrofitting existing utilities
  • The size of the unit
  • How you design the space
  • Materials
  • Permits
  • Impact fees such as parkland and school fees (depending on the size of the unit)
  • Whether you use a custom plan or pre-approved plan
  • Additional professional fees, such as building plans and construction crews
  • Landscaping
  • Furniture

The City of San Jose has a Permit Cost Estimate Worksheet for your reference.

Ways to Lower the Cost of an ADU

With that in mind, there are several ways to lower your overall project costs.

A custom plan can often cost more than a plan pre-approved by your city (if they are available), if only because it can make the permitting and inspection process take longer.

There are some cost considerations that will depend on your goals. Hiring an architect and contractor can be more expensive depending on how you use their services, but on the other hand, designing and building your ADU yourself can be more expensive than hiring professionals because you’re more likely to make mistakes that prolong the process.

The key is to strategically invest in the costs you need and avoid the costs you don’t. Hiring a specialist for your project is recommended to ensure a smooth design and permitting process, and you should certainly have specialists do specialized work like electrical wiring or utility hookups. Professionals also know how to contact companies for a project.

On the other hand, if your loved one doesn’t need a lot of space, you may be able to save money by constructing a smaller unit to suit their needs (talk to your loved one about this). If your loved one has their own furniture, you may not need to worry about furnishing the unit yourself--besides, it makes the space feel more like their own home.

Investigate Your Experts

With that in mind, it pays to investigate your experts in advance and pay for the experts you really need.

As a rule, hiring a specialist ADU contractor at the outset is one of the best investments you can make to simplify the ADU process. We’re talking about builders who specialize in ADU construction, permitting, and regulation, not house builders who are scaling their project down.

These are the people who can look at your property and understand its potential. They answer a few key questions for you:

  • Can I build an ADU?
  • What can I build?
  • Where can I build it?
  • What are my building options?

They are the ones who look at your property parameters and help you figure out the best investment for your unique living space. They also know how to design an ADU plan that is guaranteed to get approved for permits.

And most of all? They know how to assess the full cost of the project, not just the “box” cost of building the ADU. They’ll be able to tell you the realistic permitting costs, utility fees, labor costs, and other critical cost elements that you might overlook or underestimate.

The trick is to find a builder who knows what they’re doing and is more interested in building a space that truly meets your needs (as opposed to making a quick buck).

If you’re in love with the idea of a backyard cottage but your property just isn’t suited for it, a builder should tell you--and offer you good options to consider instead. They should be able to translate what you want from a living space into a new design, even if they have to change your original idea from a backyard cottage to a basement conversion or an attached ADU.

Finally, make sure to find a contractor you’re comfortable with. They’re going to spend a lot of time working closely with you--you should feel comfortable going into that process in order to build a space you’re happy with.

Quick Tip: Ask questions and look at examples. How extensive is the company’s portfolio? How many years have they been in business? Not to mention online reviews.

The Building Process

Once you’ve found the right builder for your needs, you can start to talk about the building process itself.

Review the ADU Universal Checklist

A good first step is to work with your builder to understand ADU regulations in your area. This will give you a better sense of the rulebook you’re operating with and what your options are under those guidelines. An experienced builder will be up-to-date on all the current building regulations.

Some cities, like San Jose, offer tools for regular homeowners to navigate the process. If you live in San Jose, take a look at the ADU Universal Checklist--this will be your new best friend as you progress through the ADU building process. It clearly states current regulation and walks you through the process one step at a time to ensure you create a plan that works.

The Permit Process and Submittal Package

Once your ducks are in a row, you’re ready to start looking at the permitting process. For San Jose residents, there are several documents to help with this:

Your ADU Universal Checklist can help guide you toward the permits you need for a certain ADU, but your builder is also your best resource on this front.

Acquiring a permit involves submitting your building plans to City Hall. For that, all homeowners are recommended to have the plans drawn up by a professional builder or architect--plans involve a lot of technical codes most homeowners are not aware of, and City Hall will not process incomplete or inaccurate plans.

For the submittal package, you’ll need to complete and include all of the following:

  • Building plans
  • Form 310: Building Permit Application
  • Form 312: ADU Property Owner Declaration
  • Form 302: Address Assignment Request
  • A letter from your water company stating flow data from the nearest hydrant
  • Fee payment

You, your designer or your builder can submit your ADU package, and your builder can help get the appropriate letters from your water company.

Once your plans are submitted and approved and the permit is granted, you can start construction with your builder and get well on your way to providing your loved one a cherished home.

Thinking of Building Your Own Accessory Dwelling Unit?

When it comes to providing a home, an accessory dwelling unit offers your family the opportunity to come together during difficult times. Our job is to make that dream a reality.


We’re housing advocates and expert ADU builders who have served 20 cities and jurisdictions in the San Francisco Bay Area for over 30 years. Get in touch today to let us know how we can help your family construct a home.