What Is a Detached ADU? Everything You Need To Know For 2024

Ever wish you had a little more space? With a detached accessory dwelling unit — also called a detached ADU, granny flat, or in-law suite — it’s easy to expand the amount of living space you have to work with.

For some, a simple garage conversion is all you need to create a comfortable guest room, home office, or studio. But if you are providing housing for aging parents or adult children for an extended period of time, or if you want to create a second revenue stream by creating a rental unit, a detached ADU might be the way to go. Here’s everything you need to know about building one of these small-but-mighty structures in 2024. 

Here at GreatBuildz, a free service that connects homeowners in Los Angeles, Orange, Ventura, and San Diego Counties with reliable contractors, we speak to homeowners every day about hurdles that can derail their renovation project – here are some ideas & tips to keep your project on track.

What Is The Difference Between A Detached ADU And An Attached ADU?

The biggest difference between a detached ADU and an attached ADU is that the detached ADU is a standalone structure while the attached ADU either converts part of your existing home — usually, a garage or large bedroom — or adds onto it. 

In California, ADU laws stipulate that an attached ADU can be no larger than half the size of the primary residence. So, if you own a 1,500-square-foot single-family home, an attached ADU cannot be larger than 750 square feet. A detached ADU, however, can be as large as 1,200 square feet. 

Related: Building an ADU Above Your Garage

building a modern detached adu

How Much Does It Cost To Build A Detached ADU?

Detached ADUs are usually more expensive to build than attached ADUs because you’re usually building a new house with its own foundation and support structure. It’s also possible to do a detached garage conversion, but this type of project is still likely to need all new plumbing, electricity, and HVAC. In Southern California, a one-story detached ADU usually costs $350/sf – $450/sf, so you can expect to budget between $300,000 to $400,000.

To help you gain a sense of your own ADU budget, try using an ADU cost calculator that takes into consideration not only the size of your ADU and the county you live in but also add-ons like solar panels, the type of terrain you’re building on and the current value of your home.

Because this is an expensive undertaking, many homeowners will finance their ADU project with a home equity line of credit, home equity loan, cash-out refinance, or other option.

What Regulations And Requirements Do I Need To Be Aware Of When Planning An ADU Project?

California’s ADU laws, which went into effect in January 2021, helped to clarify and create a cohesive set of requirements for property owners seeking to build ADUs on their properties. It’s important to note, though, that some municipalities have additional regulations. In the City of Los Angeles, for instance, one ADU ordinance requires there to be at least 4 feet of setback between a new unit and the property line. 

In addition to outlining where and how big you can build, ADU ordinances also spell out interior layout requirements. For example, the living room/bedroom must be at least 70 square feet, while the kitchen — including the hallways and closet — must measure at least 50 square feet. The bathroom, meanwhile, must be at least 30 square feet and include a toilet, sink, shower, or bathtub. 

You’ll also find specific rules regarding construction materials, electrical standards, and parking. No matter where you’re building — whether you’re in Orange County, Ventura County, or elsewhere in Southern California — it’s important to be aware of — and adhere to — all state and local guidelines as you proceed with your ADU project plans. If you’re not in compliance, you might be forced to correct or even demolish your project. 

Detached ADU insurance plans

What Are The Benefits Of Building A Detached ADU?

There’s no shortage of ways to put an ADU to work. Some people use them as guesthouses or as homes for aging parents. Others turn them into home offices, studios, or fitness spaces. 

You also can also use them as rental houses. Detached ADUs are especially appealing to renters because they’re larger and offer more privacy than attached ADUs. Depending on the size of the location of the unit, an ADU can bring in between $1,500 and $3,000 per month. 

Regardless of how you plan to use your ADU, adding more space to your home will increase your property value substantially. In Southern California, these projects can add between $200,000 and $500,000 to your home’s resale value — a remarkable return on investment.

What Is The Planning Process For Building A Detached ADU?

Given the complexity and expense of an ADU project, it’s important to speak with an architect early in the planning process to ensure your plans are in compliance with state and local regulations. Consulting a detached ADU contractor early in the process also is a good idea because you’ll get a clear picture of the process, costs, and timeline for the project. 

Start by speaking to a few architects/designers that are experienced with detached ADU plans (call GreatBuildz for recommendations). Most importantly, make sure the architect you’re considering is well versed in working with the city/county in which your property is located. Also, make certain they have experience designing detached ADUs locally. The cost for full plans and blueprints should be approx. $6,000 – $14,000 and should include submittal and approval at the city building department.

When asking general contractors to submit bids — which outline estimated costs as well as the scope of what is included in these costs — it’s important to be prepared with a full list of required features. Our ADU project checklist is a great resource for understanding everything that goes into a successful project and ensuring you get consistent bids.

Detached ADU Living Room

Is A Pre-Fab ADU Right For Me?

A recent trend in ADU construction is to purchase pre-fab units and have your general contractor put them together on-site. Referred to variously as modular, panelized, or factory-built ADUs, their main components are manufactured in factories and shipped directly to your home. 

At first glance, a pre-fab ADU might seem like a more affordable option than a stick-built structure. However, there often are additional costs for shipping the unit to your home and hiring a crane to lift it from the street and into your backyard. There also will be costs associated with installing a foundation for the unit to sit on and running utility lines for water, sewage, electrical, and gas. Foundation and utilities alone will cost an additional $30,000 to $50,000. There also will be labor costs for constructing the building itself.

Related: Prefab ADUs vs. Stick-Built ADUs: Which is Better?

Pre-fab ADUs do have an advantage in terms of timeline. An ADU builds using traditional construction methods typically takes between three and six months to complete, depending on the size and complexity of the project and allowing for delays due to weather or material backorders. Pre-fab ADUs, on the other hand, can be completed in as little as one to three months. 

When shopping for pre-fab ADUs, it’s important to make sure the manufacturer has received state and local approvals for the models you’re considering. This helps ensure the unit will stand up to Southern California’s environmental conditions. Additionally, if your jurisdiction requires solar systems to be installed on new buildings, you’ll want to double-check that the models you’re considering can accommodate one. 

It’s also important to understand that you might run into financing or appraisal issues. That’s because every manufacturer is different, which makes it difficult for lenders and appraisers to fully understand their construction methods. 

White ADU detached

How To Find A Great Detached ADU Contractor

Building a detached ADU is a major undertaking, and there are a number of regulations and requirements. With that in mind, it’s important to connect with experienced general contractors who know all the ins and outs of this type of project. 

At GreatBuildz, we make it easy to connect with the very best. Just give us a call and we’ll connect you with three contractors specializing in ADUs. We’ve pre-screened every professional in our network and can guarantee that each one is licensed, insured, and has a track record of success. 

We highly recommend reaching out to all three of our recommended contractors to request bids. Pay close attention not only to the bottom line but the scope of what’s included. Should you have any questions as you compare these bids, don’t hesitate to reach out. Our dedicated service team is always ready to help.

If you’re looking to renovate your home, you shouldn’t have to go it alone – GreatBuildz can help simplify your renovation experience.

GreatBuildz is a free service that connects homeowners in Los Angeles, Orange County, Ventura or San Diego with reliable, thoroughly screened general contractors and provides project support from start to finish. Call now (818.317.3567) to chat with a real person about your next remodel project or visit our website for more information: www.greatbuildz.com

Hi, we're Paul and Jon - Co-Founders of GreatBuildz. We believe everyone deserves to find a great contractor, have a stress-free renovation, and enjoy their beautiful new space. There are so many contractors out there and it's often hard to tell the good from the bad... until it's too late. We started our company to help simplify your contractor search and help you have a stress-free renovation experience. We're always available to help, no matter where you are in the process. Click here to learn more about our story.

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