What are the different types of ADUs?

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ADUs, or accessory dwelling units, go by many names and take many forms.  Sometimes called a backyard home, a granny flat, a shed house, or a casita, these terms are ultimately interchangeable.

Whatever you call it, an ADU can be a great investment and add value to your home.  However, some key differences in permitting, design, construction, and processes can have a huge impact on what the finished product looks and feels like.  Before you start your backyard house project, it’s important to get your terms straight.  

What’s true for all backyard homes?

Though there are differences in the choices you can make for your project, some things are true for all backyard homes in California because of state law:

  1. They are legal to build on any single-family zoned lot
  2. They can be up to 1,200 sqft and up to 16 ft tall
  3. Cities can’t require your home to have a minimum lot size to be eligible for an ADU
  4. Cities may not impose rules that prevent you from building any ADU that’s less than 800 sqft  
  5. Cities can’t require you to provide on-site parking, and can’t prohibit you from building an ADU in existing parking space (for example, by converting a garage) if you’re within a half-mile of transit
  6. Cities must respond to a permit application within 60 days and must evaluate your project on established, objective standards

What are the main types of ADU construction?

Prefabricated ADU Construction

Prefabricated (“pre-fab”) construction involves a unit that is built entirely off-site in a factory before being transported to your home and placed on a pre-constructed foundation. Pre-fab backyard homes are ideal for those who are looking for greater cost certainty and fast installation times. A pre-fab backyard house build can be completed with on-site construction taking as little as 2-4 weeks — however, supply chain challenges mean that production of the ADU can take up to 6 months.

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A Modular or Prefabricated Backyard Home


  • Predictable costs
  • Factory conditions provide greater quality control
  • Faster on-site construction minimizes nuisance to neighbors
  • Potential for designs to be pre-approved by your city


  • Limited choice in design and ability to customize
  • Subject to manufacturer’s capacity
  • Impacted by supply chain shortages
  • Unlikely to match the primary dwelling’s architecture and style

Modular ADU Construction

In modular ADU construction, smaller components of the ADU, or modules, are manufactured offsite, transported to your home, and put together on-site in a process similar to building with Legos. Modular construction may eventually enable a high degree of customization at a low cost, but the sector is still taking off.  


  • Low cost relative to stick-built backyard homes
  • More design options (materials, trim level, finishes)
  • Potential for designs to be pre-approved by city


  • Limited number of builders using modular limits availability
  • Unlikely to match the primary dwelling’s architecture and style

A Stick-Built Detached Granny Flat

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Stick-Built ADU Construction

Stick-built construction simply refers to traditional construction — think construction workers pouring cement and nailing boards together.  This typically involves an architect, an engineer, and a design team.  For those who want their ADU to match the look and feel of their primary dwelling, stick-built construction is likely the best approach.


  • Highly customizable
  • You can be very involved in the design process
  • Can match your existing home


  • Architecture and design costs can add up quickly
  • Availability can vary due to local contractors’ capacity to take on projects
  • The construction process can be a nuisance

What are attached and detached ADUs? What’s a garage conversion ADU? What’s a new build ADU?

Attached Backyard Home vs. Detached Backyard Homes

These terms refer primarily to whether an ADU is connected (attached) or disconnected (detached) from the main house. An attached ADU could convert an existing garage or add square footage to the main house, while a detached ADU usually involves converting a standalone garage or building new square footage in the backyard.  This distinction is important for how you’ll eventually use your ADU, but also for how existing permitting and urban planning regulations affect the approvals and construction process.

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A Detached Garage Conversion ADU
Backyard home ADU detached ADU new build house parents ADU casita granny flat guest house garage conversion prefab adu modular adu
An Attached Garage Conversion ADU

New Build Backyard Homes versus Garage Conversion

New build refers to units that are purpose-built, ground up, in empty backyard or parking space. Garage conversions take existing garages and renovate them to make them habitable.

Garage conversions tend to be significantly less expensive than other methods of building a backyard home, but the constraints of a garage can create challenges for providing adequate natural light and ventilation – it’s worth talking with us or a general contractor to get an expert perspective on whether your garage is really suitable to become a backyard home.

How Revival Homes can help

We’re here to support you at every stage of your ADU project.  If you have additional questions about ADUs, or want to discuss your project with an expert, book your free consultation with us today!


More information about ADUs

No matter where you are in your ADU journey, our educational resources can guide you through every step of the process.

How much will your ADU project cost? This post explains how to create a realistic project budget and manage your costs.
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Let’s take a look at how an ADU can help you or your loved ones live comfortably in their golden years.
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Are you thinking of building an ADU to rent? Before you get started, read our guide to becoming a successful ADU landlord.
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