What is the Best ADU Size? (How To Choose The Right Size)

Finding the right size for an ADU (Accessory Dwelling Unit) isn’t a one-size-fits-all situation.

Different projects require different approaches, but keeping the size under 750 square feet is typically a good option.

This size often avoids development limits that could complicate construction, making the process smoother and more cost-effective.

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Factors to Consider When Choosing the Best ADU Size for Your Project

Choosing the best ADU size is a very personal thing.

What works for one person might not work for the next, so here are some things to keep in mind.

Lot Size & Restrictions

The available lot size is crucial. Building a large ADU on a small plot might not just be challenging—it could be impossible due to zoning restrictions.

Always check local regulations as well, as different cities can enforce their own restrictions on ADUs, which can make the process more complicated.

Type of ADU	Maximum Size
Detached ADU	Up to 1,200 square feet
Attached ADU	Up to 50% of the primary residence, with a maximum of 1,200 square feet
Converted ADU	Typically within existing space, but can be expanded up to 150 square feet if needed to accommodate egress or entryways

Check out our guide on ADU sizes for a better idea of the restrictions that you can face with certain sizes.

Use

How you plan to use the ADU is another key factor.
A home office requires less space than a rental unit. Think about what will happen in the space to determine the appropriate size.

Budget

Your budget plays a massive role.

Larger ADUs cost more to build. If you’re on a tight budget, a smaller ADU might be more feasible and still offer great value.

Example ADU Sizes and Applications

ADUs come in many different shapes and sizes. Here are some example sizes and what they can be used for.

Detached ADU With 2 Bedrooms (1,000 sq ft)

Large ADUs approaching 1,000 sq ft tend to be multi-bedroom units that are used as rental units or to house family members or close friends.

Remember that the maximum size of an ADU in California is restricted to 1,200 sq ft, so this is getting close to the maximum size you can use.

Home Office (150-300 sq ft)

A home office is one of the most popular uses of ADUs, especially since the transition to remote work has become more prominent.

300 sq ft is usually the maximum size for this type of application, especially if you plan to use your home office for business use.

Garage Conversion (450-1,000+ sq ft)

Garage conversions are very popular choices for ADUs, and depending on the existing space, they can vary in size drastically.

Garage conversions are most commonly between 450 and 900 sq ft, but they can reach over 1,000 sq ft in some cases.

This is an ideal mid-sized option that can function as a rental unit, guest house, or anything else.

Benefits of Adding an ADU to Your Property

Let’s take a quick look at the benefits of adding an ADU to your space, no matter the size.

Extra Living Space

If you’re tight on space and need some more room, building an ADU is a fantastic option.

ADUs allow for either attached or detached space that can be used exactly how you like.

If you need a new home office to make working from home more enjoyable, that can easily be done. Likewise, if you need some rental income and have a garage that can be converted you can also create a rental unit as well.

Rental Income

ADUs can be rented out to provide a rental income.

This is very popular with ADU owners and a great way to justify the investment into building an ADU.

Utilize Unused Space

ADUs can increase property value, and especially so if you’re using up space that was otherwise not used for any other purpose.

For example, if you have some space in your backyard that has been neglected over the years and isn’t being used, an ADU is a great way to use it efficiently. It may also benefit you if you were to sell the lot.