What Size ADU Should You Build?

When building an ADU in San Diego, size matters! What size ADU will be right for you depends on several factors.  (Secret tip: when building an ADU in San Diego, bigger is not always better.)

The Basics

The basic size limitations handed down from the State of California regarding the size of an ADU are generally that an ADU can be up to 50% of the size of the primary residence, not to exceed 1,200 square feet.  That means if your home is 1,800 s.f., for example, you can build up to a 900 s.f. ADU.  If your home is 3,000 s.f., you can build up to the maximum allowed of 1,200 s.f.  Some local municipalities have exceeded the State size parameters by allowing the construction of up to 1,200 s.f. ADU regardless of the size of the main home.

Space/Lot Size

First, consider the space that you have available on your lot to build an ADU.  While many local San Diego County municipalities, including City of San Diego, Encinitas, Delmar, Carlsbad, and some others have reduced or eliminated many of the setback requirements related to building an ADU, other municipalities have not.  And, while the reduced setback requirements may provide some additional space in which to plan and build your ADU, due to some of the smaller lots that San Diego is historically known for, space may still be the limiting factor in designing your ADU.


Consider what your budget is to build an ADU.  This is important to know because more square footage comes with a bigger price tag, as would be expected.  And, in San Diego, budgets can also be stretched just because, well…San Diego!

Intended usage – How do you intend to use your ADU?

  1. Family – Around 30% of our clients have extended family, relations, or acquaintances that intend to inhabit the new ADU. An ADU is a much more economical way of providing a new independent home than the traditional method of buying a new lot, bringing in utilities, and developing as a new residence. Having family live in an ADU generally leads to larger designs and more square footage, as these ADU’s are intended for comfortable, long-term habitation.  We design most of these ADU’s in the 700 s.f. to 1,200 s.f. range to include multiple bedrooms, office/den spaces, living areas, bath, kitchen, storage, and laundry areas.  Many of them also have independent and private exterior living areas as well.
  1. Rental Income – About 30% of our clients are building their ADU’s to use as rentals to non-family members. Many of our clients start off thinking a larger two-bedroom rental might be the way to go.  However, if you examine the rental market in your area, you may find that a 2 bedroom rental doesn’t fetch as much additional rent to offset the cost of building the additional ADU square footage as you might think.  Also, it could be that many potential renters don’t really need the additional bedroom or square footage as much as they might be looking for a slightly lower rental rate.  That often has our ADU clients thinking that maybe they can rent out the 2nd bedroom to an additional renter.  That ADU rental scenario presents its own challenges and the complications of renting and governing an ADU with two separate renters.  Many of the self-contained rental ADUs we design for our clients ultimately fall between 500 s.f. to 800 s.f. and include a sizable living area, bedroom, ensuite bath, laundry area, kitchen, and ample storage.
  1. Hobbies and Accessory usage – The other 40% of our clients are using their ADU’s for purposes such as craft & hobby areas, gyms & workout areas, storage, and office space, or business usage. In order to qualify as an ADU, the constructed space has to have a bathroom, kitchenette, and living area. Fortunately, an experienced design-build contractor is usually able to incorporate these necessary features into an ADU in a way that they will integrate or, at the least, not interfere with the accessory usages that our clients are planning to use their ADUs for.

Simply put, thoughtful and efficient planning and space design based on your intended usage for your new ADU in San Diego will be the key to both meeting your needs and maximizing your budget. This is where a great design-build contractor with experience building ADU’s in the San Diego and surrounding areas can often be the best place to start your ADU experience.

Specialty Home Improvement is a family-operated design-build residential contractor, specializing in creating homes and spaces designed to suit the specific needs of our clients since 1993.  If you’re ready to start the process of building your personally designed ADU, give us a call now at 760-738-6270 or reach out through our website www.specialtyhomeimprovement.com