All About the ADU

You may have heard the term, “ADU”, and wondered exactly what it is and whether it’s an available option to add functionality and value to your own property.  ADU stands for “Accessory Dwelling Unit” and is a secondary and separate living space on a single-family property with a primary dwelling unit.  It is not to be confused with “duplex” or “two-family dwelling”, where both units will usually have their own addresses, utilities, and meters.  ADUs can be attached to the primary dwelling unit (“AADU”) or detached and often located in a rear or side yard (“DADU”).  Some other common terms for these kinds of units are mother-in-law (MIL), garage apartment, carriage house, or backyard cottage.  Each jurisdiction will have its own terminology and definition, along with regulations on when and where it can be incorporated.  For the unit to be legally rentable, the space will need to be permitted by the local jurisdiction specifically for this use.

Increased Housing Density

As the population increases, more and more jurisdictions have included allowances for ADUs and encouraged their creation to help with housing density issues.  In some areas, regulations allow multiple ADUs on the property through a combination of attached and detached units.  While some jurisdictions require the property owner to live in one of the units on the property, others don’t have this requirement adding the potential for an additional rental unit.

Key Components of an ADU

ADUs, whether attached or detached, will have their own separate entrance from the outside (AADUs will usually also have a lockable door leading into the primary dwelling unit or a “common” area).  Jurisdictions will often specify this entrance be out-of-sight or shielded from view of the street.  Separate off-street parking may or may not be required, depending on the jurisdiction.  The ADU will need its own kitchen, bathroom, controllable heating system, and fire separation from the main dwelling unit.  Electrical service is usually required to be provided through a sub-panel.  Sometimes, “common” areas can be established between the ADU and primary units where equipment panels, utility equipment, and laundry may be located, allowing tenants of both spaces access without traveling through each other’s unit.  A separate water shutoff may also be required.

ADUs will be limited in allowed area, either through maximum square footage or a percentage of the primary dwelling unit area.  For both existing un-heated spaces being converted into an ADU (such as a garage) or for new ADU construction (such as an addition or a new DADU), the ADU will often be treated as added floor area and will need to meet jurisdiction requirements for the property such as maximum gross floor area and energy code specifications.

Common AADU Locations

Some of the most common AADU locations to be established in existing homes are within a basement, within part or all of a ga

rage (“garage conversion”), or within a main living area of the primary unit – often as an addition (or “bump-out”).  For garage conversions, it is important to know whether a garage is required on the property, and separate garage space may need to be included.  For basement units, the AADU entrance may require excavation to provide a door landing and steps leading up to grade, and a window well might be needed for a bedroom egress window.  An existing “walk-out” basement is often an ideal space for an AADU.

Common DADU Locations

DADUs are most often a separate self-contained structure in the rear or side yard of a property. Jurisdictions sometimes have reduced setbacks to allow a DADU to be located closer to the property line than a primary unit.  Height and area restrictions are used to keep the structure visually reduced in relation to the primary unit, and the exterior design is sometimes required to be in keeping with the primary unit for coherence.

DADUs can also often be added over a detached garage.  An exterior stair is usually used for access, so it’s important to have the space for this element.  An upgrade to the foundation and existing walls is often needed for the added load above the existing garage walls.

Is an ADU right for you?

An ADU can add solid value to your property, and often requires a minimum of construction and permitting.  Whether you have plans to add rental income to your property, or would like for an elderly parent, care-taker, or a college-aged child to be close to your home but have their own space, creating an ADU might be the right step for you to take.

Written by:  Sarah Thompson, project architect