What is an Accessory Dwelling Unit and What Are the Requirements to Have One

Apr 22, 2025

Since my last post about “aging-in-place” on July 28, 2024, my brother-in-law’s health situation has declined further making it inevitable that he will require assistance for the balance of his life. That presents the opportunity to adapt our home to better care for him and pave the way for us to receive as much healthcare as possible in the home as we age — together we can!

Accommodations are a critical factor in home-based healthcare delivery. Obviously, the focus is on the family member needing assistance, but it extends to family members as residents and property managers as well as professional/trained staff depending on circumstances. But what options are there?

Thanks to Bill Fulkerson, whose coaching has encouraged me to be more precise and persistent as I go down the “aging-in-place” path, my next step is to learn more about ADUs. In keeping with my pattern of engaging AI to assist me, I submitted the title for this post as a prompt to Perplexity Pro. Its response follows. ¹

What Is an Accessory Dwelling Unit (ADU)?

An Accessory Dwelling Unit (ADU) is a smaller, independent residential living space located on the same lot as a single-family home. ADUs are self-contained units with their own kitchen or kitchenette, bathroom, and sleeping area, providing complete living facilities separate from the main residence [1] [4] [6].

ADUs can take several forms:

Internal: Converted portions of existing homes, such as basement apartments.
Attached: Additions to new or existing homes.
Detached: Stand-alone structures, such as backyard cottages or converted garages [1] [4] [6].

They are often known by other names, including granny flats, in-law suites, carriage houses, or guest cottages [1] [5].

General Requirements for Having an ADU

The specific requirements to have an ADU vary by state and municipality, but there are common standards and regulations

Basic Features

Must be a self-contained residence with its own kitchen, bathroom, and sleeping area [1] [4] [6].
Must be located on the same lot as the primary dwelling and cannot be sold separately from the main home [1] [2].

Size Restrictions

Maximum and minimum size limits are common. For example, in Massachusetts, an ADU can be no larger than half the gross floor area of the main house or 900 square feet, whichever is smaller[6]. In California, detached ADUs can be up to 1,200 square feet, and there are minimum size requirements as well (at least 150 square feet for any ADU) [3] [6].

Entrances and Egress

Must have a separate entrance, either directly from the outside or through a shared entry hall, and meet building code requirements for safe egress [6] [2].

Building Codes and Safety

Must comply with local building codes, utility requirements, and fire codes. This includes requirements for smoke and carbon monoxide detectors, GFCI outlets, and proper window egress in bedrooms [2] [5].

Parking

Many areas require at least one additional off-street parking space, though this can be waived if the property is near public transit or bike paths [2] [5].

Owner Occupancy

Some municipalities require the property owner (or a family member) to live in either the main house or the ADU, though some states like Massachusetts have prohibited owner-occupancy requirements to encourage more ADU development [2] [6].

Permitting

Construction or conversion of an ADU typically requires permits, including land use and building permits. ADUs without permits may need to be brought up to code and properly permitted [5].

Local Variations

Local governments may impose additional requirements, such as design standards, restrictions on short-term rentals, or further limits on size and location [3] [6].

Conclusion

An ADU is a versatile, self-contained living unit on the same lot as a primary residence. Requirements to build or convert an ADU depend on local laws but generally include limits on size, safety and building code compliance, separate entrances, parking provisions, and proper permitting. Always check your local zoning and building regulations for specific requirements in your area [2] [3] [6].