Accessory Dwelling Units in the RM: Medium Density Residential District

Accessory Dwelling Units (ADUs) are a permitted use in the RM: Medium Density Residential District of the City of Concord, provided the development complies with the standards established in [Section 18.200.180]. Detached ADUs in the RM: Medium Density Residential District are subject to specific height, setback, and size requirements, and they are not considered to exceed the allowable density for the lot.

Permissibility of Accessory Dwelling Units

Accessory dwelling units, including attached and detached units, are permitted in the RM: Medium Density Residential District, which is identified as an eligible district for these units in [Section 18.200.180]. Accessory dwelling units must be established as subordinate to a single-family principal dwelling or a multifamily principal dwelling [Section 18.200.180].

Development Standards for Accessory Dwelling Units

Accessory dwelling units in the RM: Medium Density Residential District must comply with the development standards outlined in [Section 18.200.180]. Key standards are summarized in the table below:

Feature	Standard
Minimum Size	150 square feet
Maximum Size	Lesser of 50% of the principal dwelling floor area or 1,000–1,200 square feet (depending on lot size)
Interior Side/Rear Setback	Minimum of 4 feet
Front/Corner-Side Setback	Must comply with principal dwelling requirements
Maximum Height (Attached)	25 feet (or principal dwelling height, whichever is lower)
Maximum Height (Detached)	16–18 feet (varies based on location and proximity to transit)

Size Limitations

The size of an accessory dwelling unit is limited based on the net area of the lot:

Lots less than 12,000 square feet: The maximum size is 1,000 square feet, excluding up to 230 square feet for attached garages or carports.
Lots 12,000 square feet or greater: The maximum size is 1,200 square feet, excluding up to 460 square feet for attached garages or carports.
Note: The size limitation will be waived if it restricts the accessory dwelling unit to less than 800 square feet [Section 18.200.180].

Parking Requirements

Accessory dwelling units generally require one off-street parking space per unit or per bedroom, whichever is less [Section 18.200.180]. However, parking is not required in the following instances:

The unit is located within one-half mile walking distance of public transit.
The unit is located within a designated architecturally and historically significant historic district.
The unit is part of the proposed or existing principal dwelling or a conversion of an accessory structure.
When on-street parking permits are required but not offered to the occupant of the accessory dwelling unit.
When there is a car share vehicle site located within one block.

Design and Operational Requirements

Accessory dwelling units in the RM: Medium Density Residential District are subject to specific design and operational standards, including:

Short-Term Rentals: Accessory dwelling units may not be rented for a term of less than 30 days [Section 18.200.180].
Permanent Foundation: All accessory dwelling units require a permanent foundation system in compliance with the Uniform Building Code [Section 18.200.180].
Roof and Material Consistency: For accessory dwelling units that are not pre-approved, exterior colors and materials must be consistent with the principal dwelling [Section 18.200.180].
Owner Occupancy: For building permit applications received after December 31, 2024, one of the dwellings on the lot must be the bona fide principal residence of at least one owner of the lot [Section 18.200.180].

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