Atherton Secondary Dwelling Units (SDU)
“Secondary Dwelling Unit” is the term that Atherton uses for ADUs.
Atherton regulations are in Chapter 17.52 Accessory Dwelling Units on the Municipal Code
An accessory dwelling unit is permitted provided the following standards and requirements are incorporated:
A. Maximum Number. Only one accessory dwelling unit shall be allowed on each lot or parcel.
B. Location. The accessory dwelling unit shall be attached to the existing main residence or located within the habitable area of the existing main residence or detached from the main residence and located on the same lot as the main residence.
C. Setback Requirement. Each newly constructed attached or detached accessory dwelling unit shall be located in the main building area. In addition, a newly constructed accessory dwelling unit may encroach into the side and rear yard setbacks by not more than twenty percent of the required setback. Newly constructed, detached accessory dwelling units shall meet all other applicable development standards as set forth in chapter 17.40 (Accessory Buildings and Structures).
D. Existing Garage. No setback shall be required for an existing, legal attached or detached garage that is converted to an accessory dwelling unit. A setback of five feet is required from the side and rear lot lines for an accessory dwelling unit that is constructed above a garage.
E. Passageway. No passageway, as defined in California Government Code Section 65852.2, shall be required in conjunction with the construction of an accessory dwelling unit.
F. Maximum Square Footage. The size of each such accessory dwelling unit shall not exceed one thousand two hundred square feet.
G. Floor Area. The increased floor area of an attached accessory dwelling unit shall not exceed fifty percent of the existing living area of the main residence, with a maximum increase in floor area of one thousand two hundred square feet.
H. Owner Occupancy. Either the main residence or the accessory dwelling unit, or both, shall be occupied by the owner of the property.
I. Rental Only. The accessory dwelling unit shall not be sold separate from the main residence and may be rented. Rentals must be for terms of thirty days or longer.
J. Parking Requirement.
1. Unless subject to one of the exceptions immediately below, each accessory dwelling unit shall have assigned to it one parking space for each bedroom in the accessory dwelling unit. Parking shall be accessible from a public street and screened from view of the public street. These spaces may be provided as tandem parking on an existing driveway.
2. No additional parking required. No additional parking shall be required where: (a) the unit is located within one-half mile of public transit, or (b) the unit is part of the existing primary residence or an existing accessory structure.
K. Exemption from Floor Area Limitations. The first one thousand two hundred square feet of floor area of each newly constructed accessory dwelling unit shall be excluded from maximum floor area calculations; provided, that the floor area exclusion shall not to exceed ten percent of the allowed floor area for the lot.
L. Separation Requirement. A separation of at least eight feet shall be maintained between any detached accessory dwelling unit and its main residence structure.
M. Entrance Requirements. Any new accessory dwelling unit that is incorporated into an existing main residence shall have a separate entrance facing the side or rear yard.
N. Compliance with Applicable Requirements. The accessory dwelling unit shall meet all applicable zoning and fire regulations and shall comply with local building codes. Fire sprinklers shall not be required for the accessory dwelling unit if they are not required for the primary residence.
O. Lighting Restrictions. Any lighting associated with the accessory dwelling unit shall be shielded or downlit and shall not shine onto adjoining properties.
Q. Density. The accessory dwelling unit may not exceed the allowable density for the lot upon which it is located.