Chapter 17.44 — BASEMENTS

Sections:

• 17.44.010 Purpose. 17.44.020 Definitions. 17.44.030 Locations for Basements. 17.44.040 Floor Area Calculations. 17.44.050 Lightwells, Stairwells, and Other Excavated Features in R-1B and Smaller Lots in the R-1A.

• 17.44.060 Design Requirements.

The purpose of this chapter is to provide for the regulation of basements on residential property. The intent is to regulate the location and size of basements and the orientation and screening of driveway approaches leading to basement garages. (Ord. 582 § 1 (Exh. A), 2009)

The Atherton Municipal Code is current through Ordinance 677, passed February 18, 2026.

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Title 17 Zoning | Atherton Municipal Code

Terms that are relevant to this chapter are defined in chapter 17.60 (Definitions). (Ord. 582 § 1 (Exh. A), 2009)

A. Basements under Main Buildings. Basements are permitted under a main building. The total allowed floor area for all basements under a main building shall be the same as the total floor area of the first floor of the building (e.g., maximum floor area for basements under a main building is one hundred percent of the floor area of the first floor of the main building). Additionally, the location of all basements as part of a main building shall be limited to the boundaries of the footprint of the first floor of the main building. Minor extensions of the basement beyond the first floor footprint above may be approved at a staff level for extensions that are no greater than one hundred forty square feet in combined area. However, because not all main buildings are exact rectangles, the approval authority (the planning commission as described in Table 17.06.070-1) may allow, through issuance of a special structures permit, exceptions to this rule when within the following parameters:

1. The total floor area of basements shall not exceed one hundred twenty percent of the total floor area of the first floor of the main building; and

2. A minimum of seventy-five percent of the total area of the basements shall be located directly under the footprint of the first floor of the main building; and

3. Portions of basements that are not located directly under the footprint of the first floor of the main building shall only be allowed in such locations when they are still within the extents of the main building (e.g., “squaring off” the basement).

B. Basement under Accessory Buildings. Nonhabitable basements without lightwell(s) may be located under the footprint of accessory buildings outside of the main building area with no more than one exterior stair accessing the basement level. The exterior stair must be located within the allowable buildable area for accessory buildings. No windows are permitted in these basements. Basements containing habitable space with lightwell(s) may be located under the footprint of accessory buildings outside of the main building area with a special structures permit from the approval authority with the finding that the basement will not impact heritage trees and the condition that lightwells be located on the interior side of the yard, or the lightwell shall comply with the main building setbacks if facing an exterior yard. The approval authority may impose reasonable conditions including, but not limited to, increased setbacks and limitation on the size of lightwells.

C. Garages in Basements. Garages may be located in basements under buildings located within the main building area. (Ord. 616 § 1, 2015; Ord. 582 § 1 (Exh. A), 2009)

The Atherton Municipal Code is current through Ordinance 677, passed February 18, 2026.

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Title 17 Zoning | Atherton Municipal Code

The following rules shall apply to the calculation of floor area ratio relative to basements:

A. In instances where a building has a basement and the measurement of the vertical distance between the average natural grade of the site to the finished floor of the first story measures less than two feet, the basement area shall not be included in the calculation of floor area (see figure 17.44.040-1 (Basements and Floor Area)).

B. In instances where a building has a basement and the measurement of the vertical distance between the average natural grade of the site to the finished floor of the first story measures more than two feet, the basement area shall be included in the calculation of floor area (see figure 17.44.040-1 (Basements and Floor Area)).

C. Exceptions to the requirement of this section for hillside properties (where the average cross-slope is greater than twenty percent as determined under section 16.24.050 (Lot Size Requirements) of the town municipal code) may be permitted upon issuance of a special structure permit, provided landscape screening is maintained.

Figure 17.44.040-1. Basements and Floor Area

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(Ord. 582 § 1 (Exh. A), 2009)

The Atherton Municipal Code is current through Ordinance 677, passed February 18, 2026.

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Title 17 Zoning | Atherton Municipal Code

In the R-1B district and for lots with an area of less than ten thousand square feet in the R-1A district, excavated features such as lightwells are allowed to encroach into side and rear yards up to twenty-five percent of the total required side and rear setbacks for primary buildings (see figure 17.44.050-1, Lightwell Exemption), provided landscape screening is maintained between the feature and the adjacent property line and there is no impact to heritage trees. The following provisions shall apply:

A. In residential district R-1B and for lots with an area less than ten thousand square feet in the R-1A district, an encroachment into both rear and interior side yards cannot exceed twenty-five percent of the length of the side of the building to which it is adjacent and a total area of one hundred square feet.

• B. The encroachment is measured to the face of the retaining wall, rather than to the back of the wall.

C. These provisions do not apply to accessory buildings. Lightwells for these structures may only be allowed facing towards the interior of the lot and may not exceed the minimum setbacks for the accessory building.

Figure 17.44.050-1. Lightwell Exemption

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• (Ord. 582 § 1 (Exh. A), 2009)

The Atherton Municipal Code is current through Ordinance 677, passed February 18, 2026.

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Title 17 Zoning | Atherton Municipal Code

A. The side(s) of a basement not wholly or otherwise substantially underground (e.g., greater than eighty percent) may not be located facing any side of a lot that abuts a road.

B. Except as provided in section 17.44.050 (Lightwells, Stairwells, and Other Excavated Features in R-1B and Smaller Lots in the R-1A), areas for stairways and lightwells for basements in the main building area may extend beyond the footprint of buildings, but shall be limited to the main building area.

C. Driveway approach for garages in basements shall be screened from the public right-of-way by design, landscaping, or a solid wall and gate. The driveway approach shall not negatively impact heritage trees. (Ord. 582 § 1 (Exh. A), 2009)