20% Building Coverage Threshold

The 20% Building Coverage threshold, sometimes referred to here as the BC threshold for shorthand, is a citywide HPOZ rule under which certain additions and new buildings can be processed as Major Conforming Work rather than as a Certificate Case. Where the rule applies, the project moves through staff-level review on a faster procedural track. Where it does not, the project moves to a Certificate of Appropriateness or Certificate of Compatibility.

The threshold was added to the citywide HPOZ Ordinance in 2017 by Ordinance 184903 (effective May 5, 2017) and now operates as part of the procedural framework codified at Division 13B.8 of Chapter 1A.

Building Coverage, as defined in the citywide HPOZ Ordinance, is “the area of a lot covered by roofed buildings and structures measured from the outside of the exterior wall at the ground floor, including covered porches and patios and detached and attached accessory structures over 6 feet in height.”

The definition explicitly excludes:

• Uncovered paved parking areas
• Driveways
• Walkways
• Roof overhangs
• Uncovered steps, terraces, decks, and porches
• Architectural projections not intended for shelter or occupancy

The exclusions matter as much as the inclusions. A new uncovered rear deck or patio does not contribute to Building Coverage; the same footprint with a covered roof does. A driveway extension does not count toward the baseline; a new carport over the driveway does. Designers calculating against the threshold should map the proposed scope against the definition’s inclusions and exclusions item by item.

Some adopted Preservation Plans further clarify scope at the district level — for example, calculating the baseline “excluding garages” in some districts. The applicable Preservation Plan governs district-specific carve-outs.

Under the citywide rule, an addition or new building may qualify as Major Conforming Work when all four of these conditions are met:

• Less than 20% increase in Building Coverage over the Building Coverage legally existing on the effective date of the HPOZ.
• Located outside a Street Visible Area. The addition or new building is in a non-visible portion of the lot.
• No increase in height is proposed. The addition stays within the existing roof envelope; no second story, no taller mass, no roof reshape.
• Single-structure scope. The application does not involve two or more structures.

If any one of the four conditions is not met, the project does not qualify for Major Conforming Work under this pathway, and the application moves to a Certificate Case (COA on a Contributing Structure, or CCMP on a Non-Contributor or vacant lot). The conditions are cumulative, not alternative.

The 20% calculation is measured against the Building Coverage legally existing on the effective date of the HPOZ, not against current conditions. This means the baseline is district-specific and locked at designation. Hancock Park’s effective date is 2008. Highland Park’s is 1994 (with the Garvanza expansion in 2010). Spaulding Square’s is 1993. Angelino Heights’s is 1983.

Two practical implications follow.

Improvements added after the HPOZ took effect do not raise the baseline. If a property had 1,800 square feet of Building Coverage on the effective date and a 600-square-foot addition was permitted in 2018, the 20% calculation still uses 1,800 square feet — not 2,400. A subsequent owner cannot stack additions to grow the baseline.

Unpermitted construction does not count toward the baseline either way. Building Coverage that was added without permits is not part of the legally existing figure on the effective date. Owners who inherited a property with unpermitted work should verify the legally permitted footprint with the Department of Building and Safety before calculating against the 20% threshold.

The 20% calculation is the headline number, but the three companion conditions do most of the practical filtering. Many projects come in well under 20% on Building Coverage and still fail to qualify because of one of the others.

An addition placed in a non-visible portion of the rear yard typically clears this condition. An addition with any street-visible component — including the over-the-roofline second-story condition that street-visible analysis flags — fails it. The Street Visible Area determination is made by Department of City Planning staff and is not self-classified.

“No increase in height” reads strictly. A rear addition that maintains the existing eave line and roof profile clears the condition. A second-story addition, a roof-raise, a dormer that extends above the existing ridge, or a new roof form taller than the existing all fail. The condition removes most upward-growth scenarios from the Major Conforming Work track even when the footprint expansion is modest.

The condition was added to prevent multi-building projects from being broken into smaller pieces to clear the threshold. A combined main-house addition plus new ADU does not qualify, even if each piece individually would. The citywide framework treats the package as a Certificate Case.

Before the 2017 amendments, the citywide ordinance did not have a clearly defined Major Conforming Work category for additions and new buildings tied to Building Coverage. Larger additions on Contributing properties moved to COA more aggressively. The 20% threshold created a middle path — a defined zone of expansion that, when paired with the visibility, height, and single-structure conditions, can be processed without a full Board hearing and Letter of Determination.

The trade-off is procedural, not substantive: Major Conforming Work still requires submittal through the Online Application System, application fees, and staff review against the Preservation Plan’s design guidelines. The savings are in time and in the absence of a 15-day appeal window. (Conforming Work decisions, including Major Conforming Work, are not appealable.)

Source: Citywide HPOZ Ordinance (LAMC §12.20.3) and Division 13B.8 (Historic Preservation), Chapter 1A LAMC. The 20% Building Coverage threshold was added to the citywide ordinance by Ordinance 184903 (City Clerk file 16-1157), effective May 5, 2017. The four-condition rule and the Building Coverage definition are restated in adopted Preservation Plans across the LA HPOZ system, including the plans reviewed for this entry.