• Introduction
• Definitions
• What are the Important Thresholds?
• Modification Worksheets & Plans
• How to Measure Modifications
• What is Considered Modification of Major Structural Components
• What is NOT Modification of Major Structural Components
• How to Apply the Weighting Factors

Introduction

Measuring structural modification is necessary to determine whether a discretionary permit is required to modify a nonconforming structure or a structure accommodating a nonconforming use. It is also used to evaluate structures in riparian corridors and to help determine whether a project meets the definition of “development” in geologic hazard areas.

Structural modification is based on four major structural components, established by a policy adopted by the County Board of Supervisors. The weighted value for each of the four major components is as follows:

Definitions

Reconstruction: "Modification or replacement of 65% or more of the major structural components of an existing structure within any consecutive five-year period." For projects over a property line, less than five feet from a right-of-way or within a riparian corridor, the threshold for discretionary review is 50% modification.

Major Structural Components. "The foundation, floor framing, exterior wall framing and roof framing of a structure. Exterior siding, doors, window glazing, roofing materials, decks, chimneys and interior elements including but not limited to interior walls and sheetrock, insulation, fixtures, and mechanical, electrical and plumbing elements are not considered major structural components."

What are the Important Thresholds?

An Administrative Site Development Permit is required to modify 65% or more of the major structural components in a nonconforming structure over any five-year period. For nonconforming structures over a property line, within a riparian corridor, within five (5) feet of a vehicular right-of-way or within five (5) feet of a planned, public, vehicular right-of-way improvement, the threshold for a discretionary permit is 50%. The nonconforming uses and structures section of the County Code is linked here. The permit requirements for modifying nonconforming uses and structures are summarized here.

In addition, modification of 65% establishes that one of the definitions of development in the Geologic Hazards Ordinance (Section 16.10.040(s)) is met, which means that the project may be subject to geologic review.

Any proposed increase in the height or length of a structure's nonconforming walls requires a variance application. Any proposed addition must meet site and structural standards unless a variance is approved. Information about variances may be found here.

Modification Worksheet & Plan: Calculating Structural Modifications

Modification Worksheet
PLG-215

The PLG-215 worksheet is a pdf that calculates modifications. Formulas within the worksheet sum, weight, and round values; a factored total modification summary is generated. Contextual help is available when using Adobe Acrobat.

The required Modification Plan is a separate plan sheet containing a PLG-215 worksheet along with coordinated measured roof, wall, floor & foundation plan diagrams. For a complete list of required elements for a Modification Plan, click here.

Measuring Structural Changes

The following section explains how to measure modifications, followed by what counts and and what doesn't count as a modification. 

How to Measure Modifications

Disclaimer: This method of evaluating modification of major structural components is for zoning and geologic purposes only, and does not replace or invalidate definitions or procedures of the Building Code, soils report guidelines or any other County or California code outside of County Chapters 13 and 16. Any required clarification of measurements will be provided by Planning Director or designee. To be deemed "existing," a structure must have been established legally.

Visual Aids & Illustrations

A visual aid for use with the below explanations is Modification Calculation Illustrations IB-215. 

A visual aid for preparing your plan modification sheet is Modification Plan Example IB-216. 

Roofs
Method: Divide modified roof area by existing roof area, excluding eaves and roofs over decks.

1. Measure the total area of the roof in two-dimensional plan view, neglecting slope. Sealed decks that act as roofs are included in the roof area, not in the floor area. If a sealed deck serves as a roof only to another deck, it does not count as either roof area or floor area. Do not include in this measurement:
   - Eaves.
   - Deck or porch roofs, unless the deck or porch is fully enclosed.
   - Additions. (Note: demolition or modification of an existing roof area to allow an addition counts.)
   Tip: On one-story homes, the roof area is often the same as the floor area.
2. Measure modified roof areas. The modified area of each structural member extends halfway to each adjacent member. Where roof beams, ridges or valleys are replaced or modified, the member is considered to affect an area of roof extending 16 inches perpendicular distance on either side of the member, measured from the center of the member.
3. Divide the modified roof area by the total existing roof structure area. Multiply by 100 to get the percentage of modified roof.

Walls
Modified wall segment lengths are required to be rounded up to increments of four feet. Regardless of the actual wall length, four foot increments must be used for calculations.

Method: Divide the rounded length of modified walls by the total existing exterior wall length.

1. Measure the length of existing walls. The total length is the perimeter exterior wall length on all stories, whether modified or not. Attic walls are included only if the "attic" is a story, pursuant to 13.10.700-S and Policy Interpretation ATTIC-01.
2. Measure modified length on each story. All modified wall areas are rounded up to four foot increments. There is no minimum separation between modified areas, and contiguous lengths wrap corners. Additions do not count, but demolition or modification of an existing wall to enable an addition does count.
3. Divide the total rounded up modified length (4 foot increments) by the total existing exterior wall length. Multiply by 100 to get the percentage of modified wall.

Floors
Method: Divide modified floor area by existing floor area.

1. Measure the total area of the existing floor structures of every story, including joists and slabs, whether habitable or non-habitable. Deck floors do not count. Additions do not count, but demolition or modification of an existing floor to allow an addition counts. Do not use FAR (Floor-Area Ratio) guidelines.
2. Measure the areas of modified floor structure or slab. The modified area of each structural member extends halfway to each adjacent member. For crosspieces and diagonal members, the modified area extends 16 inches on either side. Except for diagonal members, dimensions are usually measured parallel or perpendicular to joists, including locations where the dwelling wall is curved or diagonal.
3. Divide the modified floor area by the total existing floor structure area. Multiply by 100 to get the percentage of modified floor.

Foundations
Method: Modification of a linear wall foundations and pier and grade beam foundations are measured as a percentage of length; modification of a slab is measured as a percentage of area. Foundations for additions are not considered, but modification of an existing foundation to enable an addition is considered, such as new or modified footings to support a second floor.

Linear foundations

1. Measure the total length of the existing wall foundation and pier and grade beam foundation. Measure interior/exterior foundation lengths to outside face of foundation.
2. Measure the total length of the modified foundation. Contiguous supports (e.g. underpinning, piers, helical anchors, or similar) are part of the modified segment. Isolated supports are calculated as 3'-0" minimum, or the support length if larger.
3. Divide the total length of the modified foundation by the total length of the existing perimeter foundation. Multiply by 100 to get the percentage of modified foundation.

Slab foundation

1. Measure the total area of the slab, whether modified or not.
2. Measure the total area modified. Slab area must be entered in both the floor structure and foundation categories.
3. Divide the total modified slab area by the total existing of the slab area. Multiply by 100 to get the percentage of modified slab.

Other Situations
For complex foundation systems, where the above methodology does not provide an adequate approximation of modifications to a foundation, cases where new foundations area added within the footprint of the structure, cases where the loading is removed or otherwise transferred off of existing foundations, or when the above methodology does not apply, the county reserves the right to evaluate -- or to request a structural engineer to provide -- a percentage of the overall tributary area of the structure that is being affected by the foundation modifications.

What is Considered Modification of Major Structural Components

• Walls: In exterior walls of each story: the removal, replacement, reinforcement or addition of members including, but not limited to, studs, girders, headers, king studs or top plates, and also including:
  • Wall furring or sistering.
  • Balloon framing incorporated into to an existing wall - to raise a top plate or lower a foundation, for example.
  • Any length of wall that is relocated.
  • Addition of horizontal framing attached to a top plate to reinforce the story below, where no additional story or wall area is added over the reinforced top plate.
  • Special case: interior walls that function as exterior walls. An interior wall functions as an exterior wall if it establishes the inner barrier next to an actual exterior wall, thereby defining the usable area. Some dwellings are remodeled with "exoskeletons," where an interior wall is built, often with an associated foundation, just inside the existing exterior wall. In such cases, both the existing exterior wall and the adjacent interior wall will be considered part of one exterior wall, measured as one length of wall perimeter.
     
• Roofs: Removal, replacement, reinforcement, change in pitch or other modification of rafters, roof girders, rafter ledgers, beams, sealed deck floors serving as roofs to rooms below, attic roofs, cupola roof structures and similar roof areas.
• Floors: Removal, replacement or reinforcement or other modification of floor joists, slabs or related floor structures. Where a slab serves as both floor and foundation, it will count in both categories.
• Foundations: Removal, replacement, reinforcement or other modification of foundation perimeters, pier and grade beam foundations, slab foundations and other structural foundation elements on a case-by-case as determined by the Director.

What is NOT Modification of Major Structural Components

Removal, replacement or other modification of:

• Sill plates and trimmers (unless associated with relocating a wall); window sills.
• Cripple walls, including cripple wall members in existing understories, cripple walls beneath existing window sills or headers, and cripple walls placed over a top plate to raise the height of all or part of the roof. Note: although existing nonconforming walls may now be rebuilt, no new walls may be allowed within a front, rear or side yard setback without a variance, nor may the height or length of an existing nonconforming wall in a front, rear or side yard setback be increased without a variance.
• Replacement of window glazing, skylights, doors or like structures without altering the framing.
• Under an existing, unmodified header, the "filling in" of existing windows, doors (including garage doors), skylights and like openings.
• Eaves. Where eaves are sistered or replaced, only the sistered or replaced areas inside the top plate, measured from the outside wall, count as roof modifications.
• Deck walls, rails, floor structures or roofs, unless deck is part of main roof or fully enclosed. (Fully enclosed decks, porches or sunrooms are considered unheated rooms, to which modifications are counted.)
• Interior walls.
• Walls or floors of attics, understories or basements.
• Ceiling joists that are not roof rafters or floor joists for a story above.
• Removal or addition of sheetrock, wall finishes or siding.
• Floor underlayment, subfloor or finish flooring.
• Horizontal framing attached to a top plate for the addition of a new story or cripple wall. (Other modifications to support a new story, such as addition of studs or foundation upgrades, are counted.)
• Hold-downs or tie-downs for existing posts for seismic purposes.
• Structural sheathing for seismic purposes (other seismic framing/bracing, such as the addition of studs, counts).
• Channel framing / sheetrock backing for interior walls, including where attached to exterior walls.
• Modification of eaves, fascia, collar ties, crickets or similar roof features.
• Boring and notching meeting standard code requirements for electrical, plumbing and mechanical components.
• Additions are not counted, but most modifications to an existing structure to enable an addition are counted. See exception for horizontal framing for top plate (above).

How to Apply the Weighting Factors

As established by the Administrative Guidelines, modifications to each of the four major structural components will be weighted. When the spreadsheet provided above is used to calculate modification, the weighting factors are calculated automatically. The following table, which is not a calculator but provided for demonstration purposes only, shows how the weighting factors are applied to a sample structure where the floor and foundation are replaced and the walls and roof are modified by 50%. To obtain the weighted totals, the percentage of modification to each structural component is multiplied by the weighting factor. The net modification of each of the four components, rounded to the nearest whole number, is added up to obtain the total modification.

Example

In the above case, the total modification is 60%. If the project involved a nonconforming structure, the extent of modification would not require a an Administrative Site Development Permit unless also located over a property line, within a riparian corridor, within five (5) feet of a vehicular right-of-way or within five (5) feet of a planned vehicular right-of-way improvement. The 60% modification would not qualify the project for geologic review unless it met one of the other definitions in the Geologic Hazards section, 16.10.040(s).