The Mercer Island Development Code excludes that portion of the basement floor area from the Gross Floor Area which is below the existing or finished grade, whichever is lower. That portion of the basement which will be excluded is calculated as shown.

Portion of Excluded Basement Floor Area =

Total Basement Area x     Σ(Wall Segment Coverage x Wall Segment Length)

    Total of all Wall Segment lengths

Where the terms are defined as follows:

TOTAL BASEMENT AREA is the total amount of all basement floor area.

WALL SEGMENT COVERAGE is the portion of an exterior wall below existing or finished grade, whichever is lower. It is expressed as a percentage. (Refer to example.)

WALL SEGMENT LENGTH is the horizontal length of each exterior wall in feet.


This example illustrates how a portion of the basement floor area may be excluded from the Gross Floor Area. In order to complete this example, the following information is needed.

A. A topographic map of the existing grades and the proposed finished grades.

B. Building plans showing dimensions of all exterior wall segments and floor areas.

C. Building elevations showing the location of existing and proposed finished grades in relation to basement level.

Step One

Determine the number and lengths of the Wall Segments.

Step Two

Determine the Wall Segment Coverage (in %) for each Wall Segment. In most cases this will be readily apparent, for example a downhill elevation which is entirely above existing grade or will be entirely above finished grade. In other cases where the existing or finished grade contours are complex, an averaging system shall be used. (Refer to illustration.)

Step Three

Multiply each Wall Segment Length by the percentage of each Wall Segment Coverage and add these results together. Divide that number by the sum of all Wall Segment Lengths. This calculation will result in a percentage of basement wall which is below grade. (This calculation is most easily completed by compiling a table of the information as illustrated below.)

Table of Wall Lengths and Coverage

Wall Segment    Length x    Coverage =    Result

A    25′    56%    14′

B    10′    0%    0′

C    8′    0%    0′

D    25′    0%    0′

E    8′    0%    0′

F    13′    0%    0′

G    25′    60%    15′

H             48′    100%    48′

Totals    162′    NA    77′

Step Four

Multiply the Total Basement Floor Area by the above percentage to determine the Excluded Basement Floor Area.

Portion of Excluded Basement Floor Area =

=1,400 Sq. Ft. x    (25′ x 56% + 10′ x 0% . . . 25′ x 60% + 48′ x 100%)


=1,400 Sq. Ft. x 47.53%

=665.42 Sq. Ft. Excluded from the Gross Floor Area

(Ord. 17C-15 § 1 (Att. A)).