A surface parking lot must expand to increase parking space quantity. An increased quantity increases permitted gross building area (GBA), but decreases the land remaining for building footprint (BCA). The conflict is resolved by either increasing the number of building floors associated with the reduced building footprint, or adjusting other areas of the site plan. This rather simple relationship has caused endless debate because surface parking requirements affect the shelter capacity of land, and their credibility is often challenged by the history of a specific activity at a specific location. This essay will ignore activity and drill down to understand the specification topics, values, and relationships that determine parking lot area and reduce shelter capacity for any activity on any land area.
A parking requirement specifies the square feet of gross building area (GBA) permitted per parking space provided. For instance, a parking requirement of 250 means that 250 sq. ft. of gross building area may be constructed for every parking space introduced.
Figure 1 plots the increase in gross building area (GBA) potential for a 5 story building as the parking requirement declines. (Keep in mind that a parking requirement of 600 is less restrictive than a requirement of 250 because more GBA is permitted per parking space provided.) The x-axis increments represent this decline in GBA restriction. The increase from 5,912 sq. ft. to 41,382 sq. ft. of GBA shows the dramatic impact of parking requirements.
Figure 2 plots the increase in GBA potential per floor of building height as increased amounts of GBA per parking space are permitted along the x-axis. The increase from 4,966 sq. ft. to 41,382 sq. ft. shows the combined impact of parking requirements and building height options.
The increases plotted in Figures 1 and 2 can be found in Table 1. They serve to explain why parking requirements and building height options are hotly debated across the table in every planning office of the nation. Frustration prevails because the accurate prediction options and implications is not a common practice.
Line (cd) in Figure 2 shows that GBA increases decline with each floor of increased building height; and that height increases above 2 stories produce declining increases in GBA. This make these height options increasingly less cost effective when all other design specification values remain constant.
Line (ab) shows that 15,000 sq. ft. of GBA can be achieved with a 1 story building when 1 parking space is required per 600 sq. ft. of GBA, and with a 5 story building when 1 parking space is required for every 275 sq. ft. of GBA. If you look at 20,000 sq. ft. of GBA on the y-axis in Figure 2, you’ll see that it cannot be achieved with a 1 story building; given the design specification values involved. Design specifications will be explained in the section to follow.
The point is that parking requirements and building height options play a significant role in determining the shelter capacity of core land area; that shelter capacity can be occupied by any activity; and that one design category equation makes it possible to calculate its GBA potential on any core land area. (The G1 Design Category is being used for this example and applies when surface parking around, but not under the building, is chosen as a parking design solution.)
Design Specifications and Core Area
The design specification decisions in Table 2 are represented by the values entered in the boxes of Column F. Calculating core area in cell F32 is the objective, since this is the land remaining for building footprint and surface parking after percentage estimates for all other site plan areas have been subtracted.
Cell F34 in Table 2 specifies the parking requirement (a) under study, and cell F33 specifies the total surface parking lot area per parking space (s) estimated. The master equation in Table 2 uses the (a) and (s) values entered; the core area calculated (CORE); and the building height options entered in cells A42-A51 to calculate the GBA options in cells B42-B51. The GBA options from cell B7 to L11 in Table 1 were found by modifying the (a) value entered in cell F34 of Table 2. These (a) value alternatives were noted in cells B6-L6 of Table 1.
The value (a) is only one of 25 specification boxes in Table 2, and a change to one or more of the values entered will produce a different set of GBA options in Col. B. All values were held constant as the (a) value was modified to produce the GBA options from cell B7 to L11 in Table 1.
The Specification Value (s)
The value (a) entered in cell F34 of Table 2 is a straight-forward parking requirement that restricts shelter capacity. The accuracy of all parking requirements is a matter of continuing debate. It results from their effect on GBA potential and the lack of research available to justify the values involved. The only certainty at this point in time is that parking is needed for the transition to sheltered activity, and can be provided in either remote or adjacent locations.
The value (s) is equal to total parking area, excluding loading area, divided by the number of parking spaces provided. The value is easy to find when a site plan can be measured, but difficult to predict. The 18 specification decisions that combine to create the value (s) are shown in the boxes of Table 3. Each of these decisions is a variable that can be modified to alter the results calculated in cells C15-K20. The objective is to find the values in column K. The calculated result in cell K15 shows that the (s) value entered in cell F33 of Table 2 is based on a 90 degree parking layout decision.
The design specification values (l), (a), and (e) in Cells J3-J5 of Table 3 are shown in Figure 3 as dimensions of a parking lot bay. (A “bay” includes a circulation aisle and parking spaces on both sides of the parking aisle.) A parking lot may also contain parking stalls, service stalls, and circulation aisles in addition to those found in parking bays. It may also contain landscape islands to relieve the sea of asphalt. The design specification values (C%) and (L%) in cells J7-J8 of Table 3 are introduced to estimate their presence in this example. The values chosen show that a utilitarian parking lot is planned with little amenity. Parking space angle and circulation aisle options are entered in cells A15-B20 of Table 3, and each value affects the gross and net parking lot areas predicted in cells H15-J20. The number of parking spaces in one row of a bay must be estimated in Cell J6 of Table 3 to forecast parking lot area. The 10 spaces entered in Table 3 produce an average of 406 sq. ft. per space in cell K15. Thirty spaces produce an average of 389 sq. ft. and 5 spaces produce an average of 432 sq. ft. when all other specification values remain constant. Understanding this range of areas can be helpful when choosing the value to enter in cell F33 of Table 2.
The objective is to forecast average parking lot area per space in cells K15-K20 of the Planning Forecast Panel in Table 3. The value in cell K15 has been chosen for entry in cell F33 of Table 2 because it is related to a 90 degree parking angle. The area options in column K are calculated from the values entered in Columns A and B of the Planning Forecast Panel and the values entered in Cells J3-J8. A change to one or more of these values will produce a new forecast.
The point of this exercise has been to explain the scope of design specification decisions that must be correlated to produce the value entered in cell F33 of Table 2. The decisions entered in Table 3 could use refinement, but the underlying purpose is to explain the many related decisions that stand behind the production of a single parking estimate (s), and the major role this value plays in the forecast of GBA options in Table 2.
Surface Parking Coefficients
I’ve created Table 4 to illustrate the impact of (s)-value options on gross building area potential (GBA). The results are based on the surface parking equation GBA=(af/(a+fs))*CORE. When CORE is equal to 1, gross building area potential is represented by the coefficient in this equation. It becomes a function of optional (a), (f), and (s) value decisions, and expresses GBA potential as a multiple of the core area available.
Table 4 places optional (s)-values in boxes above six matrices labeled 350-800. Each matrix locates (f) values on the y-axis and (a) values on the x-axis. A larger (s)-value indicates that more parking lot area is provided per parking space. This increased average area may result from increased landscaping; more generous parking space and circulations areas, or both.
Cell L18 is part of matrix 400 in Table 4. It shows that a 5 story building with a parking requirement (a) equal to 1,000 sq. ft. of GBA per parking space can yield a total GBA equal to 1.667 times the core area available. This is possible when the parking lot contains an average of 400 sq. ft. per space (s). (The maximum building footprint area would be equal to 1.667 / 5 times the core area.) In contrast, cell B14 shows that a one story building and a parking requirement of 100 sq. ft. of GBA per parking space will yield total GBA equal to 0.200 times the core area available when (s) is equal to 400.
In matrix 800, if 100 sq. ft. of GBA (a) is permitted per average parking space (s) equal to 800 sq. ft.; the parking lot will grow much more rapidly that gross building area. This ratio limits GBA potential to 0.111 times the core area as shown in cell B50 of Table 4. If 1,000 sq. ft. of GBA is permitted per average parking space of 800 sq. ft., cell L54 shows that the ratio produces GBA equal to 1.000 times the core area for a 5 story building. If you compare the results in matrix 800 to those in matrix 400, the differences in GBA potential illustrate the impact of the (s) and (a) decisions represented. The significance of the values entered in cell F33 and F34 of Table 2 may now be more apparent.
Line 50 in Table 4 illustrates a typical trade-off decision related to the G1 surface parking design category that is not immediately apparent. In the G1 category, gross building area potential declines when average parking lot area per space (s) increases and all other design specification decisions remain constant. In many cases, more parking lot area per space indicates greater landscape provisions. These provisions soften the impact of asphalt pavement, but fewer parking spaces reduce GBA potential in the G1 Design Category. This places landscape provisions at a distinct disadvantage. It has been exacerbated because comparative options and implications have been limited by the time required to manually prepare alternate site plans. Table 2 was introduced to explain how GBA options can be forecast, compared, and evaluated by changing one or more values in its design specification template. The (s) and (a) parking values represented two of these design decisions. Tables 3 and 4 have explained the implications of choice among these values. The mathematical format involved will not eliminate argument and debate over GBA implications when landscape provisions are considered, but it can eliminate confusion and distrust around a table that has had to depend on intuition, assumption, and opinion for evaluation and conclusion. In fact, confusion surrounds every specification value entered in Table 2 at this time. This has severely compromised the leadership potential of all public and private sectors responsible for the provision of shelter for the activities of growing populations within a geographically limited Built Domain that protects their quality and source of life – The Natural Domain.