Design Decisions That Determine Single-Family Subdivision Density

I have written three essays entitled, "Design Decisions That Determine Apartment Density”; “Design Decisions That Determine Townhouse Density”; and “Design Decisions That Determine Single-Family Detached Housing Density”. They were written to identify the design topics whose values must be mathematically correlated to lead residential shelter capacity toward intensity combinations that avoid overcrowding and sprawl.

Houses provided in subdivisions have led to the term “sprawl”, but the design decisions that produced this reaction to overdevelopment have remained ambiguous, or isolated when partially recognized by zoning ordinance regulations. This essay is about the complete vocabulary of design decisions that combine to produce the shelter capacity of subdivisions -- and the implications of these choices.  

There are 27 gray cell entries in Table 1 that represent decisions collected in 3 modules entitled, “Land”, “Lot”, and “Building”. Gross land area is given in cell F3 and the objective of the table is to predict the average number of lots and gross home area options that can be constructed on the land area given under the conditions specified by the gray cell values entered and mathematically correlated. These predictions can be found in the Planning Forecast Panel at the bottom of the table, and they will change whenever one or more of the gray cell values are modified in the spreadsheet.

The Land Module in Table 1 pertains to traditional subdivision plans that provide no common open space for shared public amenities as noted by the 0% values entered in cells F13 and F14. If percentages had been entered in these cells, the remaining shelter area would have declined as a percentage of the buildable area available, but the impervious cover limit calculated in cell F12 would have remained for every lot subdivided from the smaller shelter area.

The 10 gray cell values entered in the Lot and Building Modules of Table 1 are used to find the first floor footprint area remaining on the lot area given in cell F23 after all other allocation is subtracted. This footprint area is found in cell F46. It is multiplied by the floor quantity alternatives entered in gray cells A55-A63 to find average gross home area options for the given lot area in Column B of the Planning Forecast Panel.

This footprint, or first floor area in cell F46 may seem low for a 9,000 sq. ft. lot, but it is a function of the 30% storm sewer capacity calculated in cell F12 from the 70% unpaved open space entered in cell F11. It is not only low. The area is a limit that includes all future expansion. This should indicate the critical importance of a subdivision feature that is often overlooked until storm sewer flooding produces a need to supplement the developer’s limited contribution with a public relief sewer. I cannot overstate the need to take the value entered in cell F11 seriously for this reason, along with the implications calculated in cell F12 and the related implications in the Planning Forecast Panel of Table 1. If the storm sewer capacity calculated in cell F12 had been greater, the lot area entered in cell F23 could have been reduced while producing an equal or greater footprint area. This would have produced greater shelter capacity per buildable acre and reduced land consumption for the most desired dwelling unit configuration on the planet. The downside is that too great a decrease in the value entered in cell F11 would increase the intensity calculated in Column H of the Implications Module until it prompted a flight similar to that of the original suburban migration.

Estimated lot quantity for the recipe entered in Table 1 is found in cell C55 of the Planning Forecast Panel by dividing the average lot area given in cell F23 into the shelter area remaining in cell G17. Three additional columns of predictions are also included in the Planning Forecast Panel.

The Implications Module in Table 1 is the last feature of the Table 1 forecast model. The shelter capacity of the land area given is related to the floor quantity options entered in Column A. The results are used by the formula in cell H53 to produce the related intensity values in Column H. These calculations measure the relationship of building mass and pavement to unpaved open space when floor quantity options change in Column A and the remaining gray cell values entered are constant. Any change to one or more of the gray cell values entered will produce a revised planning forecast and set of implication measurements.

The intrusion measurements calculated in Column J translate the floor quantity options entered in Column A to a compatible four-part measurement system.

The measurements calculated in Column K of the Implications Module combine the capacity, intensity, and intrusion measurements of Columns G-J into a consolidated statement of project dominance options. In other words, a project dominance value in this example is produced by correlating 18 design decisions that do not exist in isolation and must be coordinated to provide correlated leadership direction for the three physical fronts of shelter design.

From a city design perspective, land use planning with design specification correlation can optimize the use of land to shelter any activity, and is the key to correlating capacity, activity, intensity, and economic stability within limited geographic areas.

CONCLUSION

I hope that I have made the significance of comprehensive, coordinated design value decisions apparent. Our current concept of minimum, independent zoning regulations cannot lead us toward the shelter capacity and activity allocation needed to protect the physical, social, psychological, environmental, and economic welfare of growing populations within geographic limits that protect their source of life. We depend on shelter for survival but it consumes land that is our source of life. We are expected to discover the correlation required.

I have deleted most of the equations in Table 1 to simplify the illustration and have omitted a detailed discussion of Building Design Categories and Residential Activity Group classification that I have mentioned in earlier essays. If you are interested, these equations and discussions can be found in my book, The Equations of Urban Design, which is available from Amazon.com. The subdivision chapter in this book considers two fundamental Traditional and Clustered subdivision questions in depth:

1)      What lot quantity and average home area options can be accommodated in a subdivision when gross land area and minimum lot area are given? 

2)      What minimum lot area, shelter area and buildable land area is needed for a subdivision when lot quantity and average home area objectives are given?

A third question is a variation of Question 1.

3)      What average lot area and home area options are available to a subdivision when gross land area and lot quantity objectives are given?

A fourth question is solely devoted to cluster subdivisions.

4)      How is average home size affected when lot quantity remains constant in traditional and clustered subdivision plans for the same gross land area?

The answers to these questions can be examined in spreadsheets I have called forecast models. The values assigned are like a musical score. The symphony produced will remain a function of the talent available. The objective is to eliminate the dissonance produced when a score and conductor are missing.