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Monday, March 29, 2021

Design Decisions That Determine Single-Family Detached Housing Density

This discussion concerns the land required for single family detached residential activity when it occupies the G1 Building Design Category. The combination is referred to as the G1.R1 Activity Group. It may be the most desirable form of residential shelter on the planet, but the scope of demand and amount of land it consumes is becoming a serious concern. It needs an improved method of measurement, evaluation, and prediction to ensure that its land area allocation within city limits contributes to a city's average economic yield per acre that is equal to, or greater than, the average municipal expense per acre required to provide a desirable quality of life for its residents.

TABLE 1

Table 1 pertains to single family detached homes when lot area is given. It contains twenty-five gray cell locations for design value decisions / entries. These entries are mathematically correlated within the table to accurately measure or predict shelter capacity and its implications. This is more significant than may be realized because capacity per acre multiplied by revenue per sq. ft. determines the financial contribution provided by every acre in a city; and inadequate contributions produce average revenue per acre deficits that can plague our efforts to improve the quality of life provided.

The objective of the Lot Module in Table 1 is to identify the buildable lot area remaining from the gross lot area given in cell F4 after 5 design value decisions are entered and subtracted.

The value entered in cell F12 of the Lot Module deserves special mention. It is based on the storm sewer capacity planned or present for the branch line serving the area in which the lot is located. In this example 70% unpaved open space has been entered. This means that storm sewer capacity equals 30% impervious building and pavement cover when additional detention and/or retention systems are not provided. This 30% has been calculated in cell F13 from the 70% unpaved open space entered. In my experience this is one of the most overlooked issues in city planning. I have tried to draw your attention to this topic by requesting that you enter this percentage rather than the 30% impervious cover that is derived from its provision. Variances are routinely granted for building cover and pavement percentage increases that reduce the unpaved open space remaining on a given lot. These are granted with the best of intentions, but can exceed the impervious cover capacity of the adjacent storm sewer or drainage system. This often occurs because impervious cover limits are rarely recorded on plats for recall and review after the initial civil engineering installation. In these cases, variance requests can be approved to expand building cover and pavement percentages along branch storm sewer lines that were not designed to handle the increase requested when it begins to occur on multiple lots over time. The result can become a source of basement flooding, disease, decline, and decay.

DENSITY

The net density calculated in cell F14 of Table 1 also needs explanation. The discretionary values entered in cells F5-F7 and F9 have been subtracted from the gross lot area given in cell F4 to find the buildable lot area remaining in cell F11. If an unbuildable percentage of the lot had been entered in cell F5, the buildable lot area calculated in cell F11 and G11 would be less than the total lot area given in cell F4. If the total lot area were used to calculate density in this case, density would appear to be less than a calculation based on the buildable land area remaining and more lots would be permitted.

A conscious decision has been made to calculate density based on the buildable land area available because it reflects the amount of useful space available. If there had been an unbuildable ravine on the lot, for instance, it would have enhanced the view but condensed the activity present. In other words, the larger lot size would have produced a lower density calculation but increased the intensity of activity on its remaining buildable area. The bottom line is that density is only affected by the first 6 discretionary value decisions entered in Table 1 and 10 more correlated values are required to improve leadership guidance. Density can be a deceptive calculation for many reasons based on these omissions and lack of correlation.

PAVEMENT MODULE

The Pavement Module in Table 1 contains 4 discretionary design decisions that serve to further reduce the impervious cover area remaining for primary building footprint on the buildable lot. These calculations are included in cells F22 and G22.

BUILDING MODULE

The Building Module contains 5 discretionary design decisions that continue to reduce the impervious cover area available for building footprint within the buildable lot area. This continued subtraction leads to the first floor impervious area remaining in cell F32 and B41. Multiplication of the nine floor quantity options entered in cells A41-A49 by the first floor area remaining in cell F32 produces the 9 home size options calculated in column B of the Planning Forecast Panel.

PLANNING FORECAST PANEL

Column D in the Planning Forecast Panel presents total building area options that include primary home, garage, and accessory building areas. Column E presents the buildable lot area percentages consumed by the total building area options in column D. It may be a surprise to see the low percentage calculated.

It may also be a surprise to see on line 43 of the Planning Forecast Panel in Table 1 that a 60 x 120 foot lot served by a storm sewer with 30% impervious cover capacity can only support a 2 story home with 990 sq. ft. of habitable area and 1,590 sq. ft. of total building area. Cell B41 also shows that the first floor area of this home would only be 495 sq. ft. based on the 15 discretionary design values entered above. For the uninitiated this is about the size of a two car garage. The motive behind variance requests for building and pavement expansion should be apparent from this low number, as well as the threat of variance requests to installed storm sewer capacity. The mathematical justification for home size limits in relation to storm sewer capacity, unpaved open space, and 15 other variable percentages has rarely, if ever, been available, however; because the measureable implications of correlated shelter design decisions have been unavailable. They will become important considerations as populations continue to grow and consume the land like locusts of old.

IMPLICATIONS MODULE

The Implications Module beginning on line 51 calculates that the 2 story, 990 sq. ft. home mentioned above represents a shelter capacity of 9,621 sq. ft. of total building area per acre; an intensity of 0.066; intrusion of 0.400; and dominance of 0.466. These statistics are like the first blood pressure readings, however. We have an intuitive sense of the implications measured based on professional experience, but no accumulated knowledge that adds credibility to leadership recommendations.

THE 3,150 SQUARE FOOT LOT

The previous discussion involved a 60 x 120 foot lot, but the 495 sq. ft. floor plan area that emerged reminded me of a 3,150 sq. ft. lot created in 1907 that I examined in my latest book, The Equations of Urban Design. I’m quoting it here because I think its comparison to the statistics just calculated may be helpful.

The home and lot in Diagram 9.3 was built in 1907. It represents one of the first rings of migration from the central city and is an early form of an evolving suburban lot that is now part of the inner city.

The alley and detached garage represent a transition from stables, outbuildings, and remote kitchens to the automobile. Small rear yards became replacements for ample kitchen gardens. Alleys provided inadequate turning radii into garages and extended driveways consumed remaining open space for access to the garage from the street. Parking in the street was prompted by narrow lots, constrained driveways and alleys of inadequate width and turning radii. Their relative invisibility encouraged hidden behavior and indefensible space.

The home had a gravity coal furnace, electric power, public water supply, one bathroom, and was served by a public sewer that combined sanitary effluent with storm water runoff to open street inlets. At the time, it represented a significant improvement to public health and welfare, but combined sewers now tell us a different story.

Table 9.4 recites the design specification values that originally applied to this lot. Private, unpaved open space UOSL is 43.21% of the lot as noted in cell F12, but the percentage does not indicate a minimum area requirement. It is a measurement of existing condition. This means that 56.79% of the buildable lot area is impervious cover.

Impervious cover increased to 64.41% after a building addition was approved as shown in Diagram 9.4. The result was increased storm water runoff that exceeded the capacity of the combined sewer during moderate to heavy rainfalls. This increased basement and street flooding with storm water and sanitary effluent.

The driveway and garage represented 27.73% of the lot. This is an overlooked statistic but was a greater impervious area than the original building footprint. In other words, an attempt to accommodate the car and driveway reduced the alley to a garbage collection service while providing inadequate sewer capacity and sacrificing unpaved social open space. The result encouraged unsafe, on-street parking but was a step in the right direction. It provided the population with a home of their own, but did not adequately anticipate the continuing need for relief from overcrowding. Eventually, the car permitted escape to suburban areas and the migration has led to invasive sprawl as inner city homes are left to decline.

Open space on this historic lot was originally minimized to increase density within walking distance to employment since the car was a luxury. Overcrowding was exacerbated by a fifteen-foot front yard adjacent to parked cars along the street; side yards that could be as small as six inches; and a small rear yard surrounded by buildings, fence, and alley that served to complete the encirclement.

So what do the measurements tell us when entered in Table 9.4? First, the forecast of a 513 sq. ft. footprint for a two-story, three-bedroom home in cell F32 represents a floor plan equal to many two car garages today. The home area potential in Column B of the Planning Forecast Panel shows that increased floors in Column A would produce increased area in Column B, but would also produce increased levels of intensity and dominance in Columns E and G of the Implications Module. This overwhelms the open space provided in my opinion. The density calculated in cell F14 is constant because the lot area per dwelling unit does not change, but density is an inaccurate measure of the shelter capacity, intensity, intrusion, and dominance produced by increasing floor quantities as I mentioned earlier. The shelter capacity provided was 21,929 sq. ft. per acre, but the design specifications that produced this capacity also produced an intensity of 0.286 and a dominance level of 0.686. This was for a two-story building. The Implications Module shows that a five story building would produce a dominance level of 1.563 based on the values entered in the design specification template.

Design specification values are the ingredients that produce shelter capacity, intensity, intrusion, and dominance. These were intuitive design decisions in 1907 regarding lot size, home size, impervious cover, open space, and shelter capacity. The relationship of these decisions to public health, safety, and welfare could only be anticipated based on comparison to truly inadequate historic conditions. The relationship of these decisions to physical, social, psychological, environmental, and economic quality of life was not even an issue when health and safety were at risk. These evolving decisions caused residents to seek relief from health and safety solutions that still did not reach the quality of life desired. In response, market experiments with lot size and customer preference began to consume farm land and the Natural Domain in earnest.

Table 9.5 and Diagram 9.4 are included to show the implications of a 480 sq. ft. building addition that was approved for the 3,150 sq. ft. lot. The addition reflects the occupant’s desire to increase a small habitable footprint, but the additional impervious cover reduces already inadequate combined sewer capacity. It also increases intensity from 0.286 to o.422; dominance from 0.686 to 0.822; and decreases the unpaved open space percentage from 43.21% to 35.59%. The result is increased overcrowding behind an identical façade that conceals the decline in desirability. This is the path to blight that encourages sprawl.

In other words, inadequate initial home area encouraged expansion that further compromised infrastructure capacity and increased intensity pressure levels that were already excessive. These conditions were eventually abandoned by those who could afford to search for an improved quality of life with the automobile. It has been a random search for an unmeasurable “quality of life”, and experiments have been compromised again and again by well-meaning but intuitive lot sizes, variance approvals, and rezoning requests. Experiments will continue to consume the planet with sprawl and decline until we can measure, evaluate, and forecast shelter capacity, intensity, and dominance options with the power to protect our quality of life within a limited Built Domain.

I’m only telling you what you already know. The difference is that I’m translating tacit knowledge with mathematical accuracy. It adds credibility to the debate; improves the ability to evaluate options; improves the opportunity to create knowledge; and offers the vocabulary needed by leadership during the formative stages of strategic shelter, movement, open space, and life support design. When city design evaluation and decision is documented, it becomes easier to adjust and defend the result from random requests from special interest for modifications that have long range implications for our health, safety, and quality of life.”

COMPARISON

The most relevant measurements from Tables 1, 9.4, and 9.5 are summarized in columns D-F of Table 2, but measurements need observations to become useful. The first and most important is the abstract observation that the results presented in the Planning Forecast Panel and Implication Modules of these tables were produced by correlating the measurements entered in them. The second is that the unpaved open space percentages on line 12 decrease significantly as impervious areas increase on line 13 because the total cannot exceed 100%. The third is that density remains constant in cells E14 and F14 even though intensity increases in cells E42 and F42 because the number of dwelling units does not increase. It is dwelling unit area that increases. The fourth is that driveways on line 29 of Table 2 consume a great deal of the impervious lot area allocation. The fifth is that first floor area on line 32 remains low for all examples when the impervious cover limit on line 13 is not exceeded. This encourages expansion requests over time that places further demand on a sewer system that often has inadequate capacity. The sixth is that intrusion on line 43 remains constant because floor quantity on line 35 remains constant. The seventh is that intensity and dominance increase on lines 42 and 44 because of the impact produced when all design decisions entered in the gray cells of Table 1, Table 9.4, and Table 9.5 are correlated. This means that a focus on a few independent topics of zoning can easily lead to the wrong conclusions.

A growing home market will continue to experiment by consuming more land for shelter, movement, open space, and life support until: (1) Measurement, evaluation, and correlation is recognized as an essential prerequisite for shelter design leadership; and (2) Land consumption for shelter is limited to force adjustment to the geographic boundaries of a sustainable, symbiotic shelter domain. Growing shelter sprawl seeking ideal single-family dwelling unit lot sizes over the face of the planet will continue to deplete our source of life until we recognize this self-evident truth.

From a shelter capacity perspective, the quantities in cells E41 and F41 provide the most for the land consumed, but the intensity levels in E42 and F42 and the dominance levels in E44 and F44 are inner city characteristics that have prompted flight to suburban sprawl in the twentieth century.

The lot in column D of Table 2 could be considered a minimum size if it weren’t for the 30% impervious cover limit in cell D13. This limits the first floor area predicted in cell D32 and the limited home area predicted in cell D36. (I should also mention that these are maximum first floor areas that include future expansion potential.) The obvious solution is to reduce the unpaved open space percentage and increase the impervious cover percentage planned. This would increase the shelter capacity predicted in cell D41, but also increase the intensity and dominance calculated in cells D42 and D44 - as well as the cost of the storm sewer. At this point in time it is anyone’s guess if these intensity and dominance measurements represent desirable single-family detached residential lifestyle relationships -- much less a land allocation for these units that can yield revenue per acre equal to the average revenue per acre a city requires for its desired lifestyle.

The relationships in Table 2 will vary every time one or more discretionary decision values are modified in the gray cells of its parent tables. The search for values that can be a foundation for a desirable quality of life is what is meant by the search for “balance”. The fact that every building design category and occupant activity group is affected by different sets of value decisions makes the search for an economically stable and desirable quality of life a far more complicated city design challenge than presently envisioned.

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. We are expected to discover the correlation required.

I have deleted most of the equations in the attached tables to simplify the illustrations and have omitted a detailed discussion of the Building Design Category and Residential Activity Group classification mentioned in this brief essay. 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.

POSTSCRIPT

A house is the third category of shelter within the Residential Activity Group. The other two are townhouses and apartments. It is a detached building for single-family use on a legally defined lot and became governed by minimum requirements for health, safety, and welfare in the 20th century. Early plans failed to adequately anticipate the automobile however, and early houses were tightly packed to enhance pedestrian accessibility. An intuitive response to intensity and deteriorating physical conditions produced sprawling flight to suburbs providing more space for shelter, parking, movement, open space, and life support. Lot size grew to consume increasing amounts of land as populations grew to increase the need in a limited Natural Domain that was no longer a land without end.

The unanswered question that prompts sprawl remains and is not limited to housing. It seeks to understand the area required to shelter growing human activity without excessive physical intensity. The lack of an answer has led us to consume greater amounts of land that is vaguely recognized as our source of life.









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