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.
