SINGLE FAMILY HOMES

            Single-family residential subdivisions were a reaction to imploding cities full of crime, disease and despair. Implosion was fought by public health and social reform but only reversed in the 20^th century. Inner cities were left to decline, but a return to the “farm” produced subdivision sprawl. So where are we? It’s clear that populations are growing; that intensity can be overdone and that sprawl can consume the planet if ignored, but intensity is not evil. It is simply a range of possibilities that extends from the farm to the alleys of Hong Kong.

            Intensity in a single family subdivision is equal to the building mass and pavement introduced on a given lot. In fact, if “land area” is substituted for “lot”, this is the universal definition of intensity, but a concept called “balance” makes it easier to explain. Balance is a two-dimensional concept equal to the total building cover (footprint) and pavement constructed per buildable acre. This quantity has been called development cover, but civil engineers refer to it as “impervious cover”. A lot containing development cover that exceeds the impervious cover limit of its storm sewer capacity is not in “balance”. When this is the case, three-dimensional intensity simply compounds the problem. It’s a nasty issue often encouraged by zoning ordinances that ignore these design limits. I’ve mentioned this issue before, but the following examples expand on the detail provided to explain “where we are” on the issue of balance and intensity in our most popular neighborhoods.

            The first example is included for comparison. It was a subdivision lot that represented new urbanism at the beginning of the 20^th century, and was actually a generous lot size at the time. A rear alley served garages that evolved from stables, but the alley width discouraged use because turning movements were awkward at best. This particular lot, however, was wide enough for a driveway from the street, but the street was lined with the parked cars of neighbors with narrow lots. Two-way streets became one-way parking aisles and children were warned about running between cars and talking to strangers in the alley. The lots were served by a new improvement – the combined sewer, but unpredictable storms created sewer backups in basements while children played in flooded streets and Polio terrorized the nation. It was clear that sewer capacity was inadequate, but it took longer to realize that runoff was magnified by roofs and pavement. Eventually, it became evident that capacity could never anticipate every rainfall and the term 100 year storm entered our vocabulary.  

            Storm and sanitary sewers were separated, but insufficient capacity remains the most irrefutable indication of “imbalance” and excessive intensity within the built environment. The fact that storm water runoff now requires sanitary treatment simply adds an environmental dimension to a physical problem.

            Table 1 presents the statistical profile of this 1910 lot and shows that development cover was 82% of the lot and intensity was 104.5%. (Intensity is the gross building and pavement area divided by the lot size.) The remaining 18% open space is revealing. I spent the first 8 years of my life on this lot and it always felt oppressive. The open space percentage and intensity index confirm my early impressions. The combined sewer was completely inadequate to serve the amount of development cover, and the rear yard has since been covered with a room addition to increase this cover.

The lot is now part of a seriously blighted inner city neighborhood suffering from a combination of physical, social, psychological and economic problems. These problems may not have arisen from excessive intensity; but the die was cast when decline began, since there were few attributes and fewer amenities to maintain value and encourage reinvestment. This made the location undesirable and its architecture symbolizes disease in the heart of our culture. We cannot continue to live on the leading edge of improvement and leave an expanding core of blight in our wake. This is sprawl. It will remain the message until our social and economic activities are sheltered by architecture and served by movement, open space and life support systems that symbolize a new cultural / environmental awareness -- and authority to persuasively engage in the city design of urban form.

TABLE 1

The Design Characteristics of a c.1910 Lot in ft. and sq. ft.

dimensions		area
25	80	2,000		lot		LOT
18	25	450		first floor		f1
		2		floors		f
		900		home area		h
5	20	100		porch		p1
3	3	9		porch		p2
18	25	450		garage		g
8	60	480		driveway		d
3	50	150		walk		w
BCA =		1,009		0.505		(f1+p1+p2+g)
TBA =		1,459		0.730		(h+p1+p2+g)
TPC=		630		0.315		(d+w)
DCA=		1,639		0.820		(BCA+TPC)
S=		361		0.181		(l-DCA)
TDA=		2,089				(TBA+TPC)
INT=				1.045		(TDA / LOT)
BCA =	building cover area
TBA =	total building area
TPC=	miscellaneous pavement area
DCA=	development cover area
S=	project open space
INT=	intensity

            A 50 x 120 foot lot seems to have been a mid-century breakthrough that is summarized in Table 2 as an R-8 lot. Initial construction did not approach the intensity of the 1910 lot but the R-8 lot had more room for future building expansion. This was permitted to reach 50.8% development cover and 84.8% intensity based on the zoning regulations used for this example, but storm sewer design capacity is often installed with a 30% impervious cover limit. This may be adequate for initial construction but can be inadequate to serve future expansion.

The R-8 lot has seen a great number of variance requests as home size preferences have increased because fifty-one percent development cover generally requires some form of setback encroachment; but approval threatens undocumented storm sewer capacity. Boards of zoning adjustment are reluctant to deny requests, however, because: (1) they come from neighbors, (2) setback lines are arbitrary, and (3) development cover limits that protect storm sewer capacity are rarely documented.

            In Table 2, the R-8 lot was replaced by the R-7 minimum in 1968, but the 75x120 foot standard retained the setback requirements of the R-8 lot. The lot size increase attempted to accommodate attached garage floor plans and turning movements, but holding the setback lines constant increased the potential development cover area to 54.5% and the potential intensity to 100.3%, while storm sewer capacity remained undocumented. It was an inadvertent increase, meant to accommodate lifestyle changes that failed to consider future expansion and its impact on storm sewer capacity. This lot size has also seen a great number of variance requests as setback lines interrupted attempts to reach permitted development capacity and intensity.

Table 2 shows that progress was made in the second half of the 20^th century, but that we didn’t know what we were doing. The lot sizes increased without a corresponding increase in setback requirements. This inadvertently increased potential development cover and intensity as shown, even though the intent was to reduce the intensity constructed. Building height limits also grew until potential intensity INT actually exceeded that of the 1910 lot in a number of zones. This can be seen in the R-1 to R-4 zones of Table 2. The saving grace has been that maximum development capacity is rarely constructed in these low density zones, but occurs more frequently as lot sizes decrease within the R-1 to R-8 range.

TABLE 2

Development Implications of Single-Family Residential Lots

zone	frontage	build-	density	total	total	total	total	%	%	%	INTENSITY
		able lot	per	bldg	pavement	develop.	develop.	building	develop.	net	as multiple
		area	net	area	cover	cover	area	cover	cover	open	of buildable
			acre	potential	potential	potential	potential			space /	lot
										lot	area
		BLA		TBA	TPC	DCA	TDA	BCA%	DCA%	NYA%	INT
					0.25
R-1	150	130,680	0.3	204,230	11,775	95,357	216,005	0.664	0.730	0.270	1.653
R-2	150	87,120	0.5	129,870	8,321	62,159	138,191	0.654	0.713	0.287	1.586
R-3	150	43,560	1.0	60,270	4,391	30,389	64,661	0.669	0.698	0.302	1.484
R-4	125	21,780	2.0	20,257	3,026	12,703	23,283	0.565	0.583	0.417	1.069
R-5	90	15,000	2.9	10,360	1,918	9,245	12,278	0.572	0.616	0.384	0.819
R-6	90	12,000	3.6	7,350	1,688	6,938	9,038	0.525	0.578	0.422	0.753
R-7	75	9,000	4.8	7,663	1,366	4,903	9,028	0.481	0.545	0.455	1.003
R-8	50	6,000	7.3	4,100	985	3,045	5,085	0.460	0.508	0.493	0.848
c. 1910	25	2,000	21.8	1	0	1	2,089	0.505	0.820	0.181	1.045

            The Achilles Heel in Table 2 can be seen in the BCA% and DCA% columns. Building cover and development cover potential remain greater than 30%. In effect then, any property owner has permission to exceed his or her storm sewer capacity with future building and pavement expansion; and the system is at the mercy of the law of averages among the property owners served.

            Table 3 explains the limits a 30% development cover standard would impose on an R-8 lot of 6,000 sq. ft. After all other impervious cover is subtracted; the remaining first floor plan area of 250 sq. ft. explains the need to seek 40, 50 and 60% development cover. The table predicts results based on the design specification values entered in the template above, and the key forecast is the first floor plan (footprint) area available. The table is based on the concept that an impervious cover limit (imp) establishes the total development cover that can be devoted to buildings, garages, accessory buildings and all pavements. In other words, a 30% impervious cover limit means that 70% of the lot must remain as project open space, but open space has rarely been a mandated requirement. Building setback lines have created yards, but yard encroachments are often approved for garages, accessory buildings, pavement and even primary building expansion. This increases development cover over time and places storm sewer capacity at risk. This is the price of ignoring the future implications of permitted development capacity; and the problem may exist in many communities, since the zoning standards cited are not that unusual.

TABLE 3

Impervious Cover (imp) Restrictions on Development Capacity of 6.000 sq ft Lot

Design Specification Template

	total lot area in sq ft				6,000	7.26 du / net acre
	unbuildable area as %TLA				0.0%
	detached garage spaces not including driveway				2
	detached garage height in floors				1.0
	attached garage spaces not including driveway				0
	area per parking space in sq. ft.				250
	accessory buildings in TLA				2.5%
	accessory building height in floors				1.0
	accessory pavement including driveways in TLA				15.0%
	attached garage covered by home above				0.0%
Design Calculations
imp	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8
S	0.9	0.8	0.7	0.6	0.5	0.4	0.3	0.2
Max. Home Area without Garage HNG (heavy line indicates typical range of footprint proposals)
f
1.00	-950	-350	250	850	1,450	2,050	2,650	3,250
1.50	-1,425	-525	375	1,275	2,175	3,075	3,975	4,875
2.00	-1,900	-700	500	1,700	2,900	4,100	5,300	6,500
2.50	-2,375	-875	625	2,125	3,625	5,125	6,625	8,125
Total Development Area TDA
f
1.00	-300	300	900	1,500	2,100	2,700	3,300	3,900
1.50	-775	125	1,025	1,925	2,825	3,725	4,625	5,525
2.00	-1,250	-50	1,150	2,350	3,550	4,750	5,950	7,150
2.50	-1,725	-225	1,275	2,775	4,275	5,775	7,275	8,775
Intensity Coefficient = TDA / (TLA - UNB)
f
1.00	-0.050	0.050	0.150	0.250	0.350	0.450	0.550	0.650
1.50	-0.129	0.021	0.171	0.321	0.471	0.621	0.771	0.921
2.00	-0.208	-0.008	0.192	0.392	0.592	0.792	0.992	1.192
2.50	-0.288	-0.037	0.213	0.463	0.713	0.963	1.213	1.463

            Tables 4, 5 and 6 are included as further evidence of the need to predict development capacity and coordinate it with the movement, open space and life support systems that serve it. They show that lower residential densities can actually produce higher levels of intensity. This demonstrates that the answer is not simply lot size. It depends on the ability to forecast development capacity implications with design specification values, and to use these values to define future development objectives and limitations in order to protect the interrelated functions of our built environment.

            Tables 1, 2 and 3 are based on single family residential development, but are also meant to illustrate that detached homes represent a land use activity that is sheltered by the relationship of gross building area and pavement to project open space. The design specification values are unique, but the product is the same for any activity. Building mass and pavement are offset by open space to produce a specific level of shelter intensity and impervious cover. This is not the first time I’ve mentioned this, but it is the first time I’ve mentioned that all buildings are the same. Their function, size and appearance vary; but their purpose is to provide shelter. Single family housing is simply one form of shelter that produces intensity at the lower end of the range. Building cover, development cover, project open space and building height are the common ingredients of intensity. The use of these ingredients will determine the extent of our ability to shelter growing populations within sustainable limits that protect their quality of life.

            In other words, design specification templates vary with the design category chosen. The common result is building mass, pavement and open space. Values assigned to these ingredients combine to shelter activities with levels of intensity. These levels can be consistently measured, evaluated and predicted to build a foundation of knowledge for the city design of urban form.  

TABLE 4

Impervious Cover (imp) Restrictions on Development Capacity of 9.000 sq ft Lot

Design Specification Template
total lot area in sq ft					9,000	4.84	du / net acre
	unbuildable area as %TLA				0.0%
	detached garage spaces not including driveway				0
	detached garage height in floors				0.0
	attached garage spaces not including driveway				2
	area per parking space in sq. ft.				250
	accessory buildings in TLA				2.0%
	accessory building height in floors				1.0
	accessory pavement including driveways in TLA				12.0%
	attached garage covered by home above				100.0%
Design Calculations
imp	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8
S	0.9	0.8	0.7	0.6	0.5	0.4	0.3	0.2
Max. Home Area without Garage HNG (heavy line indicates typical range of footprint proposals)
f
1.00	-860	40	940	1,840	2,740	3,640	4,540	5,440
1.50	-1,040	310	1,660	3,010	4,360	5,710	7,060	8,410
2.00	-1,220	580	2,380	4,180	5,980	7,780	9,580	11,380
2.50	-1,400	850	3,100	5,350	7,600	9,850	12,100	14,350
Total Development Area TDA
f
1.00	-180	720	1,620	2,520	3,420	4,320	5,220	6,120
1.50	-360	990	2,340	3,690	5,040	6,390	7,740	9,090
2.00	-540	1,260	3,060	4,860	6,660	8,460	10,260	12,060
2.50	-720	1,530	3,780	6,030	8,280	10,530	12,780	15,030
Intensity Coefficient = TDA / (TLA - UNB)
f
1.00	-0.020	0.080	0.180	0.280	0.380	0.480	0.580	0.680
1.50	-0.040	0.110	0.260	0.410	0.560	0.710	0.860	1.010
2.00	-0.060	0.140	0.340	0.540	0.740	0.940	1.140	1.340
2.50	-0.080	0.170	0.420	0.670	0.920	1.170	1.420	1.670

TABLE 5

Impervious Cover (imp) Restrictions on Development Capacity of ½ Acre Lot

Design Specification Template
total lot area in sq ft					21,780	2.0	du / net acre
unbuildable area as %TLA					0.0%
detached garage spaces not including driveway					0
	detached garage height in floors				0.0
	attached garage spaces not including driveway				2
	area per parking space in sq. ft.				250
	accessory buildings in TLA				2.0%
	accessory building height in floors				1.0
	accessory pavement including driveways in TLA				12.0%
	attached garage covered by home above				100.0%
Design Calculations
imp	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8
S	0.9	0.8	0.7	0.6	0.5	0.4	0.3	0.2
Max. Home Area without Garage HNG (heavy line indicates typical range of footprint proposals)
f
1.00	-1,371	807	2,985	5,163	7,341	9,519	11,697	13,875
1.50	-1,807	1,460	4,727	7,994	11,261	14,528	17,795	21,062
2.00	-2,242	2,114	6,470	10,826	15,182	19,538	23,894	28,250
2.50	-2,678	2,767	8,212	13,657	19,102	24,547	29,992	35,437
Total Development Area TDA
f
1.00	-436	1,742	3,920	6,098	8,276	10,454	12,632	14,810
1.50	-871	2,396	5,663	8,930	12,197	15,464	18,731	21,998
2.00	-1,307	3,049	7,405	11,761	16,117	20,473	24,829	29,185
2.50	-1,742	3,703	9,148	14,593	20,038	25,483	30,928	36,373
Intensity Coefficient = TDA / (TLA - UNB)
f
1.00	-0.020	0.080	0.180	0.280	0.380	0.480	0.580	0.680
1.50	-0.040	0.110	0.260	0.410	0.560	0.710	0.860	1.010
2.00	-0.060	0.140	0.340	0.540	0.740	0.940	1.140	1.340
2.50	-0.080	0.170	0.420	0.670	0.920	1.170	1.420	1.670

TABLE 6

Impervious Cover (imp) Restrictions on Development Capacity of 3 Acre Lot

Design Specification Template
	total lot area in sq ft				130,680	0.33	du / net acre
	unbuildable area as %TLA				0.0%
	detached garage spaces not including driveway				0
	detached garage height in floors				0.0
	attached garage spaces not including driveway				2
	area per parking space in sq. ft.				250
	accessory buildings in TLA				2.0%
	accessory building height in floors				1.0
	accessory pavement including driveways in TLA				12.0%
	attached garage covered by home above				100.0%
Design Calculations
imp	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8
S	0.9	0.8	0.7	0.6	0.5	0.4	0.3	0.2
Max. Home Area without Garage HNG (heavy line indicates typical range of footprint proposals)
f
1.00	-5,727	7,341	20,409	33,477	46,545	59,613	72,681	85,749
1.50	-8,341	11,261	30,863	50,465	70,067	89,669	109,271	128,873
2.00	-10,954	15,182	41,318	67,454	93,590	119,726	145,862	171,998
2.50	-13,568	19,102	51,772	84,442	117,112	149,782	182,452	215,122
Total Development Area TDA
f
1.00	-2,614	10,454	23,522	36,590	49,658	62,726	75,794	88,862
1.50	-5,227	14,375	33,977	53,579	73,181	92,783	112,385	131,987
2.00	-7,841	18,295	44,431	70,567	96,703	122,839	148,975	175,111
2.50	-10,454	22,216	54,886	87,556	120,226	152,896	185,566	218,236
Intensity Coefficient = TDA / (TLA - UNB)
f
1.00	-0.020	0.080	0.180	0.280	0.380	0.480	0.580	0.680
1.50	-0.040	0.110	0.260	0.410	0.560	0.710	0.860	1.010
2.00	-0.060	0.140	0.340	0.540	0.740	0.940	1.140	1.340
2.50	-0.080	0.170	0.420	0.670	0.920	1.170	1.420	1.670