Site Planning
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- Last Updated on Wednesday, 7 May 2014
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In planning and placemaking, density is an important tool for measuring the intensity or quantity of units or items occupying a given square area, such as homes/dwelling units per acre, parking spaces per thousand square feet of retail, or households per square mile. Understanding the various kinds of densities that can be used to describe a given city, district, or land parcel helps in planning, designing and combining the appropriate types and sizes of roads, utilities, sidewalks, buildings, and landscaping to properly serve the uses and create more livable places.
Because cities and settlements are mainly comprised of housing, population and housing density are particularly relevant measures. The physical character of a residential or mixed-use neighborhood is largely determined by its housing density; crowding would be the result of excessive population density for a given housing density.
Specific building types are associated with ranges of housing densities. The illustration on the right depicts a very simple spread of residential densities with low density single-family homes ranging from 4 to 10 dwelling units per acre, medium density townhomes (with horizontally attached units) at 20 to 40 dwelling units per acre, and high density apartments (with horizontally and vertically attached units) at 50 to 100 units per acre. Real-world examples of high, medium, and low-density housing are illustrated below:
There are also building type sub-classifications which break down residential densities into finer gradations, as shown in this image from the Affordable Housing Design Advisor.
Each building type has different physical characteristics and density ranges. These ranges may vary according to local building codes and other conditions.
The residential density matrix below describes characteristics for various housing types:
It is difficult to correlate density solely to building type or height because factors such as FAR and site coverage also influence density calculations and perceptions.
For example, a high rise apartment tower with a large open-space area and a dense, mid-rise development with a smaller but better monitored open space area may have an equivalent overall site density, despite being very different in character in livability.
For other land uses (e.g. workplaces or retail areas), measures such as F.A.R., employees per square foot, and parking spaces per thousand square feet of leasable space provide additional indications of density. It can be difficult to directly compare different types of use districts solely using density measures. While height, F.A.R., and density are good starting points, the frequency of entries, building articulation, setbacks, and streetscape design largely influence street activity and livability, and should also be included in comparisons.
Density and District Form
In most urban areas, density follows a gradient of district types spanning from rural (lower density) to urban (higher density), called the urban transect. A variety of factors contribute to the density and complexity seen in different urban forms (see PDF for more information).
Finally, a sharply defined downtown core district, as shown in the image at right, is one way density can be used to create contrast, interest, and place distinctiveness.
Floor Area Ratio
A Floor Area Ratio (F.A.R.) is the mathematical proportion formed by a building’s built floor area (for all floors) divided by the area of its land parcel. For example, a one-story building with a floor plan length and width of 100 feet by 100 feet on the same size land parcel would have an F.A.R. of 1.0.
A four (4) story building with a floor plan length and width of 50 feet by 50 feet on the same 100 foot by 100 foot parcel would also have an F.A.R. of 1.0, as shown in the image at right.
F.A.R. is a limited “yardstick” that can convey a general intensity of constructed building space on a given piece of land, but it will not indicate the character of that built volume in terms of the building’s height, length, or width, or the amount of remaining open space (unbuilt land) on the parcel. As the figures below illustrate, developments with the same F.A.R. can have very different physical characteristics - that may or may not be compatible with the urban scale and density desired in various types of neighborhoods and street environments.
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- Last Updated on Wednesday, 7 May 2014
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Urban Scale is a term that describes the sense of height, bulk, and architectural articulation of a place or individual building, often in relation to the size of a human body. The use of the term “urban” also suggests the importance of the overall context of an area, and the role neighboring buildings play in establishing a sense of scale. Groups of buildings and the spaces between them establish a high, medium, or low urban scale.
The images at right illustrate the way in which buildings are grouped in various places to create a distinct urban scale. A downtown skyscraper district of a major city is clearly of a great or high urban scale. The buildings are tall, and often have substantial mass or girth.
An area with buildings predominantly between four (4) and eight (8) stories is typically considered to be of a medium scale; however the city context makes a difference. Eight stories might be considered a high-rise urban scale in a small city, whereas it could easily be thought of as a medium urban scale in a large city.
The image below, at left, is an example of medium urban scale, although these boundaries may vary by place. A one and two-story residential neighborhood, as shown in the image below/at right, includes predominantly low-height buildings, which establish a low urban scale. Increased horizontal spacing of buildings and the corresponding increase in space between buildings also contributes to the low urban scale.
The urban transect diagram shown in this PDF outlines the structure of typical urban settlement patterns, described using increasing increments of urban scale, or transect zones. The scale divides areas into six (6) transect zones, extending from T1 (rural preserve) to T6 (urban core). The figure also identifies a number of features that contribute to land use intensity and urban scale, such as setbacks, greenspace, lighting, and building materials.
The key to creating successful mixed-scale neighborhoods and districts, such as the one shown at right, lies in effectively transitioning between buildings and clusters of different scales. The appropriate relationship and transition between buildings and clusters should be outlined in a City’s building and design policies.
Specific development standards and design guidelines can help residents understand building policies and practices and provide developers with better guidance to facilitate approvals.
The figure at right depicts one example of a “form-based code” approach to setting height limits for a downtown district.
While form-based codes and height limits help establish a basis for urban scale, the architectural articulations of building height and massing largely influence the feel and urban scale of an area.
In addition to a building’s general dimensions, the articulation of the mass and facade play a critical role in defining urban scale. Buildings that lack traditional scale cues such as individual window openings and sills, visible stories, intermediate cornices, entryway features, or a building base may seem overwhelming and can be confusing with respect to scale.
For example, in the absence of the small brick side buildings in the photo at right, the actual height and scale of black glass cube building would be difficult to determine. When a large building has none or very few of the scale references listed above, it is said to lack human scale.
Buildings of extremely different heights and contrasting levels of articulation of scale (as shown at right) may create an unattractive and undesirable clash of scales that does not reflect well on either building, or on the district of which they form a part.
Misfit in urban scale, especially between low to medium scale residential buildings, as shown below, at left, can have a substantial negative impact on privacy, livability, real estate values, and neighborhood character.
With careful attention to design and articulation, large buildings can be visually broken down into smaller, more human-scaled components, as shown in the image below, at right.
Emphasizing the ground floor of a building can help create a more interesting streetscape and pedestrian environment, and sculpting back the mass of the building with upper level setbacks can help reduce the size and impact of the street wall.
Urban scale is the term used to describe some of the most important characteristics of streets, including the perceived scale of the street vertically, the sense of enclosure within a “street room” and the degree of definition of the street and its addresses as a place. The term is defined as the height of the street wall relative to the width of the street, from street wall to street wall (building face to building face). Thus, urban scale is measured using the “height to width ratio” laterally across the street.
The urban scale of a “street room” has a major effect on how the street is perceived by motorists and pedestrians. Streets with high ratios of height to width feel urban and enclosed. There is a strong sense of spatial definition and enclosure that can be supportive to placemaking.
However, streets with very high urban scale – greater than 3:1 – can begin to feel like “concrete canyons” and can be claustrophobic, especially along narrower streets. Such streets can be cold and harsh in winter. In the St. Louis region, as in many other parts of the U.S., such streets also can concentrate and redirect winds at sidewalk level in ways that are not conducive to a comfortable walking environment.
In temperate climate zones, “sun access” can be another key issue in planning and design. On east-west streets with high urban scale, the sun may not reach lower floors of buildings during winter months. On high scale north-south streets, windows of buildings may receive little direct sunlight any month of the year.
In the photo at the right, Market Street in San Francisco is an example of a high scale street. Market is a major multimodal transportation corridor and one of San Francisco’s most important commercial streets. It also feels intensely urban. However, this effect is mitigated significantly through good design (including wide sidewalks that reduce the impact of the street) and by wide variation in building height up and down the street. Although San Francisco is at a similar latitude to St. Louis (37° 40’ compared to 38° 40’), it has generally a more moderate climate (especially in the winter) and thus the shading and loss of direct sun on this east-west street is less important than it might be elsewhere.
At the other end of the range, streets with very low urban scale – less than 1:4 – exhibit little spatial definition. Such streets feel similar to certain large ballrooms in hotels or meeting rooms in conference centers. Rooms that are, say, 120 feet from wall to wall with ceilings at 20’ or less are perceived not in their entirety but as a series of spaces defined by furniture, potted plants, dividers, even changes in floor covering. Streets are experienced in a similar manner (with the sky as ceiling).
Wide, low scale streets are perceived as a series of spaces with little coherence. The street walls (buildings) do not enclose and define the street enough for it to be perceived as a place. Placemaking along such streets is difficult and requires the use of other defining elements – trees, walls, and street furniture.
An example of a low scale street in the St. Louis region is shown in the photo on the right. Here there is no enclosure at all and the street appears almost rural, even though traffic is heavy and the adjacent land uses are all commercial. This kind of environment is almost completely intolerable for pedestrians.
The perceived urban scale of low scale streets can be increased through use of street trees. For example, the urban scale of the street in the photo on the right – Main Street in Pleasanton, California – is enhanced by the street trees that increase the perceived height to width ratio above what it would be if measured by the height of the buildings along the street.
Graphic representations of common height to width ratios and photos of corresponding examples are shown below. Generally, “great streets” will have urban scale ratios that fall in a range between 3:1 at the high end and 1:3 at the low end.
3:1 Height to Width Ratio (see examples below)
Sense of spatial definition: strong; may feel like a “concrete canyon” in some settings
Skyview: very narrow; viewing tops of buildings requires changing neck angle
Lowest sun angle reaching bottom of north street wall at mid-day: 71° (east-west street)
3:2 Height to Width Ratio (see examples below)
Sense of spatial definition: strong; clear sense of enclosure
Skyview: limited; viewing tops of buildings requires changing neck angle
Lowest sun angle reaching bottom of north street wall at mid-day:
56° (east-west street)
1:1 Height to Width Ratio (see examples below)
Sense of spatial definition: high, strong placemaking potential
Skyview: limited, peripheral only
Lowest sun angle reaching bottom of north street wall at mid-day:
45° (east-west street)
1:2 Height to Width Ratio (see examples below)
Sense of spatial definition: good; sufficient for placemaking
Skyview: views of sky about equal to visual field occupied by street wall
Lowest sun angle reaching bottom of north street wall at mid-day:
27° (east-west street)
1:4 Height to Width Ratio (see examples below)
Sense of spatial definition: weak; placemaking potential is low
Skyview: three times as much sky as wall within normal range of human vision
Lowest sun angle reaching bottom of north street wall at mid-day:
15° (east-west street)
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Source: Cornell University The solar access and exposure of a building, site, or street (and district) determines the availability of sunlight for daylighting, and affects human health and mental well-being, plant growth, heating and cooling needs (including window size and shading) and performance of photovoltaic panels. Solar access and exposure can be discussed as they pertain to site design and urban/street design.
Individual Site and Building Design:
The solar access and exposure of a building and its site influence the quantity of unobstructed sunlight that can fall on a given building and/or site.
The angle of the sun above the horizon, the resulting strength of the sunlight, and the length of exposure time are affected by the building’s or site’s latitude, the time of year, cloud cover, and air quality.
Since morning solar exposure comes from the east and south and afternoon sunlight comes from the west and south, the orientation of the building or site also affects solar access and exposure.
Nearby buildings, trees, and landforms may shade a building or site and obstruct sunlight. The diagrams at right show the impact of solar angle and building orientation on the sunlight a building receives.
Urban / Street Design
From an urban/street design perspective, solar access and exposure relate mainly to the shadowing effects of groups of buildings, trees and landforms on buildings and spaces, with a particular emphasis on open spaces such as streets and plazas.
The research of William H. Whyte - The Social Life of Small Urban Spaces - has shown that solar access and exposure are key factors in creating attractive settings for pedestrian gathering in streets and plazas.
As such, urban design controls for regulating building height, bulk, setback, and related factors are important in maintaining solar access and exposure for livable, active streets and corridor spaces.
Also see the article on Urban Scale
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- Last Updated on Wednesday, 7 May 2014
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Height limits are one of many planning tools used to manage the density and character of development in urban areas. Buildings of similar height will typically have a more uniform appearance. Height homogeny can also help link buildings together. The range of human vision largely affects the perception of appropriate street space and scale. Height-to-width ratios can be used to describe the relationship between the relative amounts of visible street wall/buildings and sky.
In areas with very low height-to-width ratios, creating spatial definition can be quite difficult. Conversely, areas with height-to-width ratios at or above 3:2 can seem claustrophobic and overwhelming, and buildings may be difficult to read.
In light of the important role building height plays in shaping the streetscape, some communities choose to implement height limits. Height limits can be used to protect the architectural integrity and prominence of important buildings, mountain, waterfront, or other scenic vistas (e.g. citywide height limits implemented in San Francisco), or protect solar access to streets.
Boundaries between different height districts can be addressed in a number of ways, including building height contours, transitional height zones, point net envelope, and variable height extension.
Establish a building height minimum. Buildings with less than two-stories fail to create a sense of enclosure and space. Generally, wider streets require taller street walls, according to the ratios above. Real estate value plays a role in determining the market's desire to build up. In major downtown areas, height limits may be enforced for a number of reasons. For most other areas, where real estate values are not as high, and particularly along commercial/service corridors, building height minimums are important in creating a street-wall and welcoming pedestrian environment.
Recognize different needs for different uses. Some ordinances address maximum building height using "feet" and others use floors or stories. Specifying height limits in floors can allow retail spaces in commercial and mixed-use settings the higher ceilings desired on the ground floor (16' to 20'). Specifying a height limit in feet, for example 45', can unintentionally limit commercial buildings to three floors. Similarly, "loft" style housing is defined in part as having high ceilings, so mixed-use developments that are trying to create loft housing may conflict with height limits more often than traditional mid-century types of residential development.
Consider height requirements and/or limits in combination with density and solar access. While height limits are the most common tool for controlling building height, communities may also use solar access angles and density controls linked to lot size.
Allow for step-back. Some ordinances address overall height but also address the height of the street wall, allowing projects to step-back with subsequent stories so that the apparent scale is not as great, but the density can still be higher. Step-back design may also improve solar access.
Recognize the impact of height requirements and/or limits on building construction. Depending on local circumstances, such as building codes, fire codes, climate, seismic classification, and others, height requirements and/or limits can determine the type of construction that is possible. Generally, buildings taller than three floors must have elevators. Some cities require any commercial space above ground floor (including office) to be elevator served. Depending on local codes, the practical limit for wood frame buildings is somewhere around three to five floors. Anything above that may require steel or reinforced concrete framing. This can vary with zoning. Some ordinances address total height and others address height to the eave or drip line, allowing sloped roofs and dormers above. Ordinances should be explicit about accepted variations.
Also see the article on Urban Scale