Site analysis

The second-year studio at the Elmhurst School of Architecture opens in September with a bus ride rather than a lecture. Professor Leila Haddad, who ran a practice designing schools and libraries for fifteen years before she started teaching, takes Wren, Callum, Ingrid and Joon-ho and the rest of the class to a corner of Lowry Avenue and Marlow Street in north Minneapolis. The lot is empty except for a cracked concrete slab, a chain-link fence, a bus shelter at the corner and one enormous bur oak near the back. 'Last year you designed a house for one person,' Haddad says. 'This term you will design a building for a whole neighborhood: a library branch, a gym and hall, a community kitchen and a daycare, all under one roof. Before any of that, you are going to spend three weeks learning everything this piece of ground can tell you.'

The project is a real-feeling one. The city has bought the lot, the site of a bus garage demolished several years ago, and has promised the neighborhood a community hub. The neighborhood association, chaired by a retired machinist named Ernest Kowalchuk, has collected more than six hundred signatures asking for it. The branch librarian, Sam Okafor, and the director of a daycare that has outgrown a church basement, Rita Moreau, will act as clients alongside the city. The students will meet all three in the coming weeks. First they meet the site.

Why site analysis comes first

In Design Studio I and II the students worked on a park terrace with few neighbors and few rules. A city lot is different. It sits inside a web of streets, utilities, zoning rules, neighbors' expectations and existing patterns of use, and the building will change all of them. Site analysis is the systematic gathering and drawing of these conditions, so that the design responds to the place rather than being dropped onto it. It is also a form of respect: a neighborhood that has waited years for a building deserves an architect who has looked carefully at what is already there.

Haddad describes three kinds of information. The first is measurable and fixed: the lot lines, the slope of the ground, the soil, the utilities, the sun angles. The second is measurable but changing: traffic at rush hour, noise from the bus route, the shadow of the school across the street at different times of year, the wind in January. The third is human and harder to measure: who walks across the lot as a shortcut, where children wait for the bus, what the old bus garage meant to the people who worked there, which corners feel safe after dark. A good analysis records all three, and the third kind is often the one that shapes the best ideas.

The layers of a site

A standard site analysis is drawn as a series of layers on a common base plan, each showing one kind of information. Layers can be drawn by hand on tracing paper over a printed survey, or on separate layers in a digital model. Overlaying them reveals relationships that no single drawing shows: the path that children already take may cross the best spot for the playground; the quietest corner may also be the darkest in winter.

• Legal: lot lines, setbacks, easements, height limits, coverage limits and permitted uses.
• Physical: topography, drainage, soil, the existing slab and buried services.
• Climatic: sun path, shadows from neighbors, prevailing winds by season, snow drifting.
• Living: trees and vegetation, with species, size and condition.
• Movement: streets, the bus stop, sidewalks, the greenway path, the back lane, desire lines.
• Sensory: noise, views out, views in, smells, light at night.
• Social and historical: what the site was, who uses it now, the character of the neighboring buildings.

The lot and its rules

The lot is a rectangle 230 ft along the avenue and 165 ft deep, an area of about 37,950 ft². The city's zoning for this kind of community site sets a front setback of 10 ft from the avenue, 10 ft from Marlow Street, 15 ft from the west lot line, where houses back onto the lane, and 20 ft from the greenway at the rear. Buildings may be up to 40 ft high, and no more than 60 out of every 100 units of lot area may be covered by buildings. A setback is the minimum distance between a building and a lot line; the area left inside all the setbacks is the buildable area, sometimes called the building envelope in a zoning sense.

Two other numbers describe how much building a lot can hold. Site coverage is the building footprint divided by the lot area. Floor area ratio, often written FAR, is the total gross floor area of all stories divided by the lot area. A one-story building covering half the lot has a FAR of 0.5; a two-story building on the same footprint has a FAR of 1.0. Many zoning codes cap one or both. On this site, the coverage limit matters more than the FAR, because a community hub with a gym is a low, spread-out building.

# Buildable area, coverage and floor area ratio for the corner lot
lot_w, lot_d = 230, 165                 # along the avenue, depth to the greenway (ft)
front, side_st, west, rear = 10, 10, 15, 20
lot = lot_w * lot_d
build_w = lot_w - side_st - west
build_d = lot_d - front - rear
buildable = build_w * build_d
max_cover = 0.60 * lot
print(f"Lot area:        {lot:9,.0f} sq ft")
print(f"Buildable box:   {build_w:.1f} x {build_d:.1f} = {buildable:,.0f} sq ft")
print(f"Coverage limit:  {max_cover:9,.0f} sq ft")
gross = 26700                         # estimated gross floor area of the hub
for floors in (1, 2):
    footprint = gross / floors
    ok = "fits" if footprint <= min(buildable, max_cover) else "too big"
    print(f"{floors} story(s): footprint {footprint:,.0f} sq ft, coverage {footprint / lot:.2f}, FAR {gross / lot:.2f} -> {ok}")

Lot area:           37,950 sq ft
Buildable box:   205.0 x 135.0 = 27,675 sq ft
Coverage limit:     22,770 sq ft
1 story(s): footprint 26,700 sq ft, coverage 0.70, FAR 0.70 -> too big
2 story(s): footprint 13,350 sq ft, coverage 0.35, FAR 0.70 -> fits

The calculation settles the first big question before any drawing. If the hub were a single story, its footprint would exceed the coverage limit, and it would leave almost no room for the daycare's outdoor play area. At least part of the building must be two stories. The students also notice that the buildable area is long and fairly shallow, which suggests a building that stretches along the avenue and leaves the south side of the lot open.

Measuring and walking the site

Haddad insists that the students walk the site at least four times before the analysis is due: on a weekday morning when children are going to school, at midday, in the late afternoon at rush hour, and after dark. Each visit has a purpose. In the morning, Joon-ho counts thirty-one people crossing the lot diagonally from the bus stop toward the greenway path, through a gap in the fence that has been cut and re-cut for years. At midday, Ingrid notices that the shadow of the three-story school across the avenue does not reach the lot at all, because the school is to the north. In the late afternoon, Callum records traffic noise of about 70 decibels at the corner but only about 55 near the oak, because the distance and the slope of the ground protect the back of the lot. After dark, Wren finds that the corner by the bus shelter is well lit and busy, while the greenway is dark and empty.

Measurements are simple but important. A long tape, a hand level or a phone inclinometer, a sound-level app, a compass and a camera are enough for a first analysis. The city's survey gives the lot lines and spot elevations; the students check them on the ground and find that the survey is right about the lot but wrong about the oak, which is about 6 ft further east than drawn. Small errors of this kind are common, and they are one reason architects visit sites in person.

A desire line is a path worn by people choosing the most direct route, regardless of the official paths. Planners and landscape architects take them seriously because they show real patterns of movement. The diagonal from the bus stop to the greenway is the strongest piece of evidence on the site about how the neighborhood actually moves, and the students will have to decide whether their building blocks it, redirects it, or makes it part of the design.

Trees, soil and what is underground

The bur oak is the most valuable thing on the site. A city arborist estimates it at about a hundred and twenty years old, with a trunk about 36 in in diameter at chest height and a crown about 65 ft across. Bur oaks are long-lived, tolerate the region's cold and drought and are protected under many municipal tree bylaws. Protecting a large tree during construction means protecting its roots, which spread far beyond the trunk, mostly in the top 2 ft of soil. A common rule of thumb sets a tree protection zone with a radius of about 12 times the trunk diameter, measured from the trunk. Inside that zone, no digging, paving, storage of materials or driving of machinery is allowed.

# Tree protection zone for the bur oak (rule of thumb: radius = 12 x trunk diameter)
import math
dbh = 36                      # trunk diameter at chest height (in)
radius = 12 * dbh / 12       # in ft
area = math.pi * radius ** 2
print(f"Protection radius: {radius:.1f} ft")
print(f"Protected area:    {area:,.0f} sq ft")
print(f"Share of the lot:  {area / 37950:.2f}")

Protection radius: 36.0 ft
Protected area:    4,072 sq ft
Share of the lot:  0.11

The protected zone covers more than a tenth of the lot. Since the oak stands near the rear setback, most of the zone falls within land that would be open space anyway, but it rules out a building or a parking area at the south-west corner. Haddad points out that the tree also gives the best shade on the site in summer and is the obvious anchor for the daycare's play yard.

Below ground, the students request the geotechnical report the city commissioned. It describes about 3 ft of fill left from the bus garage, over a deep layer of stiff clay typical of the region's old lake bed. Clays of this kind swell when wet and shrink when dry, which affects foundations; the report recommends piles rather than shallow footings for a building of this size. It also notes that the old garage slab contains oil staining near its north-east corner, which will have to be removed and disposed of before construction. The city's utility maps show water and sewer mains in the avenue, a gas line in Marlow Street and overhead power along the back lane.

Neighbors, context and character

A community building belongs to its neighborhood, so the analysis must look beyond the lot lines. To the north, across the avenue, is the elementary school, a brick building from the 1950s with a busy drop-off loop. To the east, across Marlow Street, are a corner store, a brick Lutheran church with a tall steeple and a row of small stores with apartments above. To the west, behind the lane, are one- and one-and-a-half-story houses with deep backyards, many with vegetable gardens. To the south, the greenway runs along the old rail line toward the river and Kestrel Point, the park where the students built their first-year pavilions.

The students draw a figure-ground plan of four blocks around the site, with buildings in black and open space in white, and an elevation of each street frontage. The drawings show that the avenue has a mix of heights and setbacks, that the side streets are low and residential, and that there is no public building with a strong presence on the avenue between the school and the church. Ingrid writes in her notes that the hub can be the third civic building in a small cluster, and that the corner by the bus stop is where it should announce itself.

Talking to the people who know the site

Site analysis is not only observation. Haddad arranges a walk with Ernest Kowalchuk, who worked at the bus garage for twenty-two years. He tells them where the garage doors were, that the site flooded every spring before the city regraded the lane, that children have always used the gap in the fence, and that people at the bus stop have nowhere to wait out of the wind in January. Rita Moreau, the daycare director, walks the back of the lot and says the oak is the reason she wants to move here. Sam Okafor, the librarian, asks that the library be visible from the bus stop, because people who see a library are more likely to come in.

These conversations are recorded alongside the drawn analysis. They often contain the most important insights, and they also begin a relationship of trust between the designers and the community that will continue through the project.

Exercise 1: the site analysis

This exercise takes about three weeks and is done in groups of four, with each student responsible for at least two layers.

• Visit the site at least four times, at different times of day, and keep a dated log of observations, sketches, photographs and measurements.
• Draw a base site plan at 1 in = 20 ft from the survey and check its key dimensions on site.
• Draw at least seven analysis layers on the base: legal, physical, climatic, living, movement, sensory and social.
• Calculate the buildable area, the coverage limit and a range of footprints for one- and two-story schemes.
• Draw a figure-ground plan of the surrounding four blocks and elevations of the street frontages.
• Interview at least two people who know the site and record what they say.
• Finish with a single synthesis drawing that shows the five most important conditions and the opportunities they suggest.

Self-critique checklist

• Have I recorded conditions that change, such as noise and light, as well as fixed ones?
• Does every layer use the same base and scale, so they can be overlaid?
• Have I checked the survey against what I saw on site?
• Does my synthesis drawing make choices, rather than repeating every layer?
• Have I included what the neighbors told me, not only what I measured?