Urban Design & Planning: Buildings Make Streets, Streets Make Cities

Stand at the end of two streets that carry the same number of people. One is 8 metres wide between five-storey walls, and in summer it is shaded all day. The other is 40 metres wide with two-storey sheds on either side, and the sun hits the tarmac from nine to five. Neither street has a single "bad" building on it. Yet you know at once which one you would walk down, and which one you would hurry along with your head down.

That feeling is not taste. It has a geometry, and urban design is the discipline that reads it: the ratio of height to width, how often a street meets another, how much floor space sits on a patch of ground, who is allowed to build what where. Architects design buildings. Urban designers and planners design the rules and the empty spaces that the buildings will end up making between them.

The street is a room: height against width

Designers talk about a street as an outdoor room, with the building faces as walls and the sky as ceiling. The simplest number for it is the ratio of building height H to the width W between the walls. Around 1 to 1 or 1 to 2, people tend to feel "enclosed" in a comfortable way; at 1 to 5 the walls are too far apart to register and the street feels like a gap. These thresholds are rules of thumb, and different authors draw the lines at different places, but the direction is not in dispute.

That second formula is what the sun and the heat feel. A deep, narrow street sees little sky: it holds its shade by day and its warmth by night. A wide street under a high summer sun is a baking tray. Old desert cities such as the medinas of North Africa are commonly described as using exactly this, with lanes only a few metres wide between tall walls so that the street is shaded most of the day.

Worked number: a street 18 m wide with 6-storey buildings (about 18 m high) has H/W = 1, and sees cos(arctan 2) = 0.447 of the sky. Double the height to 36 m and the ratio is 2, the sky view falls to 0.243.

Blocks and corners: why short blocks make a place walkable

Zoom out. The next layer is the block: the chunk of land between streets. Block size sets how many corners there are, and every corner is a choice of route and a place for a shop. Jane Jacobs, in The Death and Life of Great American Cities (1961), listed short blocks among four conditions a lively district needs (the others: mixed uses, a mix of old and new buildings, and enough people). Her argument is that long blocks funnel everyone into the same few routes and starve most streets of passers-by.

The geometry is plain. If blocks are B metres wide and streets S metres wide, the grid repeats every P = B + S. The share of land given to streets is 1 − (B/P)², the corners per square kilometre are (1000/P)², and to go n blocks east and n blocks north on a grid you can choose among C(2n, n) shortest routes (2 for n = 1, 20 for n = 3, 252 for n = 5). Large blocks are efficient in land and cheap in pipes. Small blocks cost more street and give far more routes and shop frontage.

Grids are old and worldwide. Barcelona's Eixample, approved in 1859 from Ildefons Cerdà's plan, is blocks of about 113 m with streets of 20 m and chamfered corners. Chandigarh (planned from 1950) and Brasília (inaugurated 1960) went the other way, with large superblocks designed around the car. The unplanned medina of Fez or the lanes of an old Japanese town have no grid at all, and are still walkable. Block size is a tendency, not a law.

Density is not crowding

"High density" sounds like crowding, but the two are different things. Planners measure density as floor area ratio (FAR): the total floor area of the buildings divided by the area of the plot. It is the product of two choices: how much of the ground the building covers, and how many floors it has.

The same FAR of 2 can be two floors across the whole plot, ten floors on a fifth of it, or a 20-storey tower on 10%, leaving 90% of the ground as a park. The towers-in-a-park of mid-twentieth-century modernism and the five-storey perimeter blocks of a historic European city can have the same FAR, and feel completely different. The tower leaves open ground that often belongs to no one; the perimeter block draws a clear front (the street) and a clear back (a courtyard).

Why do planners care? Because services and shops need people near them. Transit, a corner shop and a clinic each need a minimum number of customers within walking distance. And because floor area on the same plot holds more households. Worked number: at FAR 2, one hectare (10,000 m²) carries 20,000 m² of floor; at 80 m² gross per dwelling that is 250 dwellings, and at 2.5 people each, 625 people per hectare of plot. City-wide densities are far lower, because streets, parks and rail yards count in the area too.

Public space: the city is what happens between buildings

Jacobs also gave planners the phrase "eyes on the street": a street feels safe when many people, with different reasons to be there, are looking at it from shops, windows and doorsteps. The Danish architect Jan Gehl counted instead of arguing. In Life Between Buildings (Danish 1971) he showed that when a street is made good to linger on, "optional" activities, such as sitting, strolling and watching, grow while the "necessary" ones stay the same.

William H. Whyte did something similar in New York from 1970 with his Street Life Project, using time-lapse cameras on plazas. His book The Social Life of Small Urban Spaces (1980) found that plaza use did not follow the plaza's size or shape but how much sittable space it had, and he recommended a plaza set aside very roughly a tenth of its area for sitting, or about a foot of ledge for every thirty square feet. That figure comes to me from secondary summaries, not the book itself, so treat it as a rough guide. The lesson is small and practical: a bench, a ledge, shade and a place to buy coffee do more than a grand empty square.

The same logic runs at city scale. In Bogotá the Ciclovía closes main streets to cars on Sundays for people on foot and bikes, a scheme running since the 1970s that has been copied in many cities. The "15-minute city" idea, popularised by Carlos Moreno and taken up by Paris in 2020, asks that daily needs be reachable on foot or by bike in about a quarter of an hour. A walk of 15 minutes covers roughly a kilometre, so the idea is a statement about spacing: mix the uses, and keep the blocks short.

Zoning: the rules that make the invisible shape

Land-use rules decide what may be built where. Zoning in the modern sense began with New York's 1916 Zoning Resolution, the first citywide code in the United States, written to stop tall buildings from taking the light and air from the street. It did not cap height: it forced upper floors to step back, which is why so many old New York towers have a wedding-cake profile. In 1926 the US Supreme Court, in Village of Euclid v. Ambler Realty, upheld zoning by a 6-3 vote, and the "Euclidean" model of separate zones for houses, shops and factories spread.

The rules differ widely. Houston, famously, has no conventional zoning, though deed restrictions do similar work. Many countries set the framework nationally and let cities apply it, rather than every town writing its own. Defenders say zoning prevents a factory next to a house, protects light and gives owners certainty. Critics say separating uses forces the car, and that rules on minimum lot sizes, parking and height limits make housing scarce and expensive. Both are partly right; the evidence on any one rule is local.

On a single plot, the rules become a box. Setbacks carve in from the edges; a coverage cap limits the footprint; a height limit and a FAR cap limit the stack. Whichever rule bites first is the binding one, and designers learn to ask which one it is.

Informal settlements: the city that builds itself

Everything above assumes someone with permission builds on land they own. For a large part of the world's urban population that is not how it works. The UN's 2026 report on the Sustainable Development Goals, using UN-Habitat's figures, puts about 1.13 billion people in slums or informal settlements, roughly a quarter of the urban population (the share has fallen, the number has grown). These places are on the edge of Lagos, Nairobi, Mumbai, Rio, Karachi and Mexico City, and they are not all alike. Some are dense and have been there for generations; some are new, on land nobody asked about.

Why do they form? Cities grow faster than formal housing and services are provided, and the poor build what they can afford. A city of 5 million growing at 3% a year adds 150,000 people a year, about 33,000 new households at 4.5 per home, or 91 homes a day. Unless the formal market and the state supply that many, the rest is built informally. The population doubles in about 23 years at that rate.

Policy disagrees about what to do. The architect John F.C. Turner argued from his work in Peru (Housing by People, 1976) that people house themselves better than agencies do, and that the state should provide land, water and security, then step back: "sites and services" and in-place upgrading. The economist Hernando de Soto (The Mystery of Capital, 2000) argued that giving residents formal title unlocks their assets; critics say titles alone have often not delivered credit and can push prices up. Clearing and relocation, the older approach, often moves people away from jobs and has a long record of harm. In-place upgrading, such as paved lanes, drains, water and secure tenure, is now widely preferred, but it depends on local politics, and on land.

What a graduate just sees

• A street is a room: height over width sets how enclosed it feels, and how much sky and sun it gets.
• Short blocks mean more corners, more routes, more shop-fronts; big blocks mean fewer of each.
• Density is floor area per plot, and the same FAR can be a tower in a park or a courtyard block.
• Places that work give people somewhere to sit, a reason to stay, and eyes on the street.
• Rules shape form: whichever of setback, height, coverage or FAR binds first draws the building.
• About a quarter of the urban population lives where the rules never reached, and cities that grow fast build informally.
• Plans are not neutral: who gets served, who gets cleared and who is asked are design decisions.

Where it shows up next

• The Design Studio (34/96): your building has a site, a street, a code and neighbours; the first design drawing is often the plot and its rules.
• Transportation Engineering (34/69): street width, block size and signal timing are the same decision seen by two professions.
• Construction Management (34/70): on a tight urban plot, getting materials to the site is a design constraint.
• The real world: the planning department, the developer's pro forma, the street redesign near you, and every housing debate about "where should it go".

If you want to go deeper

• 📘 Textbook: Peter Hall and Mark Tewdwr-Jones, Urban and Regional Planning (Routledge): a standard planning-school text with a worldwide history.
• 📘 Textbook: Francis D.K. Ching, Architecture: Form, Space, and Order (Wiley, 5th ed. 2023): the architecture student's visual primer, including urban fabric.
• 📘 Gentler: Jane Jacobs, The Death and Life of Great American Cities (1961), and Jan Gehl, Cities for People (2010).
• ▶️ Free course: MIT OpenCourseWare 11.001J / 4.250J Introduction to Urban Design and Development.
• 🌐 Easier first read in this library: How to Read a Building (08/10) and Transportation Engineering (34/69).