BUNJU, ON THE outskirts of Dar es Salaam, Tanzania’s largest city, has many of the hallmarks of a new African suburb. The houses are a mix of finished, half-finished and not-yet-started. Though the government has yet to pave the roads, firms have already moved in. There is an English-language private school, a pet shop, a water park, a gym and a pharmacy. Yet there is still ample evidence of Bunju’s recent past as farmland. Piles of bricks squat in patches of maize. Boys climb palm trees to fetch coconuts that they sell by the main road.

Africa, which is already quite urban, is urbanising faster than any other part of the world. By 2050 the number of African megacities (those with at least 10m people) will rise from three to 17, the second-most of any continent, forecasts Africapolis, an initiative of the Sahel and West Africa Club, a group affiliated with the OECD, a club mainly of rich countries. These include Dar es Salaam, whose population will more than double from 6.9m to 15.6m (see map). Much of the growth will be beyond current boundaries, as the city absorbs once-rural areas. These are not suburbs typical of the West. An idiosyncratic, African suburbia is taking shape—and reshaping the continent.

At some point earlier this century the population of Africa became, for the first time, predominantly urban, according to Africapolis. It classifies any agglomeration of at least 10,000 people as urban. From 2000 to 2025 Africa’s urban population soared from 248m to 829m—or from 32% to 57% of the total. (The UN, using a narrower definition, reckons that 45% of Africans lead urban lives.) Some 106 cities have at least 1m people.

To probe how African cities have changed over the past decade, The Economist used WorldPop, an open-source provider of spatial and demographic data. We took its estimates for the population density of every square kilometre in Africa, then split the continent into urban cores (at least 1,500 people per square kilometre), suburban (between 300 and 1,500) and rural areas (fewer than 300). We estimate that the total area of cities with at least 100,000 people grew by 74,000 square kilometres—slightly more than the size of Ireland. Almost three-quarters (72%) of this expanding footprint was suburban growth. When mapping cities’ expansion over time, the process resembles an ink blot seeping through paper (see map, right).

It is on the fringes where the African middle class is being made. One case study is Salisala, an area south of Bunju, assessed by Claire Mercer of the London School of Economics (LSE). She notes in a book, “The Suburban Frontier”, how it shares some characteristics of rich-world suburbs, such as the importance of cars. But whereas Western suburbs are typically planned and connected by local municipalities to services before houses are constructed, places like Salisala are part of a “self-built suburban frontier”.

In Dar es Salaam’s suburbs plots are bought on a grey market from informal brokers, many of whom speculate on future city growth. Houses go up over many years, in fits and starts as cash becomes available. This is typical in sub-Saharan Africa, where the World Bank estimates that just 5% of adults have taken out a mortgage in the past year, many in South Africa.

African cities are set to expand ever farther outward. A forthcoming World Bank report projects that from 2025 to 2050 the physical area of cities in sub-Saharan Africa will grow by 90%, slightly faster than the expected 85% growth of their populations. This may be an underestimate. Urban scholars use a rule of thumb, based on historical trends in developing countries, that when a city’s population doubles the area it occupies triples.

Yet the correlation between urbanisation and GDP per person is weaker in Africa than in Asia, argues Kurt Lockhart of the Africa Urban Lab (AUL), a group at the African School of Economics in Tanzania. Africa is urbanising while still poorer than other places were when they did so. When 40% of Africa’s population became urban, its average output per person was $1,500; the corresponding figure for Latin America was $2,500 and for East Asia $5,400.

Poverty is a cause and an effect of Africa’s sub-optimal urbanisation. From 2014 to 2020 African governments spent just $27 per person on infrastructure. The effects can be seen in the congestion clogging African cities. In Manhattan and London about a quarter of the land is given over to roads; in Dakar, Accra and Nairobi it is roughly one-eighth.

It does not help that African cities often get round to providing services only after areas are populated. Retrofitting settlements to provide them with infrastructure is three times pricier than putting it there beforehand, says the International Growth Centre, based at the LSE. It is even harder when cities grow so big that they span different local-government units. Bamako, the capital of Mali, today covers 12 administrative jurisdictions.

Onerous regulations and uncertain tenure reduce housing supply, pushing up costs. The UN reckons Africa lacks 120m houses. It notes that more than half of households that pay rent in sub-Saharan Africa spend at least 30% of their income on it, more than in any other region.

Africa, in other words, is not taking full advantage of what cities can give. Better urban planning would help. But Africa has an average of one urban planner per 100,000 people, a twentieth of the ratio in the OECD. Many complex, donor-funded spatial plans are never implemented.

The challenge is to find a sweet spot between no planning and over-planning. Patrick Lamson-Hall, who leads the urban expansion programme at AUL, says that African cities that work are doing what American and European cities did in the 19th century: predicting growth, providing a basic framework (arterial roads and housing plots), and letting the city grow organically. The use of basic plans in Ethiopian cities has been shown to raise the average income of residents relative to control groups in unplanned cities. Kenya, Mozambique, Rwanda, Somaliland and Uganda are implementing similar “urban expansion planning”.

Bunju, too, is an example of such a rudimentarily planned suburb. In the 2000s Tanzania’s government subdivided the land and sketched out roads. The scheme had flaws: plots were too large and Tanzanians grumble that the well-connected got first dibs. But recent research suggests that land values are higher than in unplanned suburbs that were used as controls.

The implication is clear. African cities are expanding fast. It is better to plan for an urban future than react after the fact. ■