Elbow room

A growing city has two places to put its new people. It can build at its edge, on farmland, desert or land reclaimed from the sea, or it can crowd more people onto the land it has already built. The split decides how far people travel to work, how much road and pipe the city must lay and how much land it takes.

Three fast-growing cities split their growth very differently between 1975 and 2020. Shenzhen and Hong Kong went from 5.7 million residents to 26.7 million, the Lagos area from 3.3 million to 13.7 million and Cairo from 9.4 million to 27.0 million. I used satellite maps of their buildings since 1975 to find where each one put its new people.

How can a map tell growing out from filling in?

I dated the land under each new resident. The European Commission's satellite record maps buildings every five years from 1975, so I gave each square kilometre of a city the year buildings first covered a good part of its ground. I then grouped the squares by that year, which splits the residents a city added into those living on land built by 1975 and those living on land built since.

The three cities split almost opposite ways. Lagos put 64% of its added residents on land already built by 1975, Cairo 49%, Shenzhen and Hong Kong only 18%. I reran the split nine ways, with squares from 90 metres to a kilometre across and with a tenth to a third of the ground covered, to check the order holds. It does. Lagos keeps 53 to 67% of its growth on old land, Cairo 32 to 56% and Shenzhen and Hong Kong 10 to 27%.

Did the state open the land in Shenzhen?

The state drew the first line. Shenzhen became a special economic zone in 1980, a strip along the Hong Kong border. The zone's built-up area grew from under 3 km² to 136 km² by 2000, according to the geographer Jianfa Shen. The growth soon ran past its fence. From 1988 the districts outside the zone drew more migrant workers than the zone itself, as factories set up on village land in Bao'an county.

When I split Shenzhen by district, most of its growth sits in that old county. Of the 11.9 million residents Shenzhen added between 1975 and 2020, 68% live in its outer districts, four in five of them on land first built after 1975. The zone was widened to take in the whole city only in July 2010.

Hong Kong grew outward by plan. Its new towns, begun in 1973 on land partly reclaimed from the sea, house about 3.6 million people today. Hong Kong took 15% of the residents added across the map, against 25% for Dongguan, the factory city to the north.

Why did Lagos fill in?

Water hems in the old city, with the lagoon on one side and the Atlantic on the other. The banks, head offices and big firms stayed on Lagos Island, Victoria Island and Ikoyi. New residents crowded onto the mainland within reach of the bridges.

Land had to be made from water. Reclamation took Lagos State from 3,577 km² to 4,050 km² before the state government suspended it in September 2025. Within Lagos State, 79% of the residents added between 1975 and 2020 live on land already built by 1975. The number of people on each km² of built-up land rose by almost a third after 1990.

The overflow crossed the state line to the north. Ogun State took 22% of the wider city's added residents, three in four of them on new land.

Did the desert towns take Cairo's growth?

Egypt has spent five decades building towns in the desert to draw people out of the crowded Nile valley. The first, such as 10th of Ramadan and 6th of October, date from the late 1970s. New Cairo followed in 2000.

I marked as desert any land with almost no green cover around it. The buildings followed the plan. Desert land took 44% of the ground Cairo built on between 1975 and 2020. The people mostly stayed behind. Desert land holds about one in ten of the residents Cairo added, while the old city and the farmland around it took the rest.

Town by town, the gap is wide. On Egypt's official estimates for 2024, the six desert towns with published plans held about 960,000 people, 11% of the 8.6 million they were built for. Most of Cairo's growth went instead into informal districts built without permits, often on farmland, which the urban planner David Sims puts at about 70% of the city.

Is Lagos unusual?

To place the three among the world's big cities, I used a simpler measure that runs on every one of them. It counts the residents on each km² of land under buildings inside a city's boundary, in 2020 against 1990. Of 499 cities of a million people or more, 53% grew more crowded on this measure.

Residents per km² in Lagos rose by a third, more than in 77% of the others. Cairo, up 8%, sits near the middle and Hong Kong fell by a tenth. The cities that spread out fastest are Chinese and Indian, led by Chengdu, where residents per km² of building land fell 60%. Most of those that filled in fastest are in Africa and the Gulf. Shenzhen has no entry of its own, because the database folds it into one city of 41 million around Guangzhou.

So why did they grow out or fill in?

Shenzhen grew out because the land beyond its zone lay open to factories and to the villages that housed their workers. Lagos filled in because its jobs stayed on the islands and water closed off the land around them. Cairo built outward by plan, but its people stayed close to the old city and its farmland edge. The split now sets how far each city's people travel to work.