Why would cities become layered without overpopulation? When I say layered I mean having a second or third ground built above ground level. With its own metros & roads. Most of the buildings below aren't connected to the buildings above. Zoning on the above level would be separate from the levels below & above. The layered cities don't have to go up that high, only one or two levels up. Overpopulation isn't an issue in this world. Not all cities have to be like this, only some. Large amounts of government resources are available to achieve this. Near future technology is available.
Mixed Population Cities
One reason this can happen is when the city is home to different races. One kind of people prefers to be firmly planted on terra firma while another race is able to fly and can thus take advantage of heights.
Such a situation could arise in a purely fantasy, sci-fi or mixed setting. Basically, the idea is to plan and construct cities that will cater to a variety of populations.
Limited Space Cities
Another reason to build up in this way is because horizontal space is limited or valuable. Take a citystate like Hong Kong or Singapore. Not a lot of real estate.
Building a second or third tier would allow for more space for the people who live and work there.
Stratification could be based on usage. You might want residential communities on the top level, where there's exposure to the climate, where they can get a sense of being "in the country" even though below their feet are several other layers of city! Commercial enterprises, sporting arenas, public venues of all kinds could easily be placed in the middle level. Services, industrial, storage and processing facilities could be placed on the low level to ease transport.
The "elite" of society live on the top level where there's lots of sun and pleasant weather because they can afford to. Government officials, owners of the facilities in the lower levels, wealthy city hoppers and the modestly well to do live above.
Service workers, functionaries and less well to do folks, foreign workers and the like inhabit the middle tier. They might visit the parks and attractions up above, but they have to scurry back to their burrows before curfew!
Laborers, mechanical operators, industrial workers, the abandoned, the poor, the downtrodden are all trodden down into the bottom level. Some of these may only be allowed topside on holidays while others may never get to see the shining city except from outside!
The old lower city was forgotten.
For the regrade, the streets were lined with concrete walls that formed narrow alleyways between the walls and the buildings on both sides of the street, with a wide "alley" where the street was. The naturally steep hillsides were used and, through a series of sluices, material was washed into the wide "alleys", by raising the streets to the desired new level, generally 12 feet (3.7 m) higher than before, in some places nearly 30 feet (9.1 m).
At first, pedestrians climbed ladders to go between street level and the sidewalks in front of the building entrances. Brick archways were constructed next to the road surface, above the submerged sidewalks. Pavement lights (a form of walk-on skylight with small panes of clear glass which later became amethyst-colored) were installed over the gap from the raised street and the building, creating the area now called the Seattle Underground...
In 1907, the city condemned the Underground for fear of bubonic plague, two years before the 1909 World Fair in Seattle (Alaska-Yukon-Pacific Exposition). The basements were left to deteriorate or were used as storage. Some became illegal flophouses for the homeless, gambling halls, speakeasies, and opium dens.
Your layered cities are like this. The old cities are still down there. New things were built on top by new governments wanting city renewal, conquerors establishing primary, disaster remediation or what have you. Whatever remains below was left to be repurposed or used as people will. These areas are not formally under the control of the topside government. Different governance structures prevail in the undercities.
There have been some very cool articles about this layered aspect of Istanbul. There is a lot of stuff underneath that city. https://www.dailysabah.com/life/history/istanbuls-mysterious-underground-waterways-pictured-for-1st-time
people are trying to out-compete one another; "no, MY building is bigger!" "no, MINE!"
social reasons, where being at the top is seen as prestigious
the cities flood on a regular basis, and the bottom floors are used for aquaponics
a large number of hostile creatures roam the land, the ground is too hard/mucky to dig bunkers and basements into, and so people go up rather then out so that the amount of building touching the ground is at a minimum
travel by personal airship is common, meaning that buildings need to be tall for airships to dock next to them
they're trying to grow crops on the sides of the buildings, and building them taller maximizes surface area exposed to the sun
buildings have mirrors on the side to support solar furnaces
larger buildings have more volume rather than surface area, making efficient housing and office space; since building out causes competition, building up is the next best option for increasing volume
it's just a popular building style
With near-future technology, new types of materials have become suitable for construction. In some cities, the ground beneath them is suitable. Instead of expanding cities and destroying the beautiful countryside it has become convenient to dig down and create a new city layer below. The original infrastructure is maintained, as much as possible. The material liberated is then used to create a new top layer on top of the original layer.
Quirks of Planning Permission and Land Rights
Land owners in your world have land rights that extend indefinitely upwards and downwards. For whatever reason its difficult/expensive to purchase or build on new land, but its permitted to make modifications to your own land.
Land owners tend to be wealthy. They pay no tax on the land they own, and they charge rent from anyone who wants to live on their land below.
The rest of the population don't own any land and are obliged to live and pay rent on the middle and bottom layers.
Imagine a world where the outside weather is generally too hot / too cold for people to survive very long unprotected. And which also includes powerful hurricanes/tornadoes.
This does two things:
- Since living almost entirely indoors with windows that don't open is normal, living in a middle of a giant city-block with virtual windows instead isn't much of a downside. Some people may even prefer it, perceiving it as safer.
- If the hurricanes are powerful enough, a single large fortified block can probably withstand them better than a bunch of separate buildings. To limit the area which needs to be build, the multi-floor design could make sense.
If you go into future technology, you could also have weather control facilities which dampen the effects of extreme weather, but only in a limited radius, and are vastly expensive to build/operate, such that it's worth building multi-level to fit the city in the radius of a single one.
Cities are sinking. Capitalists solution: sell a new real estate, layered on top.
Most modern cities are built near large body of water, so I think this works?
The city is sinking. The capitalists do not care about investing for returns that 1) come in a very long time and 2) not in the form of money, so they ditched the idea of environment-saving projects entirely.
How can I make money on this situation? AHA!
The big players of the city land management i.e. shortsighted planners, (possibly bribed/lobbied) government agencies, real estate investors, think it's just in their interest to stack another layer on top of the existing city. "Grand new real estate, prestigious because why not, and sells at very low price of one billion dollars!1" Said their advertisements.
Sure, it will be expensive to build this one massive concrete bed2 several dozen meters in the air. But don't worry, it will be built by siphoning government subsidies! Of course, the revenues will benefit the capitalists..
It will be an overpriced megablock for the first buyers, but the capitalists will keep advertising and growing this "new layer of city" thing. Common middle-class people is "lured" by the idea of prestige or decent, comfortable living on this new area. Supply and demand (plus maybe competition to sell this new hyped area) will drive the prices to somewhere within reach of upper-middle class, making the area the "dream" for middle and lower-class.
Fast forward several decades after this boom: the city now has an upper layer of concrete, with the richer families of the community naturally being/wanting to be on the top layer. The capitalists made their fat stacks, the city is sinking even more, but hey, who cares about those peasants? "If they want to live in the comfortable, currently-far-from-sinking higher area of the city, they should work hard, invest, make money, etc etc to get that! Decent life comes with hard work!" The lower part becomes a swampy wasteland just several more decades later..
1 Just an illustrated price for some terribly overpriced real estate but still within reach of some very rich people.
2 Could be concrete, could be handwavium. Could be very expensive, could be moderately expensive. But the idea stands.
Efficiency and specialisation
High population density is more efficient. Average commutes are very short and a very specialised society needs a lot of interactions. Stratification might be due to some special functions being assigned to each level.
Overpopulation might not be there, but a government planning every detail might still take it into account as a possible future (or past experience?) problem.
The above city implies a plan made in advance in every detail. It is a lot more different than the current cities which start small and grow in a chaotic manner. Even a plan like the one done for Brasilia would not be detailed enough. It must be a city designed in this way and built in this way from scratch.
Demands for surface land:
In your world, there is something special about the surface that makes its usage in high demand. Perhaps there is a valuable agricultural product that only grows on this world, and is exported to other worlds/off-world colonies. Or the production is inefficient, such that vast quantities of production are needed to make small quantities of the final product. The form of the substance is open to interpretation - imagine, for example, a drug that infinitely extends life, but requires greater and greater doses as people age. The profit from this would be so high that every available inch or arable land would be used for crops. Your cities would be off coasts, over mountains, or centrally located for the industrial demands of processing the crop.
Another alternative is that the people are extreme ecologists, and are desperate to maintain a minimal impact on the environment. Hyper-urbanization could be a model the government is trying to encourage, so other cities adopt this and become less of a disruption to the ecosystem.
As a result, people cluster into dense population centers. But since we want to entice people to live there, we create entirely new levels to allow folks to enjoy a less-dense lifestyle while still increasing the density of the cities immensely.
- Another reason for this is seen in Minneapolis. The city built a network of skyways in the city buildings so people didn't need to expose themselves to the harsh winter weather. As a result, the second floor of most buildings instantly became the most valuable real estate in the buildings. Your city dwellers could have some unpleasant environmental factors that have encouraged people to network between large structures, then gradually enclose platforms to make a new layer. Once this happened once, developers might do it again to try and repeat the success.
With all of these, they will happen if it makes economical sense. If it costs more to have layers with no benefits to have layers it won't happen.
On Earth, what happens in lower layers will affect the upper layers. If the lower layer can't support the upper layers it would be disastrous. This would mandate strict building and engineering considerations which can be costly.
The atmosphere is really cold or toxic etc
This would require cities to be enclosed. So the city is more likely to be an archology/megastructure then a collection of buildings.
Underground is cheaper then above ground.
It is economical to build deep, Ie in a large asteroid or small moon, once you get beyond some number of layers there will be specific layers that interconnect the city. When building underground layers are best way to go. This happens with large coal and salt mines that are not pit mines.
Already tall structures
Super tall tree world. I.e. trees that are large and hundreds of meters tall. So it costs less to build with the structures then to remove them and build. Multiple layers will form when it is more efficient to have transport layers at multiple certain heights then transferring up/down to one transport layer.