In the early 1900, Mumbai was growing as an industrial city, and needed workers from Maharashtra and Gujarat. Most of these workers came to the city as single males or after leaving their families back in their villages. Housing needs of these workers were met by chawls. Chawls were built by private builders.
A Chawl is a group of one or two room housing units laid out along two corridors. The corridor in the front of the house provides the entry doors to the units. The corridor in the back provides a passage to common sanitary facilities that are shared by the units on the floor. Chawl buildings are typically 3 to 5 storied. The building can be linear or C shaped.
If a housing unit has two rooms, then the first room is living and sleeping room, and the second room is a kitchen as well as study room. This type of layout suited well for a migrant worker who needed a cheap place to stay so that he could send money back home to the family. However, due to lack of affordable housing at the next level, these workers continued to stay in the chawls even after their families moved in with them. The whole housing unit is around 300 - 400 sq-ft.
Most of the chawls were build before the second world war. They were constructed with teak wood with wooden flooring.
Chawls are disappearing slowly from Mumbai as more options for self contained apartments are becoming available. Due to better transportation, people are willing to stay further away from Mumbai and commute. However, while the Chawls ruled, they contributed to bringing the society closer together. Public festivals were nurtured by these communities.
Famed Marathi author P. L. Deshpande has written a wonderful novel about Chawl living named - Batatyachi Chawl. It's a classic.