What was a solution to scarce living/working space in cities during the industrial revolution?

Massive infrastructure plans where made to make new "apartment complexes"  that could house the massive amounts of people immigrating into the cities at the time. Also the sub-burbs where created in-order to house people outside the city so that it wouldn't be too crowded. 

If your answers look like this a) expansion through conquestb) development of a calendar c) development of a written language d) creation of a large road workthen the answer is dhope it