cardboard city, n. — Green’s Dictionary of Slang

cardboard city n.

also cardboard condominiums

[the original Cardboard City was situated on the London’s South Bank, but increasing homelessness has meant that the term now applies to any such gathering]

the cardboard box ‘homes’ that are used as shelter by the homeless.

1993		I. Welsh Trainspotting 252: Two million on the dole already [...] Aw they perr buggers livin in cardboard city.
1994		R.P. McNamara Times Square Hustler 122: It is common to see people sleeping in corners [...] in numerous cardboard boxes, or ‘cardboard condominiums’.
1999		D. Healy Sudden Times 185: We might [...] saunter through cardboard city and back again by underground from Waterloo.
2007		(ref. to 1980s) N. ‘Razor’ Smith Raiders 215: There existed at that time (i.e. 1980s) a shanty-town of homeless dwellings constructed mainly of cardboard boxes [and k]nown as Cardboard City.