Up TO 85pc of flats inspected in the capital have not met minimum housing standards, according to a new report.
A draft Dublin City Council/ Royal Institute of Architects' document has found the planning system has failed to regulate large houses being split into smaller flats.
The report also claims a "two-tier" standard has arisen between new apartments and older flats.
The report follows a three-year intensive inspection programme to crackdown on slum conditions being experienced by many in the city's private rental sector.
It found nine-out-of-10 flats converted from single homes that were inspected did not have planning permission, and fewer than half of their tenancies were registered with the Private Residential Tenancy Board (PRTB).
Private flats must comply with just 15pc of the standards of general housing, but despite this 84pc failed to meet these basic standards.
While flats are not required to meet the size rules of new apartments, they were found to be half the general minimum size requirement of 55sqm.
Independent councillor Mannix Flynn said the lack of accommodation in the city means people are having to accept appalling living conditions.
"We are going back to the days of the tenements," said Mr Flynn.
"Families are being squashed into dark rooms with poor sanitation and it is a catastrophe.
"We have the Government handing out rent allowance to people, but no inspection system of the accommodation these families are living in," he explained.
The report recommends flats to be required to meet general housing standards, and the inspection programme should be rolled out nationally.
It also recommends the role of the PRTB should be broadened, and landlords should be required to register their properties, not just tenancies, with the board.