The entirety of Wednesday's meeting of the city council's planning committee will be dedicated to applications seeking permission to increase the size of homes to be able to cater for at least seven people.
It follows in the wake of advice from the chairman of the Portsmouth & District Private Landlords' Association to either sell or enlarge smaller HMOs due to concerns a 'perfect storm' in the sector made them financially unviable.
Martin Silman said licensing proposals being considered by the council, combined with 'record' rents for family homes and reduced student demand had made it more difficult to own three- or four-bedroom HMOs.
'The net impact of these points is that the most in need will have nowhere to live,' he said in the association's letter. 'The number of small, nice homes for three or four nurses or dockyard contract workers will decrease and the number of mega HMOs will rise in response to the high demand and high prices that this will create.'
All but one of the 12 applications being considered by councillors have been recommended for approval with planning officers saying they complied with its policies.
The other, a scheme for the change of use of an HMO in Prince Albert Road, is set for refusal with a report saying the proposed communal area was too small to accommodate seven residents.
The planning applications are all required to be considered by the committee at the request of council leader Gerald Vernon-Jackson, although several have also been objected to by people living nearby.
There are an estimated 6,000 HMOs in the city, the equivalent of about a quarter of all private rental accommodation. Under current rules, 1,226 are mandatorily licensed, while its HMO register lists a total of 4,200.
Proposals being considered by the council to introduce an additional licensing scheme would increase the number requiring formal registration of three- and four-bedroom HMOs for the first time.
They were brought forward due to concerns about living conditions in many homes across the city with council research showing almost a third of those inspected having serious hazards.