Dozens of people had objected to the Port Solent scheme warning it was 'not in character' for the area and would set a precedent for the loss of 'family housing' at the exclusive marina residence.
Speaking at Wednesday's planning committee meeting, David Jump, who owns another of the Oyster Quay flats with his wife warned it would also 'increase the pressure' on neighbours in terms of parking.
However, a report by Portsmouth City Council planning officers recommended the councillors approve the change, saying it was 'acceptable' and complied with both local and national planning policy.
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'Having regard to all material planning considerations and representations it is concluded that the proposed change of use is acceptable and would be in accordance with the relevant policies of the Portsmouth Plan and the objectives of the National Planning Policy Framework,' it said.
Councillors were told the flat had been used as a shared home – or house of multiple occupancy – for a decade until 2018 when this use was challenged by legal action, however this has since ended.
'We see a lot of these applications for HMOs,' committee member and Portsmouth City Council leader, Councillor Gerald Vernon-Jackson said. 'The planning system means that any applicant who gets turned down has the ability to appeal and we know from bitter experience that turning things down here where we have no planning reason to do so just means we are giving people false hope because we know it will get through at appeal.
'That costs the council money - and that's taxpayers' money - and we shouldn't be in the business of telling people we are going to turn this down knowing it would be given planning permission at appeal.
'We have absolutely no option in this case because the planning law is absolutely clear. We couldn't say 'no' to this even if we wanted to. All the policies we have, have been passed.'