Proposals have been submitted for an extension at the Flying Bull Academy which would house three new specialised classrooms and therapy and sensory rooms - more than doubling its special needs capacity.
The project is being led by Portsmouth City Council as part of efforts to cater for a predicted rise in the number of primary school-aged children needing this support.
The planning application, submitted earlier this month, includes a single-storey extension to the academy's existing school building, building on land currently used for car parking.
The academy, which is part of the University of Chichester Multi-Academy Trust, said the lost spaces had already been reprovided elsewhere in the school grounds.
'Flying Bull has provided a unit for many years catering for children of primary school age,' a statement submitted with the application said.
'This project will increase the capacity of the unit from 14 to 32 by providing three additional classrooms. This will allow for a more appropriate number of children per class and more needs-specific classroom settings.'
It added that, unlike with mainstream schools, these places would 'not necessarily' be fully used at all times.
Funding is being provided by Portsmouth City Council and it will also oversee construction of the extension.
Its aim is for the extra places to be available by the start of the next school year.
'It has been forecast that in the coming years that there will be an increase in the number of primary age pupils with social, emotional and mental health difficulties, or needing alternative provision,' the council's deputy leader, and cabinet member for education, Suzy Horton, said.
'The aim of the centre is to enable the children who attend to return successfully to their mainstream school.'
She said the project would 'ensure that Portsmouth is prepared' is prepared for rising numbers of children needing these places.
The planning application will be considered by council planning officers in the coming weeks.