Student HMO (Houses in Multiple Occupation) Properties fall under C4 of the Use Class Act which are defined as small shared houses occupied between three and six unrelated individuals, as their only or main residence, who share basic amenities such as a kitchen or bathroom. Certain councils need planning approval where article 4 is in operation. Manchester council requires one of these, but Salford council currently does not. Therefore you do not need planning permission to convert a Use Class C3 (Family Dwelling) to become Use Class C4 if Article 4 is not in operation. However if you want to convert a house into a student HMO with more than 6 bedrooms then this falls under the Sui Generis Use Class which will require planning permission.
For further information take a look here:
https://www.planningportal.co.uk/info/200130/common_projects/9/change_of_use
and here
https://www.gov.uk/browse/housing-local-services/planning-permission