Use Class C4 (houses in multiple occupation) relates to small shared houses occupied by between three and six unrelated individuals, as their only or main residence, who share basic amenities such as a kitchen or bathroom.
Use Class C4 (houses in multiple occupation) can benefit from limited permitted development opportunities to change use without requiring a full planning application. There may be conditions and limitations to such a change however, and further detailed information about this can be found here on the Planning Portal website. This website also details other restrictions to permitted development that may be in place, such as planning conditions or Article 4 Directions, and it is recommended you get confirmation from the local planning authority about changes of use proposed using permitted development rights.
If a C4 premises does not benefit from permitted development rights, planning permission may be sought to change it to another use. You can learn more about this process here.
Should an HMO have 7 or more residents, then it is considered to be 'sui generis' (i.e. outside any classification) and does not benefit from any permitted development rights.