This depends on how many rooms the HMO will have post-works and if there is an article 4 directive implemented by the local authority. If there is no article 4, then planning is required for 7 or more rooms. Alternatively, if you convert the house to a 6-bed HMO and there is no article 4 directive, then this should be allowed under permitted development.  Always check with the local authority, architect, or planning consultant.