If you construct both the loft conversion and the single-storey rear extension in accordance with the conditions that apply to PD, you do not have to apply for permission. Instead, apply for 2 Lawful Development Certificates, one for the loft conversion and one for the single-storey rear extension. These will certify that the works are lawfully done under PD rules.
If one or both are not constructed in accordance with the conditions that apply to PD, apply for planning permission. If both do not comply, they can be submitted under one application.