Anyone who designs extensions is a designer. It could be a chimney sweep, or some world class Architect, the only criteria is that they know what they are doing.
Planning consultants may well have some design knowledge, but typically they just know about planning law and policy, so might not know how to actually design with any flare or to building regulation standard. So IMO, you don't want a planning consultant unless they are knowledgeable about all the other things involved in designing a loft conversion.
You need someone who knows about planning, knows about building regulations, and knows how to design to your needs. This is typically anyone who draws plans for extensions for a living, but as with all professions, there are good and bad ones, so it's a case of picking a good one.