It depends what you are looking for.
If you want help submitting or opposing aplanning application, you would normally go to a planning consultant; private firms who employ qualified town planners (sometimes ex local authority planners) who know about planning law, polcies, regulations etc.
For help with legal challenges to a planning decision, planning consultants would probably have contact with planning lawyers, barristers etc.
All these people cost a lot of money, and are probably not needed when considering a domastic extension.