Why the searches do not answer this
A conveyancer's searches are addressed to the property. They are precise and they are narrow, and the narrowness is the point: a standard enquiry asks whether a road scheme is proposed within a stated distance, not what the developer intends to do with phase four.
Reviewing the planning documentation for a whole development is a different job, with a different cost, and it falls outside what a conveyancer is instructed to do. The Lexsure wording below says that in terms, which is unusually direct and worth reading as written — the limits of the retainer are part of what you are being told.