I have a distinctly strong feeling of deja vue reading this - pretty damn sure I've answered this once before.
When you are a developer, the planning authority can ask for whatever it wishes (within reason) as a condition for granting planning consent. This is the condition they require from you.
Look at it this way - you make a shed-load of money from the development gain on the land - less the modest cost of the road; they benefit from getting a bright shiny spanking brand new road at your expense, for the benefit of the community at large.
Please show me where you reckon 'the Transport & highways state a maximum of 5 houses can be served by a private/ unadopted road'. This is hogwash (a technical term).