A site-specific flood risk assessment is normally required where a development is in or adjacent to Flood Zone A or B, where the site has a history of flooding, where a watercourse crosses or bounds the site, or where the planning authority requests one because of local flood risk or surface water concerns.
The trigger is risk, not just zone. Sites within Zones A and B will almost always need an assessment, but so will sites beside a river or stream, sites downstream of a culvert, sites with recorded flooding, and sites where a strategic flood risk assessment in the development plan flags a concern.
Planning authorities can also request an assessment as further information after an application is lodged. Commissioning the work at design stage is usually faster and cheaper than responding to a further information request later.