Flood zones and what they allow.

Zone 3a is land assessed as having a 1 in 100 or greater annual probability of river flooding, or 1 in 200 or greater from the sea. Development is not banned there, but it has to earn its place. First the sequential test asks whether there are reasonably available sites at lower risk, and if there are, the answer is usually no. If the site survives that, the exception test asks two further questions and both have to be answered. Zone 3b is a different proposition: it is the functional floodplain, land where water has to flow or be stored in times of flood, and its whole purpose is to reduce flood peaks downstream. Building homes on it removes storage that other people are relying on, which is why it is the one part of the system that behaves like a genuine prohibition.

The trigger most people know is the zone, and it is the main one: a site-specific assessment is expected for development in zone 2 and zone 3. The trigger people miss is that zone 1 is not automatically clear. Zone 1 describes river and sea flooding only, and a site can sit in it while flooding regularly from surface water, groundwater, a watercourse too small to be mapped, or a failing sewer. Where an authority has identified critical drainage problems, or another source of risk is in play, an assessment is required in zone 1 as well. The practical point is that the flood map answers one question about one source, and treating it as a clearance for the whole subject is how sites get caught out late.

Householder development is treated proportionately, and the guidance is explicit that an assessment should be proportionate to the risk actually anticipated rather than a fixed document. In zone 1 with no other source of risk, an extension is rarely troubled by any of this. In zone 2 or 3 it usually is, and what the authority wants is short and practical: what the finished floor levels are, whether the work displaces any floodwater, whether the occupants can get out safely, and what happens to the surface water the new roof and paving will shed. Many authorities publish a simplified form for exactly this situation, which is worth finding before commissioning anything longer.

Three bodies have a hand in it and they answer different questions. The lead local flood authority, which is the county or unitary authority, deals with surface water, ordinary watercourses and groundwater, and is a statutory consultee for major development. The Environment Agency deals with main rivers and the sea. The water company deals with connection to the public sewer, and it is entitled to refuse or restrict a surface water connection where the network cannot take it. A scheme can satisfy the planning authority and still be held up by the last of those three, which is why the connection question is worth asking early rather than at the point of discharging the condition.

Schedule 3 would have created a separate approval body for drainage, sitting alongside planning permission and with its own consenting process. It has been discussed for well over a decade, hopes of commencement in 2025 were not met, and a House of Commons research briefing in February 2026 confirmed it remains uncommenced, with ministers preferring to deliver the same outcome through planning policy. An amendment during the passage of the Planning and Infrastructure Bill that would have brought it into force was opposed by the government and withdrawn. What this means in practice is not that sustainable drainage is optional. It means the requirement reaches a scheme as a planning condition rather than as a separate consent, so the place to look is the decision notice, not a second regime.

This is the single most common gap between what an applicant checks and what the authority holds. The publicly available flood map distinguishes the broad zones, but the split between 3a and 3b is made locally, in the strategic flood risk assessment that the planning authority prepares to support its local plan. A site can appear simply as high risk on the national map and be functional floodplain in the council's own evidence base. Because 3b behaves close to a prohibition for new homes, establishing which side of that line a site falls on is worth doing before anything is bought, drawn or promised. It is a document request rather than a survey, and it is usually free.

A condition requiring details to be submitted and approved before development begins is exactly what it says. Beginning work without it does not simply create a technical breach to be tidied up afterwards; it can mean the permission was never lawfully implemented, which matters enormously if the permission is close to expiry or if the site is being sold. Drainage details are also among the slower conditions to discharge, because they usually need calculations, sometimes ground investigation for infiltration rates, and frequently a consultee response. Applying to discharge them at the same time as ordering materials, rather than after, is the difference between a condition and a delay.