Flood resistance and resilience a council will condition

Flood resilient construction splits into two ideas that councils treat differently, and mixing them up is why so many applications come back with a request for more information. Resistance keeps water out of the building. Resilience accepts that it gets in and limits what it ruins. A house in Flood Zone 3 will usually be conditioned to have both, with resistance handling shallow water and resilience handling anything deeper.

The reason resistance is limited to shallow depths is structural rather than a matter of cost. Hold water out of a dry building and the pressure difference across the wall and floor grows with depth. Above roughly 600 millimetres that pressure can crack masonry or lift a slab, and the repair is far worse than a wet carpet. So water exclusion strategies stop at shallow depths and the strategy changes above them.

What sits in each category

Resistance measures are the visible ones. Demountable barriers across doorways and gates, flood doors with compression seals, sealed or rendered external walls below the design level, covers or automatic closures on airbricks, non-return valves on foul and surface water drainage so that sewage does not back up through the ground floor, and sealing around service entries where a pipe or cable passes through the wall.

Resilience measures are the ones nobody sees. A concrete floor slab with closed-cell insulation rather than a suspended timber floor. Lime plaster or a water-resistant board on the lower part of the wall instead of gypsum plasterboard. Sockets, consumer units and boiler controls raised well above the design flood level. Kitchen units on legs so the carcass can be lifted out and the floor dried. Solid or engineered doors rather than hollow core. Chemical-free ways to dry a wall, which means avoiding cavity insulation that holds water.

Then there is the measure that outperforms all of them: raising the finished floor level. Where the site allows it, setting the ground floor above the design flood level including the climate change allowance is worth more than every barrier on the list, and it is the measure councils condition most consistently.

How the level actually gets set

The number in the condition comes out of the Flood Risk Assessment, and it is expressed in metres above ordnance datum rather than as a height above the ground.

The sequence is: establish the design flood level for the relevant annual probability, add the appropriate climate change allowance for the river basin district and the development lifetime, then add a freeboard, commonly 300 millimetres, to cover uncertainty in the modelling. That gives a finished floor level. The condition then reads that the finished floor level of the development shall be set no lower than a stated level in metres AOD, and it is usually a pre-commencement or pre-slab condition.

Climate change allowances are revised. Use the current published figures, state the date of the version you used, and say which allowance band you applied and why.

Where raising the floor is impossible, as it often is on an extension to an existing house, say so plainly and set out what is being done instead. Councils understand that a rear extension cannot sit 600 millimetres above the kitchen it opens into.

Standards, certification and what to specify

Two documents do the work here. The British Standard for flood protection products covers manufactured barriers and closures and there is a certification route for products tested against it. The CIRIA code of practice for property flood resilience covers the whole process from hazard assessment through design and installation to maintenance, and it is the document a competent installer will work to.

Specify by performance, not by brand. A drawing note saying "flood door to threshold level 8.42m AOD, tested product with independent certification, installed by a competent installer in accordance with the manufacturer's instructions" survives a change of supplier. A note naming one product does not.

Two details are missed more often than the rest. Non-return valves need access for maintenance, so the chamber has to be somewhere reachable. And a barrier is useless if it lives in a garage the occupier cannot get to when the water is rising, so show the storage position on the plan.

How the measures are secured

Most councils use a condition rather than a legal agreement for householder and minor schemes. The wording matters because it decides who does what and when.

Good wording states the level in metres AOD, names the measures by performance, requires them to be installed before first occupation, and requires them to be retained thereafter. Weak wording asks for a scheme of flood resilience measures to be submitted and approved, which pushes the whole design into a discharge of conditions application and adds eight weeks after permission.

Where a condition is drafted loosely, ask for it to be tightened before the decision is issued. That sounds counter-intuitive and it is worth doing: a precise condition you can discharge on the drawings already submitted is better than a vague one that requires a fresh application.

A flood warning and evacuation plan is the other common requirement, and for anything with a sleeping risk the council will want to know that safe access and egress exists in the design event, rather than only that the building stays dry.

Where drainage sits in this

Property level measures deal with water arriving from a river or the sea. They do nothing about water the development itself generates, which is the province of the Drainage Strategy. The two documents overlap at the non-return valve and at the question of whether a surface water outfall can discharge when the receiving watercourse is in flood.

If the outfall is drowned during the design event, the site needs storage for the volume that cannot discharge. Say where that storage is and how deep it gets. A surface water scheme that only works in dry weather is not a scheme.

What this costs and who pays

Property level measures are usually a small proportion of the build cost of a new dwelling and a larger proportion of the cost of a modest extension, which is why householders resist them. The argument that works with a client is the insurance one. A house that has been flooded and repaired conventionally floods again and is repaired again. A house repaired resiliently dries out in weeks rather than months, and the occupier moves back sooner.

Avoid quoting a figure per property. Prices for barriers, doors and valves move, installation varies with the construction of the building, and a number in a planning document tends to be quoted back at you.

What to put on the drawings

• Existing and proposed ground levels and finished floor levels, in metres AOD, on the plans and the sections.
• The design flood level and the climate change allowance used, annotated on a section through the building.
• Locations of barriers, flood doors, airbrick covers and non-return valves, with a schedule.
• Height of sockets, consumer unit and boiler above finished floor level.
• Storage position for any demountable equipment.

None of this displaces the prior policy questions. The Sequential Test asks whether the development could be located somewhere at lower risk, and it is applied before mitigation is considered at all. Where it is passed and the development is still in a higher risk zone, the Exception Test asks about wider sustainability benefits and about safety for the lifetime of the development. Resilience evidence belongs in the second of those, not the first.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it. To see which flood and drainage documents a site is likely to need, enter your postcode.

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