Flood resistant and resilient construction: what to specify
Below about 600 mm you can try to keep water out. Above that, the sensible strategy is letting it in and designing so the house dries out and comes back into use quickly.
Flood resilient construction and flood resistant construction are two different strategies, and a planning condition that asks for both without saying which applies where is the reason so many of these conditions are hard to discharge. Resistance keeps water out of the building. Resilience accepts that water will get in and limits the damage it does, so the property dries out and comes back into use in weeks rather than months.
For a householder, the practical question is usually which of the two the council expects, at what depth, and whether the specification will satisfy both building control and an insurer.
Resistance and resilience as different strategies
Resistance measures work against a head of water: demountable or automatic door barriers, flood-resistant doors and windows, sealed or waterproof external render and mortar, non-return valves on foul and surface water drainage, and airbrick covers.
Resilience measures assume water arrives inside: solid concrete floors with tile or resin finishes rather than timber and carpet, closed-cell insulation, lime or cement render and cement board rather than gypsum plasterboard on the lower part of walls, raised electrical sockets and consumer units, kitchen units on stainless or plastic legs with water-resistant carcasses, and raised service entries.
The two are usually combined. Barriers hold back a shallow flood, and resilient internal fabric means a deeper one costs a fraction of what it would otherwise.
The distinction matters commercially as well as technically. Resistance measures are visible, priced as products and easy to point at. Resilience is mostly decisions about materials taken during a refit, which cost little extra at the time and a great deal if taken later.
Where resistance stops being appropriate
The commonly applied limit is around 600 mm of water depth against the external wall. Above that, published guidance, including BS 85500 and the property flood resilience code of practice, cautions against trying to hold water out, for two reasons.
Structural loading is the first. Water exerts hydrostatic pressure, and a solid wall holding back a metre of water on one side with a dry floor slab on the other is carrying loads it was never designed for. Floors can lift and walls can be pushed.
Seepage is the second. Masonry is not watertight. Water finds its way through mortar joints, under slabs and up through service ducts, so a sealed building slowly fills anyway, and does so behind sealed finishes where it cannot dry out.
Above 600 mm the sensible strategy is to let water in and out, which means controlled flooding of the ground floor with resilient finishes and a plan for drying. That is counter-intuitive to most homeowners and it is what a specialist will recommend.
Building regulations and planning conditions compared
The two regimes ask different things and it is worth being clear which is driving your specification.
Approved Document C addresses flood resistance and resilience for new buildings, with guidance on floor construction, materials and services, and requirement C2 deals with resistance to moisture. Building control checks the built form against that guidance.
Planning conditions are about the acceptability of the development on the site, and they follow from the Flood Risk Assessment. Typical wording requires flood resistance and resilience measures to be submitted and approved in writing before occupation, and implemented as approved. What a council will and will not accept in that submission is set out in flood resistance and resilience measures conditioned, which covers the council's side of the exchange.
The failure mode is a condition discharged with a page of generic bullet points. Officers want a schedule tied to a drawing.
Specifying so the condition is dischargeable
Write the submission as a schedule, room by room and element by element, with the design flood level stated on the drawings.
- State the design flood level, its source and its return period, including any climate change allowance applied.
- Mark that level on the existing and proposed elevations and sections.
- List each element below the level, with the proposed specification and the standard or product performance it meets.
- State the height of sockets, switches, consumer unit and boiler above finished floor level.
- Describe the drainage protection, including non-return valve locations shown on a plan.
- Set out the operational arrangements for any manual measures: who deploys the barrier, on what warning, and how long it takes.
That last point catches people out. A demountable barrier that depends on somebody being home is not a reliable measure for a property with an hour of flood warning, and a council may say so. Automatic barriers or passive measures avoid the argument.
Where the scheme also needs a surface water strategy, the Drainage Strategy should cross-refer to the flood levels used here, so two documents do not quote different figures.
Insurance and Flood Re
Flood Re is the reinsurance scheme that allows insurers to offer affordable cover on homes at flood risk, funded by a levy on home insurers. Two limits matter to householders.
Properties built after 1 January 2009 are excluded from the scheme. That is a deliberate policy choice intended to discourage new building in flood risk areas, and it is worth knowing before buying a new house on a site that floods.
The scheme includes a Build Back Better element, under which participating insurers can pay up to £10,000 on top of the repair costs to fund resilience measures after a flood claim. Not every insurer participates, and the measures funded have to be agreed, so ask the question at claim stage rather than assuming.
Check the current position on the Flood Re website before relying on any of this, because scheme rules and the transition arrangements towards its planned end have been revised more than once.
Retrofit on an existing home
Retrofit is where most property flood resilience work actually happens, and it follows a survey rather than a catalogue.
A property flood resilience survey establishes the flood mechanism, the likely depth and duration, the construction of the building and the routes water takes into it. Those routes are frequently not the front door. Airbricks, service penetrations, the gap under a suspended timber floor and backflow through the toilet are all common, and a survey that only looks at doors and windows will miss most of them.
Duration matters as much as depth on an older property. Water standing for two days against solid brick walls saturates the masonry, and drying that out properly takes months of controlled dehumidification. Rushing the drying and replastering early is the most common reason a repaired house smells of damp a year later.
Sequencing that works: survey, then drainage protection and service entry sealing, then openings, then internal resilience as and when rooms are refitted. Doing the internal work first, before the water route is understood, is money spent in the wrong order.
Listed buildings and properties in conservation areas need care, because external barriers, render changes and visible valves can require consent. Get that checked before specifying, since a flood door in a listed cottage is a listed building consent application in its own right.
One honest limitation. Resilience reduces damage and recovery time, and it does not stop the flood. A house that floods every other year with 800 mm of water is a case for relocation of living space upstairs, not for a better door seal.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required. Enter your postcode on PlanScope to see which flood documents your property is likely to need.
Frequently asked questions
What is the difference between flood resistance and flood resilience?
Resistance keeps water out, using barriers, sealed doors, waterproof render and non-return valves. Resilience accepts that water will enter and limits the damage, using solid floors, closed-cell insulation, cement based finishes, raised sockets and water-resistant fittings. Most schemes combine the two, with resistance handling shallow events.
At what depth does keeping water out stop working?
Published guidance commonly identifies around 600 mm as the point beyond which resistance becomes inadvisable, because hydrostatic pressure can damage walls and floors and because masonry seeps in any event. Above that depth the recommended approach is controlled entry of water with resilient internal fabric and a drying plan.
How high should sockets be in a flood risk property?
Above the design flood level, with the height stated in the submission rather than described in general terms. Sockets, switches, the consumer unit and the boiler are the usual items raised, and the design flood level including any climate change allowance is what sets the figure for your property.
Does Flood Re cover a new build house?
No. Properties built after 1 January 2009 are excluded from the scheme, which was a deliberate decision intended to discourage new development in flood risk areas. Cover for such properties is arranged on the open market, and the availability and price depend on the individual insurer's assessment.
Will my insurer pay for resilience measures after a flood?
Possibly. The Build Back Better element of Flood Re allows participating insurers to pay up to £10,000 towards resilience measures on top of the repair costs after a flood claim. Not all insurers participate and the measures have to be agreed, so raise it when the claim is made rather than afterwards.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.