Safe access and egress: dry routes and alternatives

Safe access and egress in flood risk terms means a route that people can use during the design flood event without being exposed to dangerous depths or velocities, and that emergency services can use to reach the site. It is assessed separately from the building itself, and it fails schemes that otherwise pass the sequential and exception tests. A dry route is the benchmark. Where a dry route cannot be provided, the argument moves to hazard rating, and that is where most negotiation happens.

The question a lead local flood authority asks is simple. If the design event happens at three in the morning, can people leave, and can a fire appliance get in.

What the guidance expects of a route

National flood risk guidance requires development to be safe for its lifetime, and access and egress is one limb of that. The assessment should look at the route from the building to a point outside the floodplain, in the design event including the relevant climate change allowance, and it should consider who is using it.

The design event depends on the source. For fluvial flooding it is normally the 1 per cent annual exceedance probability event with a climate change allowance applied; for tidal and coastal flooding it is the 0.5 per cent event with the appropriate allowance. Climate change allowances are revised periodically, so check the current published allowances for your river basin district and epoch rather than copying a figure from an older report.

Three outcomes are possible. A dry route, meaning no flooding along its length in the design event, which is what authorities prefer. A safe but wet route, where shallow slow water is crossed and the hazard rating supports use. Or no acceptable route, where the answer is either a redesign or a refusal.

Depth and velocity hazard ratings

The hazard rating used across England comes from Defra and Environment Agency research, and it combines depth with velocity and a debris factor:

hazard rating = depth × (velocity + 0.5) + debris factor

Depth is in metres, velocity in metres per second, and the debris factor reflects the likelihood of debris in the flow and the vulnerability of the people exposed. The resulting number falls into bands:

For a residential scheme, expect the authority to want the route within the lowest band, or to accept the second band only with a good reason and supporting measures. Care homes, schools and anything with a vulnerable occupancy get pushed to the lowest band, because the bands themselves are keyed to mobility.

Velocity is the part people ignore. A 200 mm depth on a flat car park is a different proposition from 200 mm running across a road at 1.5 metres per second, and the rating shows that: the second case lands around 0.9 before any debris allowance.

Dry routes, safe refuge and assisted evacuation

A dry route is not always achievable on an infill site in a historic town, and the guidance does not treat that as automatically fatal. What it does is shift the weight of the argument.

Safe refuge within the building can support the case: sleeping accommodation above the design flood level, a floor area at that level able to hold the occupants, and a means of signalling to rescuers. Refuge works for a block of flats with upper floors. It works poorly for a bungalow, and for single-storey development in Flood Zone 3a many authorities simply refuse.

Assisted evacuation is the weakest argument available. National guidance is clear that the ability to evacuate people is not a substitute for locating development where it is safe, and fire and rescue services do not welcome being written into a developer's flood plan as a design feature. Use it as a residual measure, with a flood warning and evacuation plan, never as the primary answer.

Register the site for flood warnings and say so in the assessment. It costs nothing and it tells the officer you have thought about the operational side.

Routes crossing land you do not control

This one derails applications late. A route shown across the neighbouring supermarket car park is worthless unless you have a right to use it, and a condition cannot create one.

Acceptable routes are those over the public highway, over land within the application site, or over land where a recorded public right of way exists. Anything else needs an easement, and the authority may want it in place before permission rather than as a Grampian condition, because the flood safety case depends on it.

Check the highway extent properly. Adopted highway boundaries rarely match the kerb line, and an access strip that looks public on an aerial photograph is often in private ownership. Order the highway records at the same time as the topographic survey.

Vehicular access for emergency services

Pedestrian and vehicular access are assessed differently, and the second is where local practice varies most. Fire and rescue services want to bring an appliance close enough to work, and the depth at which that becomes impossible is a matter of local judgement rather than a national number.

What helps is asking early. Many authorities have a strategic flood risk assessment that sets local access criteria, and some fire services publish a position on operational access in flood conditions. Where a scheme depends on the answer, put the question to both the lead local flood authority and the fire service at pre-application stage and record the reply in the Flood Risk Assessment.

Where the access serves a basement, the case is harder again, for reasons we set out in basement development and the flood risk objection.

Where an authority will accept a compromise

Patterns emerge from decisions, and it is fair to describe them as indicative rather than as rules.

• Redevelopment of a previously developed site in a town centre, where the sequential test has been passed on regeneration grounds, attracts more flexibility than a greenfield edge-of-settlement scheme.
• Upper floor residential over commercial ground floor is easier to defend than ground floor living space, because refuge is built in.
• A route that is dry for the 3.3 per cent event and shallow in the 1 per cent event is often accepted, particularly where flood warning lead times are long.
• Fast-responding catchments with short warning times get treated more strictly, because evacuation before onset cannot be relied on.
• Where the exception test is engaged, access and egress usually appears in the safety limb rather than as a separate matter, which we cover in the two limbs of the exception test.

Groundwater flooding sits outside this framework and needs its own treatment, since depths build slowly and velocities are low but duration is long. Our note on groundwater flooding evidence covers how to assess it.

Evidence and drawings to submit

Officers cannot accept an assertion that access is safe. Give them the working.

1. A topographic survey with levels to Ordnance Datum along the whole route, not just at the site boundary.
2. Design flood levels for the relevant events, from Environment Agency modelled data or site-specific modelling, with the source and date stated.
3. A route plan showing depth at intervals along the route, with the exit point out of the floodplain marked.
4. A longitudinal section of the route against the design flood level, which is the drawing that persuades.
5. Hazard rating calculations at the controlling points, with the velocity source and debris factor stated.
6. Land ownership or highway extent evidence for the route.
7. A flood warning and evacuation plan where refuge or assisted evacuation forms part of the case.

The longitudinal section is the single most useful drawing and the one most often missing. It takes an afternoon to produce and it answers the officer's question directly.

One honest uncertainty: hazard rating thresholds are applied with judgement, and two neighbouring authorities can reach different conclusions on the same numbers. Where the margin is fine, get the question resolved at pre-application rather than discovering it in a consultation response eight weeks in.

Enter your postcode on PlanScope to see which reports your site is likely to need, including the drainage and flood documents that sit alongside the access case.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.