Solar Farm Flood Risk Assessment & Drainage

A solar farm flood risk assessment (FRA) is the site-specific report an English local planning authority needs before it can grant permission for a ground-mounted solar scheme where flooding, or the way the site drains, is a concern. It sets out the flood risk to the array and its infrastructure, shows the development will not raise risk elsewhere, and pairs with a surface water drainage strategy for the parts of the site that need one. In Wales the equivalent document is a Flood Consequence Assessment, and a separate SuDS approval applies.

Unda prepares these reports for solar developers, EPC contractors and planning consultants across England and Wales, from a few megawatts on a single field to schemes in the hundreds. The work is rarely about the panels flooding. It is about where the transformers, substation and battery containers sit, whether floodwater can still move across the site once the rows are in, and whether the way the array changes runoff and erosion has been thought through and evidenced.

Since 17 August 2026 that report is written to a different rulebook. The new National Planning Policy Framework moved flood risk into Chapter 18, as Policies F1 to F9, and moved the flood zone definitions, the vulnerability classification and the compatibility matrix into a new Annex F. Solar farms are still essential infrastructure, and are now named in Annex F, Table 2 in their own right.

What is a solar farm flood risk assessment?

It is a technical planning document that assesses every source of flooding affecting a ground-mounted solar site, judges the risk over the development's lifetime, and demonstrates that the scheme is safe and does not increase flood risk elsewhere. It is read by planning officers and, where the site is in a flood zone, by the Environment Agency. On larger schemes it sits alongside a drainage strategy for the compounds and access, and a land-management approach for the array itself.

An assessment for a solar farm has to answer a specific set of questions that the Environment Agency and lead local flood authority raise again and again. In our experience the recurring concerns are these.

• Location of the transformer, inverter and substation units. These are the vulnerable, valuable parts of the scheme, and where they sit within the floodplain is the first thing a reviewer checks.
• Location of the panels within the floodplain, and whether they, and the perimeter fencing, would interrupt the conveyance of floodwater across the site.
• Safe access and egress, for both operation and maintenance and for emergencies during a flood.
• Any increase in impermeable surfaces, and whether that has been attenuated back to the rate the field discharged before.
• Development within statutory buffer zones to rivers, watercourses and ordinary watercourses.
• Landraising or built platforms that would reduce the flood storage capacity of the site and need compensating.
• Increased surface-water runoff and erosion, particularly channelised flow where panel rows run down a slope.
• Containment for the battery compound, where storage is co-located, so that firefighting water cannot reach a watercourse or a soakaway.

Do solar farms need a flood risk assessment?

In practice, almost always, and the August 2026 Framework widened the net rather than narrowing it. Policy F4 carries the old triggers into policy and drops the qualifier that used to limit them. A site-specific FRA is required for development in Flood Zones 2 and 3, and in Flood Zone 1 for sites of a hectare or more, for land the Environment Agency has flagged with critical drainage problems, and now for any land a strategic flood risk assessment or the Flood Map for Planning shows to be at risk from any source, now or in the future. The old wording only caught proposals introducing a more vulnerable use. That qualifier has gone.

Two things follow for solar. Ground-mounted arrays are large by nature, so a scheme sitting wholly in Flood Zone 1 is usually caught by the one-hectare trigger anyway. And the Flood Map for Planning update of 28 May 2026 added surface water extents with climate change allowances, so fields the old maps showed clear now sit inside a mapped risk area. Land that passed a validation check last spring may not pass one now. Our guides cover when an FRA is required and how to read the Flood Map for Planning; you can screen a site with our free flood risk map and postcode checker.

If you are appraising land before purchase, that combination — a widened trigger and a bigger mapped surface water extent — makes early flood screening part of the acquisition checklist rather than a planning-stage formality.

Do solar panels cause a loss of floodplain storage?

This is the question that separates a solar FRA that clears review from one that comes back, and it is the one most reports answer with an assumption rather than a calculation. The honest answer is that the panels themselves rarely displace much flood storage, but you have to prove it, and storage is not the only test being applied.

The developer position is well established: panel tables are mounted on posts, the lowest edge sits clear of the ground, floodwater passes underneath, so no floodplain volume is lost. That reasoning is usually right. It is not self-evident, and the Environment Agency has said so — on the Helios Renewable Energy Project in May 2025 it recorded that no evidence had been supplied to justify the assumption that the floodplain lost to the arrays was negligible, and asked for volumetric displacement to be determined by the area-volume method, including for the 2080s epoch.

There is a second problem with leaning on the void beneath the panels. National Planning Practice Guidance is explicit that while stilts and voids may be an appropriate way of mitigating flood risk to a structure, they should not normally be relied on to compensate for lost floodplain storage. A panel table on posts is functionally a void.

Storage and conveyance are two different tests, and on a solar site the second is usually the sharper of the two.

The modelling point is not theoretical. On Peartree Hill the Agency noted that it usually asks for panel upstands to be considered within hydraulic models, through roughness or flow constriction layers. On One Earth in September 2025 it went further, asking for the impact of submerged panels on flow routes to be assessed or the panels removed, and noting that voids should be a last resort because of blockage risk. Where the modelling is done properly the numbers are usually reassuring, millimetres rather than hundreds, but the number has to exist.

What genuinely displaces storage is the built infrastructure, not the array: the substation and DNO compound, inverter and transformer plinths, any control building, battery containers and their bunds, and any landraising to lift plant clear of the design flood level. Those are the elements to locate sequentially on the driest ground, and the elements that need level-for-level compensatory storage where they cannot be. Access tracks are normally compacted granular material laid at grade, which keeps them out of the storage argument and the impermeable-area argument at once.

Where a solar farm sits in Annex F, Table 2

Solar farms are essential infrastructure. Annex F, Table 2 of the August 2026 Framework names them in their own right, separately from the electricity network infrastructure entry that used to carry them by implication, so the classification no longer has to be argued. It is worth checking which version a colleague or consultee is quoting: some legacy reports and older strategic flood risk assessments still treat an unoccupied array as less vulnerable, and the government's own Planning Practice Guidance on flood risk was last updated in September 2025 and still refers to the superseded Annex 3.

Annex F, Table 3 pairs the classification with the flood zones. Most arrays sit in Flood Zone 1, where the label makes no practical difference. It bites at the wet end of the site.

In Zone 3a the essential-infrastructure label is the more demanding of the two, because it triggers the Exception Test where a less vulnerable use would not. In Zone 3b, the functional floodplain, it is the only route in at all. The classification also raises the climate change allowance: essential infrastructure attracts the higher central peak river flow allowance rather than the central one, so the design life and the percentage have to be justified explicitly and pulled live for the catchment. Our guides to the Annex F vulnerability classification, flood zones and climate change allowances set out the detail.

The Sequential and Exception Tests for solar farms

Where any part of a site is in Flood Zone 2 or 3, expect the local planning authority and the Environment Agency to want the flooding tests applied. The August 2026 Framework renamed them — the Sequential Test is now Policy F5 and the Exception Test is Policy F6 — and in doing so handed solar applicants a better argument than they had before.

The change that matters is the area of search. Policy F5 caps it: the area the test is applied to should not be greater than the anticipated catchment of the development in terms of its likely occupiers or users. Authorities that asked for district-wide or county-wide searches no longer have policy behind them. A viable solar scheme needs the solar resource, a landowner and, above all, a workable grid connection with an accepted offer at a named point. That is the catchment, and it is a narrow one. Setting it out properly, with the connection agreement as evidence, is the difference between a sequential case that survives and one that invites a two-county alternative-site trawl.

1. Apply a sequential approach within the site as well as across it. Steer the vulnerable parts of the scheme to the land at lowest risk: inverters, transformers, substation, control building and any battery compound on the higher, drier ground, with the array over the lower-lying land.
2. Define and evidence the area of search under Policy F5. Tie it to the grid connection point and the technical constraints on the scheme, rather than accepting an administrative boundary as the default.
3. Check whether the site is exempt at all. The test is not applied where a site was sequentially tested at allocation, nor where a site-specific FRA demonstrates clearly that the risk is surface water only and the design keeps people safe for the development's lifetime without increasing risk elsewhere.
4. Pass the Exception Test where Zone 3a or 3b cannot be avoided. Policy F6 now runs to three limbs: wider sustainability benefits, safety for the lifetime of the development, and no increase in flood risk elsewhere. The renewable-energy benefit supports the first; finished levels for the electrical plant, flood-resilient design and the drainage strategy support the other two.

Both tests are their own discipline, and the wording of a solar case has to be right first time, particularly since April 2026, when most planning appeals moved to a written representations procedure that generally excludes evidence not before the council at determination. A sequential case you meant to strengthen at appeal is a case you have lost. We cover the tests in full on our Sequential and Exception Test page and in the background guide to what the two tests involve.

Surface water drainage for solar farms: are the panels impermeable?

This is the question that decides a solar farm drainage strategy, and there is a real split between authorities on it. The defensible mainstream position in the UK is that the panel area is not an impermeable surface. Rain sheds off the panels onto pervious ground beneath, the support posts occupy a negligible footprint, and empirical work has found panels over grass do not materially change runoff volume, peak discharge or the time to peak. Several authorities put this in writing: Central Bedfordshire treats the array impact as nil and the site as roughly ninety-five per cent permeable, and Essex cites a runoff increase from panel cover alone of about a third of one per cent.

The real hydrological effects are the concentration of flow and the erosion at the panel drip line, and the risk of channelised flow where rows run straight down a slope. So the design response is to control where water goes and protect the soil, rather than to attenuate the panel area as if it were a roof. The new hard surfaces are drained conventionally; the array is managed through layout and land management. The two jobs are different, and treating them the same is the most common mistake we see corrected at consultation.

These principles are consistent from site to site, and they are the ones the more solar-literate authorities ask for by name.

• Lay panel rows parallel to the contours wherever possible, so runoff moves as sheet flow rather than concentrating into channels down the slope.
• Intercept and slow flows with SuDS between and around the array — grass buffers, filter strips, swales and, where channels would form, gravel-filled cut-off drains or berms to re-spread flow.
• Run interception swales along the downstream boundary of each array block to store, attenuate and infiltrate; a two-metre-wide, 0.3-metre-deep swale following the contours is a sound default.
• Keep a good vegetated sward across the site and avoid bare or compacted ground, which is the single biggest driver of runoff and erosion on a solar site.
• Decompact after construction and inspect the sward through the first seasons to confirm it establishes and stays uncompacted.
• Leave existing watercourses and ditches alone. Policy F8 now says development should not enclose a watercourse without compelling reasons and should remove culverts and renaturalise channels where it can, which is the first time deculverting has carried national policy weight.

Conventional SuDS, designed to the discharge hierarchy with attenuation and, where relevant, treatment, belong to the substation and DNO compound, the inverter and transformer stations, any control building, the battery compound and permanent hardstanding. These are attenuated back to the local greenfield runoff rate for events up to the one-in-100-year storm plus climate change, with the compounds carrying the water-quality risk on the site.

Policy F8 changed the weight behind all of this. The 2025 National Standards for Sustainable Drainage Systems were guidance with no legal force; national policy now requires design in accordance with them, and applies that duty to every development rather than only to major schemes. Two practical consequences for solar. The discharge hierarchy has five priorities, not four, and collection for non-potable use now sits above infiltration at the top of it — a live option where a scheme has a welfare building or panel-washing demand. And where infiltration is proposed it has to be evidenced with BRE 365 infiltration testing, which you can scope with our free infiltration rate calculator and soakaway ground-conditions checker. The wider approach draws on CIRIA C753 and is set out in full on our surface water drainage strategy page.

Because the runoff shedding off inert glass panels is not treated as contaminated, the array does not normally need dedicated water-quality treatment. The material risks are sediment and erosion from bare ground, which makes sediment control largely a land-management task. That dovetails with biodiversity net gain: the species-rich grassland and sensible grazing or mowing that deliver the habitat uplift also maintain the infiltrating, erosion-resistant surface the drainage strategy relies on, so the drainage strategy and the land-management plan should be written to be mutually consistent.

Battery storage on a solar site: fire water is a drainage problem

Most solar schemes now come forward with battery energy storage co-located, and a BESS compound raises different questions from the array. A solar farm drainage strategy that covers only the panels, tracks and compounds is incomplete the moment a battery is added. The array is a runoff and erosion problem. The battery compound is a containment problem, and it is the part of a solar site most likely to draw a late objection from the Environment Agency or the fire and rescue service.

The reason is simple: fighting a lithium-ion battery fire means applying water for a long time. At Carnegie Road in Liverpool in September 2020, the first significant incident of its kind in the UK, defensive firefighting ran for 59 hours with water as the sole medium, and hydrogen fluoride was confirmed as a by-product where water met the cells. The run-off was caught, by luck, in a gravel soakaway under the containers. No lead local flood authority would accept a soakaway as the designed containment route for contaminated firewater.

The number that sizes the problem comes from the fire service, not the drainage calculation. The National Fire Chiefs Council guidance, now in its December 2025 second edition, expects a flow rate of no less than 25 litres per second at any hydrant on site, or an equivalent static supply of around 180,000 litres. That volume has to go somewhere.

What that means for the drainage strategy, and what a reviewer will look for:

• Containment volume stated and calculated, not assumed. On One Earth the detention basins were sized to attenuate a one-in-10-year event plus 228 cubic metres of firewater, and the Environment Agency still picked up internal contradictions in the design. The figure has to be traceable to the suppression system and the hydrant supply on your site.
• An isolation route on the outfall. Automatic penstock or drain closure valves so contaminated water is held rather than discharged. The NFCC guidance expects site plans to show drainage runs, pollution control features and fire water containment such as bunded or kerbed areas.
• Bunded, impermeable compound surfacing, with the containment sized around the bund rather than relying on the wider SuDS network to cope.
• No infiltration under the battery compound. Soakaways and infiltration basins belong elsewhere on the site; the current Environment Agency pollution prevention guidance and CIRIA C736 both turn on keeping contents out of surface water and groundwater.
• Attenuation and containment reconciled. The same basin cannot be full of attenuated storm runoff and available for firewater at the same time. State which duty governs and show the arithmetic.
• Levels for the containers themselves. Where any part of the compound sits within a modelled flood extent, plinth or bund levels with stated freeboard, so the asset stays operational and the containment stays effective in a flood.

Two points of currency worth getting right, because a lot of published material has them wrong. Grid-scale battery storage is not currently regulated under the Environmental Permitting Regulations, as the Environment Agency confirmed on a solar and BESS scheme in November 2024, but Defra consulted on modernising industrial permitting in autumn 2025 and standard rules permits covering fire plans and firewater containment are the expected direction. Treat that as current, not settled. And the applicable standard for grid-scale systems is BS EN IEC 62933-5-2:2020, not PAS 63100:2024, which covers battery storage in dwellings.

Ordinary watercourse consent for tracks and cable crossings

A field-scale solar site almost always has to cross a ditch somewhere, for an access track or a cable route, and that consent is easy to leave until it is late. Any works, including temporary works, that obstruct flow in the channel of an ordinary watercourse need land drainage consent under section 23 of the Land Drainage Act 1991. The lead local flood authority grants it, except inside an internal drainage district, where the drainage board does; works affecting a main river are the Environment Agency's under a separate permit. Determination takes two months, after which consent is treated as granted.

Environment Agency consultation responses have flagged multiple access road crossings of ordinary watercourses on solar and battery schemes and recommended early engagement with the lead local flood authority. Identify every crossing at layout stage rather than at discharge of conditions, and design out the enclosure where you can: a clear-span or bottomless crossing avoids the culverting that Policy F8 now discourages, and is easier to consent than a pipe.

Local variation: what your lead local flood authority will expect

There is no single national number a solar farm must drain to. The requirements are assembled locally, from the lead local flood authority as statutory consultee on surface water, the local plan drainage policy and the strategic flood risk assessment, with national policy over the top. We pull the specific LLFA guide, local plan policy and validation list for every site rather than assume a standard applies.

• Essex is the most solar-specific, and its logic is the one above. It asks for a drainage strategy and a land-management strategy, contour-parallel rows, cut-off drains where channels would form, a maintained sward and no culverting. It is the model we take as our default method.
• Suffolk publishes solar standing advice, with a rule of thumb of perimeter swales or filter strips roughly every fifth row, existing flow routes kept, and an FRA where the site is over a hectare or in a flood-risk area.
• Devon and Cornwall are often misremembered as having a special solar drainage rule. They do not; their dedicated material is landscape and renewable-energy guidance, and the drainage numbers still come from the generic LLFA SuDS guide.
• Discharge floors vary from place to place. Essex restricts discharge to the one-in-one-year greenfield rate and rejects the QBAR method, while Devon has abolished a fixed minimum and agrees rates site by site. The number always comes from the consenting authority, never from a previous job in a different county.

Solar farms in Wales: FCA, TAN15 and SAB approval

A Welsh site changes three things: the flood policy and map, the name of the assessment, and a separate mandatory SuDS approval. The August 2026 Framework does not apply in Wales. Policy sits in Planning Policy Wales and the revised Technical Advice Note 15, in force since the end of March 2025, which replaced the Development Advice Map with Natural Resources Wales' Flood Map for Planning. Welsh climate change allowances were reissued in March 2026 and are not interchangeable with the English figures.

The second consent is what catches schemes out. Any construction with a footprint of 100 square metres or more needs its SuDS approved by the local authority's SuDS Approval Body against the statutory Welsh standards, before construction begins and separately from planning permission. A solar farm will exceed that trigger, so the approval has to be planned into the programme, and what counts as construction area for a post-mounted array is worth scoping early. Our Flood Consequence Assessment and TAN15 guidance pages cover the Welsh route in full.

Is this the right assessment for you?

Solar work sits within a wider family of flood and drainage services, and the right document depends on your site and stage. If a solar farm FRA is not quite what you need, these are the closest neighbours.

• A different renewable technology? See our wind farm flood risk assessments and biogas and anaerobic digestion assessments, or start at the renewable energy flood risk hub.
• A standard planning scheme, not solar? Our flood risk assessment for planning covers residential, commercial and mixed-use development.
• A site in Wales? You need a Flood Consequence Assessment to TAN15, plus SuDS Approval Body sign-off.
• Only the drainage in question? Go straight to our surface water drainage strategy or drainage strategy services, or the foul drainage strategy for the compounds.
• Facing the flooding tests head-on? See our Sequential and Exception Test reports.
• A groundwater or detailed-modelling question? We offer groundwater flood risk assessment and flood modelling, and BREEAM assessments where a scheme is chasing the credit.
• Discharging a condition after approval? We handle the discharge of drainage conditions.
• Not sure which you need? Tell us the site and the stage and we will point you to the right report. You can also browse our work by area, by river or by local authority.

What our solar farm assessment covers

We scope each report to the scheme, but a solar farm FRA and drainage strategy from Unda typically brings together the following.

What a solar farm flood risk assessment costs

We price to the scheme rather than a template, because the same field can be a two-week job or a three-month one depending on what the maps show. These are the things that move the fee, in roughly the order they matter.

• Which flood zones the red line touches. A site wholly in Zone 1 with no mapped surface water is a different job from one clipping Zone 3a, where the Exception Test and a compensation case come into play.
• Whether published mapping is good enough. Where the Flood Map for Planning is too coarse for the decision, hydraulic modelling is needed, and that is the largest single variable in any flood fee.
• Site area, watercourse crossings and whether a drainage strategy is needed as well as the FRA, including infiltration testing to evidence it.
• Whether battery storage is co-located, which adds the containment design and usually a further round with the lead local flood authority.
• England or Wales, since a Welsh scheme carries a Flood Consequence Assessment and a separate SuDS Approval Body submission.
• The stage you are at. Screening at option or acquisition stage is cheaper than unpicking an objection after refusal, and since April 2026 it is also the only stage at which most new evidence can be introduced.

Tell us the site and the stage and we will give you a fixed fee against a defined scope, with the exclusions written down. One of our consultants replies within 60 minutes.

How we prepare your assessment

1. Scope the site. We check the live Flood Map for Planning, the local SFRA and the LLFA's requirements, confirm the Policy F4 trigger and agree what the report and any drainage strategy must demonstrate.
2. Engage the consultees early. We liaise with the Environment Agency and lead local flood authority so the principles are proportionate to the scheme and agreed before they become an objection.
3. Work with your design team. We fit the flood and drainage response around the array layout, grid connection and compound positions, keeping it achievable and cost-effective.
4. Write the assessment. We produce the FRA, and the surface water drainage strategy where required, with the classification, tests, climate allowances, layout and land-management approach evidenced.
5. Support it to determination. We respond to consultee comments and, where an objection has already landed, set out the case to resolve it.

Why choose Unda for a solar farm flood risk assessment

Unda is a chartered flood risk and drainage consultancy, and the approach we use on solar is settled and defensible rather than argued afresh each time. We treat the array as the land-management and flow problem it is, and reserve conventional attenuation for the parts of the site that need it, which keeps schemes proportionate and the consultees on side. We work on everything from single-field arrays to schemes consented by Development Consent Order, including co-located battery storage, and we engage the Agency and the lead local flood authority early rather than reacting to an objection.

Reports are written to Chapter 18 and Annex F of the August 2026 Framework, not to the superseded Annex 3 numbering that a good deal of published material still carries. A Senior Flood Risk Consultant signs off every report, so the advice that reaches a planner or a consultee always has a named specialist behind it.

Frequently asked questions

To discuss a solar farm flood risk assessment or drainage strategy, talk to our experienced flood risk consultants on +44 (0) 1293 214444 or email enquiries@unda.co.uk. We work across England and Wales, and one of our consultants will get back to you within 60 minutes.