Walkthrough: a riverside mill converted to flats

This is an illustrative walkthrough rather than a real scheme. The building is a four storey grade II listed water-powered textile mill of about 1840, on a river bank, with a cobbled yard between the mill and the lane. The proposal is 22 flats, with the ground floor given over to parking, plant, cycle and bin stores. The mill sits in Flood Zone 3a. Nothing about the structure can move, which is what makes the flood case and the heritage case the same problem.

The order in which the evidence is assembled matters here more than on almost any other kind of scheme, because the ecology survey season, the flood modelling and the heritage recording all sit on the critical path.

The building, its listing and its flood zone

The mill is listed, so listed building consent is needed for the alterations as well as planning permission for the change of use and the works. The list entry describes the exterior and the surviving cast iron internal frame, and the interest includes the plan form of the open floors, the loading doors on the gable and the wheel pit.

The Environment Agency Flood Map for Planning places the site in Flood Zone 3a, with the river channel and the wheel pit immediately adjacent. A site-specific Flood Risk Assessment is therefore required, modelled rather than taken from the published mapping, because the mapping cannot resolve levels across a yard with a retaining wall and a historic channel running through it.

Residential use is classified as more vulnerable in flood risk terms. The existing mill use is less vulnerable. The conversion therefore increases the vulnerability classification of the building in a Flood Zone 3a location, and that single fact drives the next section.

Why the sequential test does not apply but the exception test does

Planning Practice Guidance on flood risk sets out the circumstances in which the sequential test need not be applied, and a change of use is one of them, with a narrow exception for changes to caravan, camping, chalet, mobile home and park home sites. So this scheme does not need to demonstrate that there is no reasonably available site at lower flood risk, which saves a great deal of argument.

The exception test is a different matter. For a more vulnerable use in Flood Zone 3a, the exception test is required, and the change of use exemption does not carry across. Applicants conflate the two tests regularly, and an application that claims exemption from both will be returned.

The exception test has two limbs and both have to be satisfied.

The first limb is where the heritage case does real work. A listed building whose fabric is deteriorating because it has no viable use is a recognised sustainability benefit when a conversion secures it, and that argument has to be evidenced with a condition survey rather than asserted. A separate Exception Test (Flood Risk) document setting out both limbs, cross-referring to the flood risk assessment and the heritage statement, is clearer than burying the test in an appendix. Where a sequential test is nonetheless requested by the authority, the position on the Sequential Test (Flood Risk) should be set out with the guidance reference rather than argued informally.

Climate change allowances are the technical heart of the flood case. Use the current published peak river flow allowances for the relevant river basin district and the appropriate epoch for a residential lifetime, and state which allowance and which epoch were used. Those figures are revised periodically, so check the current schedule at the point of modelling rather than reusing a figure from an earlier job.

Finished floor levels inside fixed fabric

On a greenfield site the answer to a flood level is to raise the floor. In a listed mill the floor is a historic structure spanning between cast iron columns, and raising it destroys the thing the listing protects.

Four moves are available, and the scheme uses three of them.

1. Put no residential accommodation on the ground floor, using it for parking, stores and plant, which are less vulnerable uses
2. Design the ground floor for water entry and recovery, with lime plaster or no plaster, solid floors, raised electrical services and no fitted finishes that cannot be dried
3. Raise sleeping accommodation and all habitable rooms above the modelled flood level including the climate change allowance
4. Fit demountable flood barriers to the openings, as a supplementary measure rather than the primary strategy

Point four needs care. Barriers depend on warning time, on somebody being present and on maintenance, so they support a strategy and cannot be the strategy. The Environment Agency's standing advice treats resistance measures as secondary to locating vulnerable uses above the flood level.

The electrical strategy follows from the flood level rather than from the usual riser logic. Distribution boards, lift motor rooms, pumps and the communal heating plant all have to be above the modelled level, which in a mill with low floor-to-floor heights at ground level means plant goes on the first floor or in a new enclosure at a higher level, and both options have heritage consequences.

Safe access and egress across the yard

The yard floods before the building does, which is typical of riverside sites and often the issue that generates an Environment Agency objection.

The assessment has to establish the depth and velocity across the route between the building entrance and the nearest land above the flood level, for the design event including climate change. Where a dry route exists, say so and show it. Where it does not, the position is that occupants shelter in place above the flood level, and that requires the building to be safe for the duration of the event, with a flood warning and evacuation plan, signed up to the Environment Agency's warning service, and a management arrangement that survives a change of managing agent.

In this walkthrough the lane rises away from the river, so a dry route exists for the modelled event but not for the plus-allowance scenario. The scheme therefore provides both: a route that works in the design event and a shelter-in-place arrangement above it, with the plan secured by condition and referenced in the leases.

Heritage significance and the loss of fabric

The conversion needs new openings for windows in the long elevations, two new stair and lift cores, and compartmentation between flats that did not exist in an open mill floor. All of that is loss of fabric.

The heritage case rests on three propositions, each of which needs evidence in the Heritage Statement. The significance of the mill lies principally in its external form, its cast iron frame and its relationship to the watercourse, rather than in the open volumes of each floor. The conversion secures the optimum viable use of a building that has no realistic alternative. And the harm has been minimised by the specific design choices made, with the rejected options recorded.

Practical moves that reduce harm in buildings of this type: place new cores in the least sensitive bays, usually at a later extension or a previously altered end. Reuse existing window and loading openings rather than cutting new ones wherever the plan allows. Keep compartment walls clear of the iron columns so the frame remains legible, and expose rather than box the structure within the flats. A written and photographic record of the interior before works, and of the frame as found during the strip-out, is normally required by condition.

Industrial sites also carry a contamination history. A Phase 1 desk study is a standard requirement here, and the experience in a conversion where contamination was found mid build is the reason to do the investigation before the contract is signed rather than after.

Bats and swifts in the roof structure

A mill roof with a slate covering on open timbers, gable loading doors and a long history of vacancy is good bat habitat and good swift habitat. Both need dealing with before permission, not after.

The survey sequence is a preliminary roost assessment, which can be done at any time of year, followed by emergence and re-entry surveys in the active season between May and September, with the optimal window from May to August. The number of visits depends on the roost potential identified at the first stage. Birds nesting in the structure are protected while nesting, and the breeding season runs broadly from March to August.

Protected species survey work and any mitigation licence require a suitably licensed ecologist under the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017. That is a legal requirement rather than good practice, and an AI-assisted draft is a starting point for that ecologist rather than a substitute for them. Where a European protected species licence is needed, it is applied for after planning permission is granted, because Natural England's tests include the existence of the permission, and the licence process adds weeks to the programme.

A Bat Survey that starts in October has lost a year. On a scheme like this the ecology commission should be the first thing ordered, before the architect is appointed.

Biodiversity net gain also has to be checked rather than assumed. The statutory regime includes a de minimis exemption where the development affects less than 25m² of on-site habitat or less than 5m of linear habitat, and a conversion entirely within an existing footprint with no works to the yard may fall within it. Where the yard is relandscaped, it will not.

The document pack and the order of consents

The submission comprised the application forms and plans, the flood risk assessment with the hydraulic modelling appendix, the exception test statement, the heritage statement with a condition survey and a statement of significance, the preliminary roost assessment and emergence survey report, a Phase 1 contamination desk study, a structural appraisal of the frame, a drainage strategy, a construction management plan and a planning statement.

The consent order ran as follows.

1. Pre-application discussion with the conservation officer and the Environment Agency, in parallel
2. Ecology surveys through the summer
3. Planning application and listed building consent application submitted together
4. Permission and consent granted with conditions
5. European protected species licence applied for
6. Conditions discharged in the order the pre-start requirements demanded
7. Building Regulations application, then works

The single lesson from schemes of this type is that the river and the bats set the programme, and the design team sets everything else around them.

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