Floodplain Compensation FRA Addendum, Alveston, Stratford-upon-Avon

Floodplain Compensation Addendum to a Flood Risk Assessment, Replacement Dwelling on the River Avon, Alveston
Location: Ferry Lane, Alveston, Stratford-upon-Avon | Planning application: 26/01935/FUL, Stratford-on-Avon District Council | Services: Flood Risk Assessment Addendum, Floodplain Compensation

The Site and the Objection

The site, of about 1,500 m², lies on the south-west bank of the River Avon, a Main River, which forms its north-eastern boundary. Application 26/01935/FUL is for a self-build dwelling on the existing foundations of a demolished house and the alteration of an existing Boat House annexe, both with ground floors at 39.33 mAOD over open undercrofts. The design flood level is 38.73 mAOD, the 1 in 100-year (1% AEP) level plus a 20% allowance for climate change from the Environment Agency's River Avon model, as set out in the applicants' Flood Risk Assessment by another consultant.

The Environment Agency objected on 9 September 2026 (PA-0010759/01) on floor levels and lost floodplain storage, and asked for the storage gained and lost to be tabulated in height bands of 200 to 250 mm, with plans and cross-sections of existing and proposed ground. Its second response, of 23 October 2026 (PA-0010759/02), accepted the design flood level and the floor levels, 600 mm above it, but maintained the objection on storage. The submitted layout surrounded both buildings with about 144 m² of raised decking, planters, steps and landings on blockwork dwarf walls, and the original assessment had credited the voids beneath the buildings. The Environment Agency's letter states that stilts and voids "should not normally be relied upon for compensating for any loss of floodplain storage", the position in paragraph 049 of the Planning Practice Guidance.

Our Appointment

We were appointed on 26 October 2026 to prepare an addendum on floodplain storage only, answering the objection. A first band check on the submitted layout, counting the decking and planters solid, found 62.4 m³ lost below 38.73 mAOD, 32.8 m³ of it in the top band, which could not be replaced without lowering the parking area and the walled garden. On 28 October 2026 the applicants revised the site plan (560-PL-102 Rev F): all decking and planters were removed and replaced by self-supporting open-tread galvanised steel steps with a small platform at each external door, five sets at the house and two at the Boat House, with four balcony posts. The new structures cover 31.8 m² below the design flood level.

Method

Existing ground was taken from the topographical survey spot levels, with no LiDAR. The depth between the lowest ground beneath a new structure (37.92 mAOD) and the design flood level was divided into four bands: 37.90 to 38.10, 38.10 to 38.30, 38.30 to 38.50 and 38.50 to 38.73 mAOD. Every step, platform and post was counted as solid over its full plan area from ground to 38.73 mAOD, with no credit for the space beneath it or for the undercroft voids. Storage created was calculated from a triangulated ground model of the survey sampled on a 0.1 m grid, and a surplus in one band was not used to offset another.

Storage Lost and Created

The new structures displace 15.54 m³: 0.46, 1.72, 6.05 and 7.31 m³ from the lowest band to the top. Compensation is to be provided by lowering about 89 m² of higher ground: raised planters and paving in the south garden (zone A1, about 27 m², cut to 38.45 to 38.49 mAOD), the turning area south-east of the house (zone B, about 55 m², falling to an outlet at 38.00 mAOD) and a shallow grip on the river side (zone E, about 7 m²). The zones create 0.81, 3.46, 7.88 and 10.72 m³ by band, 22.87 m³ in total, so every band is in surplus. The tightest band relative to the loss is 38.30 to 38.50 mAOD (+1.83 m³, 30%). If the whole area beneath the first-floor balcony were built as a terrace, the loss would rise to 18.49 m³ and every band would still balance. About 26.4 m³ is to be excavated and removed from the floodplain, and the lowered ground drains back to the river through the outlet, so no enclosed hollow is formed. The drive, the parking area and the walled garden are not lowered.

Access and Constraints

The original assessment relied on the removed decking for safe access, so the addendum restates the route out: from the entrance platform, over the parking area at about 38.67 to 38.83 mAOD and along the drive, which rises to 39.38 mAOD at Ferry Lane and is not lowered. It also sets out where lowering deepens the design flood (about 0.37 to 0.73 m across the turning area), the drains crossing the cut zones, the tree root protection areas to BS 5837, the retained edges for the structural engineer, and a possible Environment Agency Flood Risk Activity Permit for work within 8 m of the river.

Outcome

The addendum (Issue 1, 30 October 2026) concludes that floodplain compensation is to be provided on a level-for-level, volume-for-volume basis up to and including the 1 in 100-year plus climate change level, as the Environment Agency's response PA-0010759/02 requested, with the plan, level-for-level graphs and sections in two A3 drawings. It was prepared for the applicants to send to the case officer with a request to re-consult the Environment Agency. At the time of writing the application is undecided and the Environment Agency has not responded to the addendum.

More on these services:
Floodplain compensation for extensions: https://www.airandfloodconsultants.co.uk/flood-compensation-for-extensions
Floodplain compensation calculator: https://www.airandfloodconsultants.co.uk/post/floodplain-compensation-calculator
Flood risk assessments: https://www.airandfloodconsultants.co.uk/floodrisk