SuDS Strategy — Dental Surgery Extension with 13.5m³ Attenuation, Lydney, Gloucestershire

SuDS Strategy for a Dental Surgery Extension with 13.5m³ of Attenuation Storage, Lydney
Location: The Coach House, Victoria Road, Lydney, Gloucestershire, GL15 5DG | Client: Punit Shivhare | Services: SuDS Strategy

Responding to the Lead Local Flood Authority

The commission arose from the Flood Risk Management Officer's requirements on the planning application. Gloucestershire County Council, as Lead Local Flood Authority, sought a drainage strategy demonstrating compliance with the discharge hierarchy for a commercial premises in a built-up part of Lydney, on a site in Flood Zone 1 with no watercourse within 20m of the development.

For a scheme of this scale the practical question is a narrow one: where does the surface water go, at what rate, and can that be shown to be no worse than the existing situation. The answer here was helped considerably by the fact that the scheme reduces hard surfacing rather than adding to it.

Working on behalf of Punit Shivhare, we prepared the SuDS Strategy in support of the application for the erection of a single storey rear extension with associated internal alterations to the dental surgery, together with alterations to the front entrance and the conversion of a garage to a cycle and bin store.

Site Characteristics and Drainage Baseline

The Coach House stands on Victoria Road in a built-up part of Lydney, in a mixed commercial and residential setting. We drew on the Environment Agency Flood Map for Planning, the Risk of Flooding from Surface Water mapping, BGS geological and soil mapping and the Forest of Dean Strategic Flood Risk Assessment. The site lies within Flood Zone 1, with no main river or ordinary watercourse within 20m, so surface water is the governing consideration and the discharge hierarchy is the substance of the case.

The Baseline and the Reduction

The existing arrangement is a largely hard-surfaced yard and forecourt draining without attenuation. The proposed development results in a decrease in impermeable area from approximately 330m² to 200m², a reduction of 130m², or around 39%.

That reduction is the foundation of the strategy. Converting 130m² of hard surfacing back to permeable and landscaped surface produces betterment before any engineered feature is introduced, and it means the drainage design is improving on the baseline rather than compensating for a loss.

The Drainage Design

The strategy specifies 180m² of permeable paving over a 300mm drainage layer at 25% porosity, providing 13.5m³ of sub-surface attenuation storage.

Two points are worth drawing out. First, the permeable paving area of 180m² is deliberately close to the whole of the retained hard-surfaced area, so that the treated catchment is effectively the entire external surface rather than a fraction of it. Second, the sub-base does two jobs: it provides attenuation storage, and it provides infiltration where the underlying soil conditions allow, which places the design correctly at the top of the drainage hierarchy rather than defaulting to a discharge-and-attenuate solution.

The surface water network is designed to the 1-in-100-year storm event with a 40% allowance for climate change.

A Commercial Premises with a Public Function

A dental surgery is a building with a continuous stream of visitors, including patients with limited mobility. Permeable paving suits that setting well: it removes standing water and ponding from the forecourt and parking area in ordinary rainfall, which is an access and safety benefit quite separate from the flood risk case. Where a drainage feature earns its place on more than one ground, it tends to survive value engineering.

Outcome

In line with NPPF requirements, and subject to the measures set out in the strategy, the proposed development can proceed without being subject to significant flood risk, and the implementation of suitable surface water management techniques means the development will not increase flood risk to the surrounding area. Forest of Dean District Council granted permission on 18 August 2026 under reference P1568/25/FUL, with the SuDS Strategy and a revised version both on the application file.