Flood Risk & Drainage Strategy for replacement dwelling in Pinner, London

Client and project overview

The project was originally referred to GeoSmart via a previous client. As the project progressed, we worked closely with our client Martin McGahon, a planning consultant at Town and Country Planning Ltd., as well as directly with the end client.

The proposal involved the demolition of an existing bungalow and the construction of a new replacement dwelling with habitable roof space at 68 Moss Lane, Pinner, HA5 3AU, within the London Borough of Harrow.

Site constraints and opportunities

The site presented several significant environmental and technical challenges:

• High flood risk: Located within Flood Zones 2 and 3, with a documented history of flooding.
• Surface water risk: The Environment Agency’s surface water flood maps identified parts of the site at high risk.
• Proximity to a watercourse raised concerns for ecology and maintenance access.
• Single point of access via a bridge created evacuation challenges.
• Limited infiltration potential and complex ground levels hindered conventional drainage options.

Local Authority requirements

Harrow Council’s Planning Department required a rigorous demonstration that the proposed development would be safe from flooding and would not increase flood risk elsewhere. Key conditions included:

• An emergency access/egress plan, accounting for the risk of the bridge being unusable during a flood.
• Fluvial and pluvial flood storage compensation, exceeding the usual requirements.
• A full Flood Risk Assessment (FRA), compliant with National and Local Standards, addressing all sources of flooding (fluvial, pluvial, groundwater, and artificial).
• A detailed sustainable drainage strategy, aligned with the drainage hierarchy and providing adequate on-site storage.
• Ensuring the building was set back from the watercourse to protect ecological integrity and allow maintenance access.

GeoSmart’s approach

Thanks to our extensive experience with Harrow Council and similar developments, we proactively developed a comprehensive suite of reports from the outset. This allowed us to anticipate and address potential objections quickly:

• Our consultants maintained regular contact with both the Council’s drainage officer and the planning case officer, enabling us to adapt to feedback in real time.
• We proposed a pragmatic evacuation solution by engaging with neighbouring landowners, creating alternative egress routes should the bridge become impassable.
• We utilised the latest datasets, including EA modelled flood outputs, Strategic Flood Risk Assessment (SFRA), LLFA mapping, BGS records, and GeoSmart’s own groundwater data, to accurately model site-specific risks and interactions with the development.

SuDS and Flood Risk expertise

Our drainage strategy was informed by both national and local best practice. Using GeoSmart’s proprietary SuDS Infiltration Map (SD50) we assessed the viability of infiltration techniques, accounting for:

• Soil permeability
• Groundwater levels
• Gradient and available space
• Foundation proximity and structural constraints

We then designed a SuDS scheme that provided:

• Storage for the 1 in 100-year rainfall event plus climate change
• Space-efficient attenuation features
• Enhanced biodiversity and amenity value

Balancing policy compliance and project viability

We worked closely with the architect and client to ensure that our recommendations:

• Could be realistically implemented within the final site layout.
• Did not compromise design intent or lead to unsustainable cost increases.
• Included compensatory flood storage in soft landscaping areas, while maintaining usability.
• Avoided triggering a Flood Risk Activity Permit by refining the development footprint near the watercourse.

Outcome and benefits

• Planning permission granted, subject to conditions.
• Improved flood resilience, achieved via raised finished floor levels and resistance measures.
• SuDS solution provided stormwater attenuation and contributed to sustainability objectives.

This project highlights GeoSmart’s ability to navigate complex planning challenges with precision, working collaboratively with consultants, clients, and local authorities to unlock development opportunities in sensitive locations. By combining in-depth technical knowledge with proactive engagement and tailored solutions, we helped secure a viable, policy-compliant scheme that balances flood resilience, sustainability, and design ambition, ensuring long-term value for the client and the local community.

“James Robinson at GeoSmart was excellent at dealing with Harrow council and quite honestly, I’m not sure we would have obtained planning for our new build project had he not been part of the team. I can’t thank James and GeoSmart enough for being quick to respond to the council and our client when requested.”