Flood Risk Assessment in Harrow

Harrow is one of the few London boroughs where every watercourse begins within its own boundary, and every planning application answers to the same council twice over. The London Borough of Harrow decides the application as the local planning authority and reviews the drainage as the Lead Local Flood Authority, so a scheme's flood risk assessment and its surface water drainage strategy are read by the same team, against the same evidence base.

Developers, architects and planning consultants come to Unda for a flood risk assessment and drainage strategy in Harrow for planning permission, for a property purchase or mortgage, for insurance, and alongside sequential and exception tests where a site falls in a higher-risk flood zone. Whatever prompts it, the report has to satisfy Harrow's own Local Plan, its Strategic Flood Risk Assessment and its Lead Local Flood Authority requirements before it will be accepted.

Harrow adopted an entirely new Local Plan on 24 March 2026, replacing policies that had stood since 2013 — so a flood risk assessment or drainage strategy built around the old policies is already out of date.

• Decision-maker. Harrow Council is both the local planning authority and the Lead Local Flood Authority for its area, as with every London borough.
• Current policies. Policy CN3 (Reducing Flood Risk) and Policy CN4 (Sustainable Drainage) in the Harrow Local Plan 2021–2041, adopted 24 March 2026.
• Evidence base. Harrow's Strategic Flood Risk Assessment Level 1 (2009) and Level 2 (October 2024), the latter testing 18 allocated development sites in detail.
• Drainage expectation. A greenfield runoff rate of 5 litres per second per hectare, backed by a completed London Sustainable Drainage Proforma at validation.
• Local quirk. Harrow's validation list asks for a surface water drainage strategy within the flood risk assessment for all new development, including a single dwelling.

Who decides flood risk and drainage in Harrow's planning system?

Harrow Council wears two hats on any application that touches flood risk. As the local planning authority it determines the application against the Harrow Local Plan 2021–2041. As the Lead Local Flood Authority — a role every London borough holds for its own area under the Flood and Water Management Act 2010 — the same council reviews the surface water drainage strategy as a statutory consultee. The Environment Agency is consulted on the borough's main rivers and any development in Flood Zone 2 or 3, using its own standing advice and flood risk assessment guidance, and Thames Water is the sewerage undertaker for the wastewater network, so a drainage strategy connecting to the public sewer needs its agreement too.

• Local planning authority. London Borough of Harrow — determines every planning application in the borough.
• Lead Local Flood Authority. Also the London Borough of Harrow. It reviews surface water drainage and SuDS as a statutory consultee, the arrangement in every London borough rather than the county/district split found across most of England.
• Environment Agency. Consulted on main river flood risk and on any development in Flood Zone 2 or 3.
• Sewerage undertaker. Thames Water — agreement in principle is needed for any foul or surface water connection to the public sewer; clean water supply comes from Affinity Water.

Does a Harrow planning application need a flood risk assessment?

A site-specific flood risk assessment is required for a Harrow planning application wherever the site falls in Environment Agency Flood Zone 2 or 3, in either of the two surface water bands Harrow maps as its own Flood Zone 3a and 3b, in a designated Critical Drainage Area, or on a site of one hectare or more in Flood Zone 1 — and Harrow applies this to minor development, householder extensions and changes of use, not only to full or major applications.

Harrow has adopted the Environment Agency's Flood Map for Surface Water as a second set of flood zones for planning purposes, labelling the 1-in-100-year surface water extent as its own Flood Zone 3a and the 1-in-30-year extent as its own Flood Zone 3b — reusing the same zone numbers the Environment Agency applies to river and tidal flooding, but for an entirely different source of water. A site can screen as low risk on the national river Flood Map for Planning and still need a flood risk assessment because it falls inside one of Harrow's own mapped surface water zones.

Missing one of these documents is the most common reason a Harrow application does not validate in the first place — see Unda's guide to why planning applications get invalidated for flood risk and drainage and the wider guide on when a flood risk assessment is required.

Harrow's SuDS and drainage requirements

Harrow expects new development to reduce surface water runoff to a greenfield rate of 5 litres per second per hectare from every impermeable area, following the national SuDS hierarchy the London Plan carries into local policy — infiltrate first, then discharge to a watercourse, and only as a last resort to sewer — and to submit a completed SuDS proforma confirming how the strategy meets that standard.

• Discharge rate. Greenfield runoff of 5 litres per second per hectare from all impermeable areas, with any shortfall against that rate justified in the drainage strategy rather than excused by an existing high run-off site.
• SuDS proforma. Harrow, like every London borough, requires a completed London Sustainable Drainage Proforma at validation, summarising the discharge route, pre- and post-development runoff rates, SuDS components and maintenance arrangements — see Unda's guide to what a SuDS proforma is and when it's required.
• Foul and surface water separation. Much of Harrow's drainage network is a historic 'dual pipe' system, so proposals must keep surface and foul water separate and demonstrate agreement in principle with Thames Water for any sewer connection.
• Water efficiency. Major residential development is also expected to meet the London Plan's target of 105 litres or less of mains water per person per day.
• Land Drainage Byelaws. Any works to, or discharge into, an ordinary watercourse in Harrow needs separate consent under the council's own Land Drainage Byelaws.
• Common objection triggers. Defaulting to sewer discharge without ruling out infiltration or a watercourse outfall, missing exceedance routing for storms beyond the design standard, and a SuDS proforma that doesn't cross-reference the calculations behind it — see Unda's guide to resolving an LLFA objection.

A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports. They often travel together in a Harrow planning submission, but they answer different questions: is the site safe, should the development be here at all, and where does the surface water go. Unda's surface water drainage strategy and SuDS design service covers the second half of that answer.

Harrow's Local Plan flood and drainage policies

Harrow adopted an entirely new Local Plan on 24 March 2026, covering 2021 to 2041 and replacing the Development Management Policies adopted in 2013. Two policies in the new plan's climate and nature chapter now govern flood risk and drainage for every application: Policy CN3 (Reducing Flood Risk) and Policy CN4 (Sustainable Drainage), alongside Policy CN5 (Waterway Management) for the borough's rivers and streams.

What does Policy CN3 require in Harrow?

Policy CN3 requires a Harrow planning application to demonstrate that development will be resistant and resilient to all relevant sources of flooding, not just the source that first triggered the flood risk assessment. Following main modifications agreed during the Local Plan's examination, the policy also stops any increase in the built footprint or the flood vulnerability classification of existing development within Flood Zone 3b — the functional floodplain — so an extension or redevelopment on a 3b site cannot leave the flood risk position worse than it already is.

What does Policy CN4 require for drainage in Harrow?

Policy CN4 requires a sustainable drainage system that controls the rate and volume of surface water runoff, sized and designed so it can be maintained for the lifetime of the development. Where a site is affected by land contamination, the policy specifically requires that SuDS infiltration components must not discharge to ground in a way that would put groundwater resources at risk — a requirement added during examination in response to Environment Agency concerns.

These policies replace Policy DM9 (Managing Flood Risk) and Policy DM10 (On-Site Water Management and Surface Water Attenuation) from Harrow's 2013 Development Management Policies, which no longer determine planning applications now the new Local Plan is adopted — worth checking if an older report, objection or pre-application response still cites them.

Harrow's Strategic Flood Risk Assessment and evidence base

Harrow's flood risk evidence base runs in two tiers: a Level 1 Strategic Flood Risk Assessment from 2009 covering the whole borough, and a Level 2 assessment published in October 2024 that tests 18 allocated development sites in detail against the sequential and exception tests.

• Level 1 SFRA (2009). Maps borough-wide flood risk from all sources and underpins the Local Plan's flood zone evidence.
• Level 2 SFRA (October 2024). Site-specific modelling and mitigation recommendations for sites allocated in the new Local Plan, including safe access and egress routes and compensatory flood storage advice for each site.
• Surface Water Management Plan (2012). Identifies Harrow's Critical Drainage Areas, including Roxeth.
• Functional floodplain. Flood Zone 3b is defined and mapped through the SFRA and feeds directly into Policy CN3's restriction on increasing built footprint or vulnerability there.
• Joint context. Harrow also sits within the West London Strategic Flood Risk Assessment, shared with Barnet, Brent, Ealing, Hillingdon and Hounslow.

How Harrow applications get invalidated on flood risk and drainage

Harrow's local validation list will not register a planning application without a site-specific flood risk assessment wherever the site falls in a mapped flood zone, and without a surface water drainage strategy for every new development, however small — a stricter local position than councils that only ask for drainage information from major schemes.

• FRA required even for minor development. Extensions, outbuildings, hard surfacing and changes of use within a mapped flood zone all need a flood risk assessment, not only full applications.
• Drainage strategy required for every new dwelling. Harrow's checklist asks for a surface water drainage strategy within the FRA for all new development, including a single dwelling.
• Read the evidence base together. Harrow expects the flood risk assessment to reference the local validation checklist, the Surface Water Management Plan, the Local Flood Risk Management Strategy, the SFRAs and the Land Drainage Byelaws together, not any one document in isolation.

How Harrow behaves in practice

Harrow investigated a significant surface water flood under Section 19 of the Flood and Water Management Act 2010 after roughly a month's rainfall fell in five hours across the Roxeth Critical Drainage Area on 23 September 2024, and the resulting feasibility study is now shaping how the council reviews drainage proposals in that part of the borough.

• Roxeth Critical Drainage Area. The September 2024 storm prompted a Section 19 investigation and a published feasibility study the council now draws on when reviewing nearby drainage proposals.
• Section 19 thresholds. Harrow investigates a flood as significant where it affects five or more homes, two or more business premises, a piece of critical infrastructure, or blocks a transport link for a set number of hours depending on its category.
• Growth context. Regeneration around Wealdstone and the wider Harrow & Wealdstone Opportunity Area brings redevelopment of previously developed sites, which the council links directly to reducing future flood risk in its Local Plan evidence.
• Get it right first time. Since April 2026's planning appeal changes, a technically weak flood risk assessment or drainage strategy is harder to fix on appeal than at application stage — see Unda's guide to the new planning appeal rules and flood risk.

Flood risk in Harrow's area

Every watercourse in Harrow rises within the borough itself, so the dominant flood risk here is surface water and the smaller order tributaries it feeds, rather than a single defining river — Harrow sits at the very top of the catchments it drains into.

The Woodridings Stream drains the north-west, Yeading Brook and the Roxbourne the south-west, Edgware Brook the north-east, and the Wealdstone and Kenton Brooks the east, all ultimately reaching the Thames through the Crane, the Brent or the Colne. Around 93km of watercourse runs through the borough, and just under half of it is culverted beneath streets and buildings, a legacy of Harrow's early-to-mid twentieth century suburban expansion that concentrates flood risk at pinch points a flood map alone won't show. For the fuller day-to-day flood risk picture across Harrow and its neighbouring boroughs, see Unda's area guides.

Flood risk and drainage services for every situation in Harrow's area

A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports. They often travel together in a Harrow planning submission, but they answer different questions: is the site safe, should the development be here at all, and where does the surface water go.

Unda prepares flood risk assessments for planning in Harrow's area, flood risk surveys for property purchase, flood risk assessments for insurance, and flood risk assessments for BREEAM, alongside sequential and exception test reports, groundwater flood risk assessments, flood modelling, and surface water drainage strategies and SuDS design.

• Detailed design and conditions. Where a Harrow permission is conditioned on drainage, Unda prepares the detailed scheme through discharge of drainage planning conditions and, at the outline stage, preliminary drainage design.
• National policy context. Read how the NPPF and the September 2025 Planning Practice Guidance update apply on Unda's NPPF flood risk guidance page.
• Free checking tools. Screen a Harrow postcode on the Flood Risk Map and the Live River Levels Map, check the ground beneath a site on the Geology & Ground Conditions Checker, and size a drainage scheme with Unda's drainage calculators and BRE 365 infiltration rate calculator.

How Unda helps in Harrow

Unda has prepared flood risk assessments and drainage strategies across London and the wider South East since 2014, working directly with Lead Local Flood Authorities, the Environment Agency and local planning authorities on exactly this kind of dual-role council. Our flood risk consultants know how Harrow's own surface water flood zones, its SuDS proforma and its new Local Plan policies fit together, and build reports to satisfy the same team reviewing both halves of the application. Read more about Unda.