Flood Risk Assessment and Flood Warning and Evacuation Plan — Children's Home Change of Use, Stanmore
We prepared a site-specific Flood Risk Assessment and Flood Warning and Evacuation Plan for the conversion of a dwellinghouse (C3) to a children's home (C2) at 2 Sitwell Grove, Stanmore, Harrow, HA7 3NB, on behalf of Guardian Roots. The London Borough of Harrow declared application PL/2135/26 invalid on 14 August 2026 because the site falls within a climate change flood zone, and on 18 August 2026 its Infrastructure Team, as Lead Local Flood Authority, confirmed a medium surface water categorisation and directed that the submission comprise flood data and emergency planning information only. The site is in Flood Zone 1, but the climate change 1% AEP surface water extent covers most of it, so surface water governs. Both uses are More Vulnerable, so vulnerability does not increase, and the evacuation plan answers the changed occupancy profile. The submission resolved the validation objection.
Flood Risk Assessment and Flood Warning and Evacuation Plan for a Children's Home Change of Use, Stanmore
Location: 2 Sitwell Grove, Stanmore, Harrow, HA7 3NB | Client: Guardian Roots | Services: Flood Risk Assessment; Flood Warning & Evacuation Plan
An Invalid Application, and the Scope That Resolved It
Planning application PL/2135/26 sought the conversion of an existing dwellinghouse (Use Class C3(a)) at 2 Sitwell Grove, Stanmore, into a children's home (Use Class C2). The London Borough of Harrow declared the application invalid on 14 August 2026, on the ground that the site falls within a climate change flood zone and a Flood Risk Assessment is therefore required. Working on behalf of Guardian Roots, we prepared the site-specific Flood Risk Assessment and the accompanying Flood Warning and Evacuation Plan.
An invalid application is a specific problem with a specific remedy, and it is worth being precise about both. Harrow's Infrastructure Team, acting as Lead Local Flood Authority, confirmed on 18 August 2026 that the site is categorised as medium risk for surface water flooding, and directed that the assessment be prepared in line with the Joint West London Strategic Flood Risk Assessment and Harrow Local Plan 2021-2041 Policies CN3 to CN5.
Because the proposal is a change of use only, with no extension, no increase in footprint, no additional hardstanding and no external development, the Council directed that the submission comprise flood data and emergency planning information only. We prepared the report to exactly that scope.
That distinction matters. A change of use cannot increase surface water runoff: there is no additional impermeable area, no altered drainage connection and no change to overland flow paths, so a drainage assessment would have had nothing to assess. What a change of use can do is alter who is inside the building and how vulnerable they are, which is precisely why the emergency planning element carries the weight here.
Site Characteristics and Flood Risk Profile
We drew on the Environment Agency Flood Map for Planning, the Risk of Flooding from Rivers and the Sea dataset, the current Environment Agency Surface Water Spatial Planning products including the climate change allowance, the Joint West London Strategic Flood Risk Assessment and records of historic flooding at the address.
The site lies within Flood Zone 1, an area with a low probability of flooding from rivers and the sea. The Flood Map for Planning records no Flood Zone 2 or Flood Zone 3 within the site boundary, and the Risk of Flooding from Rivers and the Sea dataset places no risk band across the site.
Surface water is the governing source. On the current Environment Agency Surface Water Spatial Planning products the site lies within the 1-in-1000-year (0.1% AEP) extent under present-day conditions over most of its area. Including the Environment Agency climate change allowance, the 1-in-100-year (1% AEP) extent covers most of the site. That shift, from a rare event today to a 1% annual probability event over the lifetime of the development, is the reason Harrow raised the requirement, and the assessment addresses it directly rather than resting on the Flood Zone 1 designation.
The overall surface water flood risk to the proposed development is assessed as medium. Tidal, fluvial, groundwater, sewer and artificial-source risks are all low, with no incident recorded at the site.
Vulnerability and the Occupancy Profile
A dwellinghouse and a children's home both fall within the More Vulnerable classification, so the proposed use does not increase the flood risk vulnerability of the development. That is a straightforward conclusion but an important one, because it removes any question of the change of use pushing the proposal into a category that would require the Exception Test in this location.
What does change is the occupancy profile. A children's home has residents who may need assistance to respond to a flood warning, and staff who need a defined procedure to follow rather than a general awareness of risk.
The Flood Warning and Evacuation Plan
The plan sets out the flood warning arrangements available at the site and the lead time attached to them, the trigger points at which the plan is activated, the responsibilities of the staff on duty at the time, the route to safe refuge, and the actions required at each Environment Agency warning code. It is issued with a one-page flood information sheet for display on site, so that the procedure is available to staff at the moment it is needed.
Outcome
Subject to the measures set out in the report, the proposed development is considered to be at an acceptable level of flood risk for its lifetime, and flood risk at the site and at surrounding sites is not increased. The submission answered the Council's stated scope precisely, resolving the validation objection without loading a change-of-use application with drainage assessment it did not require.