Flood Risk and Extensions: London Data by Borough
Over 300,000 London properties sit in flood risk zones. If yours is one of them, extending gets more complicated and more expensive. We mapped Environment Agency flood data against London boroughs to show where flood risk hits hardest.
TL;DR
If your property is in flood zone 2 or 3, you'll need a Flood Risk Assessment before extending. This costs £750-2,500 and adds 2-4 weeks to the planning timeline. Your extension must incorporate flood-resilient construction: raised floor levels, water-resistant materials below the flood level, non-return valves on drainage, and sometimes flood barriers. These measures add £5,000-15,000 to the build cost. Permitted development rights are not affected by flood risk, but Building Regulations require flood resilience measures regardless.
15%
London in flood zone 2+
Environment Agency
£750-2,500
Flood Risk Assessment cost
Industry average
300mm
Typical raised floor level
EA standing advice
£5-15k
Flood-resilient construction cost
Mayfair Studio cost model
Sources: Environment Agency flood map data, planning.data.gov.uk, Mayfair Studio cost model
Flood Zones Explained
The Environment Agency classifies all land in England into flood zones based on the probability of river and sea flooding, ignoring the effect of existing flood defences:
- Zone 1:Low probability. Less than 1 in 1,000 annual chance of flooding. No restrictions on extensions. Most of London falls in this zone.
- Zone 2:Medium probability. Between 1 in 100 and 1 in 1,000 annual chance. Flood Risk Assessment required for extensions over a certain size. Householder extensions are generally acceptable with appropriate resilience measures.
- Zone 3a:High probability. Greater than 1 in 100 annual chance. FRA required. Extensions must pass the sequential test (demonstrate they can't be located elsewhere). More onerous design requirements.
- Zone 3b:Functional floodplain. Land where water has to flow or be stored in times of flood. Extensions are generally refused unless they are water-compatible development.
London also faces surface water flood risk (from heavy rainfall overwhelming drains), which is separate from the river/sea flood zones. Surface water flooding is increasingly common and can affect properties well outside the mapped flood zones.
Flood Risk by London Borough
Using Environment Agency flood map data and planning.data.gov.uk flood risk zone datasets, we mapped which London boroughs have the most area in medium and high probability flood zones. The Thames and its tributaries (Lea, Wandle, Brent, Ravensbourne) are the primary drivers.
| Borough | Flood zone 2+ area | Main river risk | Surface water risk | Impact on extensions |
|---|---|---|---|---|
| Richmond | High | Thames | Moderate | FRA required for many properties |
| Kingston | High | Thames, Hogsmill | Moderate | FRA required for riverside areas |
| Hammersmith & Fulham | High | Thames | Low-moderate | Riverside properties affected |
| Greenwich | High | Thames | Moderate | Thames Barrier protects but FRA still required |
| Tower Hamlets | High | Thames, Lea | Moderate | Docklands area heavily affected |
| Wandsworth | Moderate | Thames, Wandle | Moderate | Riverside and Wandle corridor |
| Hounslow | Moderate | Thames, Crane | Moderate | Western areas near Crane valley |
| Lewisham | Moderate | Ravensbourne | High | Surface water a major issue |
| Southwark | Moderate | Thames | Moderate | Northern riverside areas |
| Merton | Moderate | Wandle | Moderate | Wandle corridor properties |
| Barking & Dagenham | Moderate | Thames, Roding | Moderate | Thames-side industrial conversion areas |
| Newham | Moderate | Thames, Lea | Moderate | Lea valley and docklands |
| Lambeth | Low-moderate | Thames | Moderate | Riverside only |
| Enfield | Low-moderate | Lea, Turkey Brook | Moderate | Eastern areas near Lea valley |
| Camden | Low | Fleet (culverted) | High | Surface water, not river flood zones |
| Islington | Low | None significant | Moderate | Minimal river flood risk |
| Hackney | Low | Lea (eastern edge) | Moderate | Eastern boundary only |
| Barnet | Low | Dollis Brook | Low | Very localised |
| Harrow | Low | None significant | Low | Minimal flood risk |
| Bromley | Low | Ravensbourne | Low-moderate | Very localised corridor |
Source: Environment Agency flood map data, planning.data.gov.uk • Data as of March 2026
Table shows boroughs with notable flood risk. Boroughs not listed have minimal flood zone 2+ areas. Surface water risk is assessed separately and can affect any borough.
How Flood Risk Affects Planning Permission
Flood risk does not remove permitted development rights. You can still extend under PD in a flood zone, subject to the usual size and position limits. However, if you need planning permission for any reason (conservation area, Article 4, or exceeding PD limits), flood risk adds extra requirements:
- 1.Flood Risk Assessment (FRA). Required for all planning applications in flood zones 2 and 3, and for sites larger than 1 hectare in flood zone 1. For householder extensions, a site-specific FRA typically costs £750-1,500. More complex sites (near watercourses, in zone 3) can cost £1,500-2,500.
- 2.Sequential test. In flood zone 3, the local authority must be satisfied the development cannot reasonably be located in a lower-risk zone. For householder extensions, this is usually straightforward since you can't move your house, but it still adds administrative time.
- 3.Environment Agency consultation. The planning authority must consult the EA for applications in flood zone 3. The EA has 21 days to respond. This can extend the planning timeline by 3-4 weeks if the EA requests further information.
- 4.Conditions. Even if permission is granted, it will likely come with conditions requiring flood-resilient construction, finished floor levels above the predicted flood level, and sustainable drainage (SuDS).
Extra Costs: Flood-Resilient Construction
Even under permitted development (where no FRA is needed for planning), Building Regulations still apply. If your property has flooded before or is in a known flood risk area, Building Control may require flood resilience measures. Here's what they cost:
| Measure | What it involves | Cost | When required |
|---|---|---|---|
| Flood Risk Assessment | Site-specific report by qualified assessor | £750-2,500 | Planning apps in zones 2/3 |
| Raised floor level | Floor raised 150-300mm above predicted flood level | £1,500-3,500 | Zones 2/3 or EA recommendation |
| Water-resistant materials | Closed-cell insulation, cement render, ceramic tiles below flood level | £1,000-3,000 | Zones 2/3 |
| Non-return valves | On all drainage connections to prevent backflow | £300-800 | Zones 2/3 and surface water risk areas |
| Sump pump | Automatic pump to remove any water ingress | £800-2,000 | Zone 3 or history of flooding |
| Flood barriers | Removable barriers for door/window openings | £500-1,500 per opening | Zone 3 or EA recommendation |
| Electrical rewiring | Sockets and consumer unit raised above flood level | £500-1,500 | Zones 2/3 |
| SuDS drainage | Sustainable drainage: soakaways, permeable paving | £1,500-4,000 | Most extensions (not flood-specific) |
Source: Mayfair Studio cost model, CIRIA flood resilience guidance • Data as of 2024/25 rates
Total additional cost for a comprehensive flood-resilient extension: £5,000-15,000 depending on flood zone, predicted flood depth, and specification level. The FRA cost is in addition to the build cost uplift.
Timeline Impact
Flood risk adds time at two stages: planning and construction. Here's how the timeline changes:
| Stage | Standard (no flood risk) | With flood risk (zone 2/3) | Extra time |
|---|---|---|---|
| Pre-planning research | 1-2 weeks | 2-4 weeks | +1-2 weeks (FRA commissioning) |
| Planning application | 8-12 weeks | 10-16 weeks | +2-4 weeks (EA consultation) |
| Building Regs approval | 2-4 weeks | 3-5 weeks | +1 week (flood measures review) |
| Construction | 10-16 weeks | 12-18 weeks | +2 weeks (raised floor, specialist materials) |
| Total project | 21-34 weeks | 27-43 weeks | +6-9 weeks |
Source: Mayfair Studio project data • Data as of 2024-2026
Surface Water Flood Risk: The Hidden Problem
Surface water flooding (from overwhelmed drains during heavy rain) is increasingly common in London and doesn't follow the same flood zone classifications. Properties well outside flood zones 2 and 3 can still be at significant surface water risk.
For extensions, surface water risk matters because: (a) your extension adds impermeable area, increasing runoff from your site, and (b) Building Regulations increasingly require sustainable drainage (SuDS) to compensate. Most London boroughs now require SuDS for any extension that adds more than 5m² of hard surface.
Typical SuDS solutions for residential extensions include permeable paving (£80-120/m²), soakaways (£800-2,000), rain gardens (£1,000-3,000), and green roofs (£80-150/m²). These costs apply even in flood zone 1 if the borough has adopted a SuDS policy.
Frequently Asked Questions
Can I extend my house if it's in a flood zone?
Yes, in almost all cases. Flood zones 1 and 2 rarely prevent extensions. Flood zone 3a requires a Flood Risk Assessment and may require flood-resilient construction, but householder extensions are generally acceptable. Only flood zone 3b (functional floodplain) is likely to prevent an extension entirely.
Does flood risk affect permitted development rights?
No. Your permitted development rights are not affected by flood zone classification. You can build the same size extension under PD whether you're in flood zone 1 or flood zone 3. However, Building Regulations still require flood resilience measures, and you may need a Flood Risk Assessment for a planning application if your extension exceeds PD limits.
How much does a Flood Risk Assessment cost for an extension?
A site-specific Flood Risk Assessment for a householder extension typically costs £750-1,500. More complex sites near watercourses or in flood zone 3 can cost £1,500-2,500. The assessment must be prepared by a qualified professional and submitted with your planning application.
Which London boroughs have the most flood risk?
Richmond, Kingston, Hammersmith & Fulham, and Greenwich have the most area in flood zones 2 and 3, primarily due to the Thames. Tower Hamlets and Newham are also significantly affected along the Thames and Lea. For surface water flooding, Lewisham and Camden are particularly vulnerable despite having less river flood risk.
Will flood risk affect my extension's insurance?
Potentially. If your extension is in a flood risk area, your buildings insurance premium may increase. The government's Flood Re scheme covers most residential properties built before 2009, but extensions to properties in high-risk areas can affect premiums. Check with your insurer before starting work.
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