Flood risk in SL9 (Chalfont St Peter)
Of the 869 postcodes in SL9 – covering Chalfont St Peter and Gerrards Cross – 4 (0.5%) have their centre in Flood Zone 3 and 17 (2%) in Flood Zone 2, on the Environment Agency’s Flood Map for Planning.
The Zone 3 share is well below the England average of 5.6%, and below the Buckinghamshire figure of 1.1%.
Most of the district is in Flood Zone 1, but the postcodes in Zones 2 and 3 tend to cluster along rivers, streams and low-lying ground – so it’s worth checking the exact postcode rather than relying on the district picture.
Flood zones only cover rivers and the sea, and ignore flood defences. Surface water from heavy rain can affect any street, including in Zone 1.
Flood zones by postcode sector
Each sector is the first part of the postcode plus the first digit after the space. SL9 9 has the highest share in Flood Zone 3 (1.8% of its postcodes).
| Sector | Postcodes | Zone 3 | Zone 2 | Breakdown |
|---|---|---|---|---|
| SL9 0 | 215 | 0% | 0% | |
| SL9 1 | 14 | 0% | 0% | |
| SL9 7 | 179 | 0% | 0% | |
| SL9 8 | 234 | 0% | 0% | |
| SL9 9 | 227 | 1.8% | 7.5% |
Main rivers in Buckinghamshire
River Thames – Marlow and the southern boundary
River Great Ouse – Buckingham, Newport Pagnell and Olney
River Wye – High Wycombe and Bourne End
More on the area in our Buckinghamshire flood risk guide.
How these figures are worked out
We took the centre point of every live postcode in SL9 from OS Code-Point Open and tested it against the Environment Agency’s Flood Map for Planning, as published in the flood-risk-zone dataset (version of 5 November 2026). A postcode counts once, whether it has one home or forty, and a postcode whose centre is just outside a zone still counts as Zone 1. Updated November 2026.
For one address, the free postcode check looks at the zones around the postcode, and the full report adds surface water, reservoirs, recorded floods and defences.
