Phase 1 Contamination Risk Assessment — Office-to-Residential Conversion, Chertsey
We prepared a Phase 1 Preliminary Risk Assessment for the change of use at 2 Guildford Road, Chertsey — a site within a Source Protection Zone, with a historic gasworks complex less than 100 metres to the southeast and former railway sidings on the boundary.
Phase 1 Contamination Risk Assessment — Office-to-Residential Conversion in Source Protection Zone, Chertsey
Location: 2 Guildford Road, Chertsey, KT16 9BJ, Runnymede Borough Council | Client: Arkidraft Ltd | Services: Phase 1 Preliminary Risk Assessment
When a Change of Use Triggers a Full Contamination Desktop Study
Office-to-residential conversions don't always require a contamination assessment. But when the site sits within a Source Protection Zone, overlies a Principal aquifer with high groundwater vulnerability, and a historic gasworks complex lies within 100 metres, the planning authority will want to see one.
We were appointed by Arkidraft Ltd to prepare a Phase 1 PRA for 2 Guildford Road — a proposed change of use from office to residential studio flats, in accordance with the NPPF (2025) and EA's LCRM guidance (2025).
The Contamination Context
The site lies within a Source Protection Zone, overlying the Shepperton Gravel Member — a Principal superficial aquifer. The dominant off-site source is a historic gasworks complex 46–99 metres to the southeast. Former railway sidings (0.12 hectares) adjoin the boundary.
Desktop Study Methodology
The assessment drew on BGS mapping, EA aquifer and SPZ records, Groundsure Insight report (ref GS-B9F-3HU-8Q6-K7V, April 2026), historical OS mapping from the 1870s, UXO screening, and radon mapping. A Conceptual Site Model evaluated metals across contact, ingestion and inhalation pathways.
Risk Assessment Findings
Despite the proximity of the gasworks and railway land, contamination risk was classified as low across all linkages. No industrial activity had taken place on the site itself. Made Ground was unlikely. No basement construction proposed. Controlled-waters linkages carried Severe consequence but low overall risk.
Outcome
The Phase 1 PRA gave Runnymede Borough Council the basis to support the application without requiring Phase 2 intrusive investigation.