An extension is a footprint, a set of dimensions the council expects you to work within, a foundation and a structure. We build it, coordinate the applications and the inspections, and appoint the structural engineer for the calculations. This page sets out the local rules, the flood position and the ground conditions that shape an extension in Staines-upon-Thames.
How deep a rear extension can go depends on the house type
Appendix C of the Spelthorne Design Code sets the depth by house type. It states that up to 4 metres on detached and semi-detached properties, and 3 metres on terraced properties, are usually acceptable subject to appropriate design and use of materials. Census 2021 recorded 415 households in detached houses in the Staines ward and 210 in the Staines South ward. Which depth figure applies to you follows the house type rather than the street.
Two further limits shape the back of the house. Two storey extensions with a footprint greater than 50 per cent of the original house are likely to require particular care in design and justification. The code also sets a minimum retained garden length of 10.5 metres in Inner Suburban and Suburban areas, rising to 15 metres for three storey development. It sets a minimum back to back distance of 21 metres, rising to 30 metres for three storey development.
Side extensions have set in, set back and width rules to meet
The Spelthorne Design Code requires single storey side extensions to be set in from the plot boundary by at least 250mm, and set back from the main building line by at least 300mm. Appendix C explains that the set in allows for construction of foundations within the property boundary and avoids roofs, gutters and downpipes overhanging neighbouring properties. Subordinate multi storey side extensions must be set back from the main building line by a minimum of 1 metre.
Two more figures govern the shape. Side extensions should only exceptionally exceed two thirds of the width of the host building. Flat roofed extensions, and the height of pitched roofs nearest the property boundary, should not normally exceed 3 metres in height. The Design Code was adopted as a Supplementary Planning Document on 17 March 2026, so these are current requirements rather than draft guidance.
Flood risk decides floor levels and whether the footprint can grow at all
Spelthorne Borough Council's Strategic Flood Risk Assessment states that the section of Staines-upon-Thames which lies within the borough is almost entirely situated in a region of Flood Zone 2 and 3, medium to high probability. It gives the reason as the town's position on the banks of the River Thames. It states that of the four primary towns in Spelthorne, Staines-upon-Thames is that with the greatest risk from fluvial flooding. Surrey County Council is the Lead Local Flood Authority for the town under the Flood and Water Management Act.
Local Plan Policy E3 follows from that. Within Flood Zone 3b the footprint of a building should not be increased unless level for level floodplain compensatory storage can be provided. Finished floor levels shall not be lower than the existing and where possible should be raised, with a written justification required if raising is not possible. Sweeps Ditch matters on some plots. It heads at the junction of High Street and South Street, runs across Elmsleigh Car Park and under the railway, then passes behind Drakes Avenue and towards Knowle Green. The council maintains it as a land drain up to Laleham Road, at which point it is classed as main river.
An archaeological assessment can be required before the ground is opened
Approximately two thirds of the Staines Conservation Area is covered by an Area of High Archaeological Potential, so there is potential for the remains of earlier buildings and structures to survive beneath existing buildings and streets. Local Plan Policy PS3 requires an archaeological assessment with a planning application where development affects, or has the potential to affect, an Area of High Archaeological Potential, a County Site of Archaeological Importance or a Scheduled Monument. It applies the same requirement to all other sites over 0.4 hectares.
The council's appraisal records that the area was likely chosen as a Roman crossing point of the Thames because the subsoils are gravel rather than alluvial, providing solid foundations for bridge structures. Foundation depth for your extension is still decided by the engineer from a trial hole, not from a map. We dig one, record what is in it, and pass it on before the design is fixed.
- Levels, drainage and landscaping made good around the new build
- First fix and second fix plumbing and electrics throughout
- Scaffolding, site set-up and safe access on tight London plots
- Roof works and structural alterations at roof level
- Party Wall Act notices for excavation near a neighbour's wall
Trusted House Extensions in Staines-upon-Thames
An extension in Staines-upon-Thames is constrained before it is designed, and that is the useful part to know early. Depth, set backs, roof height, garden length and back to back distance come from the Design Code. Floor levels, footprint increase and compensatory storage come from Local Plan Policy E3 and the flood position of the address. We check all of it at the survey, so the drawing starts from what is achievable rather than being cut back after a refusal. We then appoint the structural engineer, build to the calculations issued, make the building control application and coordinate the inspections. GS Renovation Ltd is fully insured, registered in England, company number 14611254, and quotations are free and without obligation.
- Garden levels and access made good where the build meets outside
- Waste cleared as we go, not left piled in the garden
- Site left safe and secure at the end of every day
- Dust screens and floor protection in occupied homes
- Brick and mortar matched to the existing elevation
- Permitted development checked before you pay for a planning application