An extension is a planning job, a building control job and a build, and they run on different clocks. We survey, appoint a structural engineer where the design needs one, coordinate the building control application and the inspections, and run the trades to a written programme. We do not produce structural calculations, and we are not architects or planning consultants. This page sets out what the Royal Borough asks for, and what the ground and the plot patterns in Windsor mean for a design.
What the Royal Borough asks for before a Windsor extension application is valid
The borough formally adopted its National and Local Validation Requirements on 5 February 2026, and published the policy document in March 2026. Under those requirements a Design and Access Statement is only required in a conservation area where the floor area of the proposals exceeds 100 square metres. A Heritage Statement is required if the property is statutorily or locally listed, is within a conservation area, or affects a heritage asset and its setting, under Borough Local Plan Policy HE1. Under Policy NR3 a tree survey compliant with BS5837:2012 is required for any tree with a diameter greater than 75mm within influencing distance of a proposed development.
Building control is a separate application and it stays with the council. The Royal Borough runs its own building control service and describes it on its website as a local, independent, impartial, not for profit service. It asks for two days' notice before work commences, one day's notice for foundation excavations and for steel beams before encasing, and seven days' notice before occupation. We make that application and book those dates against the build programme.
Which plan applies to your street depends on where in Windsor you are
The adopted Borough Local Plan 2013 to 2033 was adopted by the Royal Borough on 8 February 2022. Paragraph 3.3.1 records that approximately 82 per cent of the borough is Metropolitan Green Belt, with 27 conservation areas, over 950 listed buildings, 17 scheduled monuments including Windsor Castle, and 12 registered historic parks and gardens. Designation is therefore an early question on an extension here rather than a late one.
There is a neighbourhood plan layer as well. The Windsor Neighbourhood Plan was made part of the development plan on 29 June 2021, after a referendum in May 2021 in which 88 per cent of those voting supported it. It covers the majority of the residential areas of Windsor but excludes the town centre, the Castle, Home Park and the riverside around Alexandra Gardens. The Old Windsor Neighbourhood Plan was adopted on 17 December 2019, following a referendum in October 2019 in which 93 per cent of those voting supported it. The Central Windsor business plan, Windsor 2030, has not progressed, and the council records that the originally designated forum has expired.
Foundation depth is a different question in Dedworth than it is by the river
British Geological Survey 1:50,000 mapping shows London Clay Formation as the bedrock beneath Dedworth, including Dedworth Road, Dedworth Drive, Wolf Lane, Smiths Lane, Testwood Road, Ruddlesway, Gallys Road and Dedworth Green. The same mapping shows no superficial deposit above that clay at Wolf Lane, Testwood Road, Dedworth Green and along Ruddlesway and Smiths Lane. Alluvium appears only on the low ground at the northern ends of Smiths Lane, Gallys Road and Ruddlesway. A clay at foundation level, and any tree within influencing distance of it, are matters for the engineer to design around.
Elsewhere the picture changes again. BGS 1:50,000 mapping shows Head deposits, described as clay, silt, sand and gravel, over the Victoria Street, Frances Road, Kings Road and southern Sheet Street part of Windsor, and at the eastern end of Grove Road. Around Clewer Village church at the northern end of Mill Lane it shows Alluvium, while Mill Lane itself, Maidenhead Road and Arthur Road are mapped as Shepperton Gravel Member, and no superficial deposit is mapped at Springfield Road. Foundation depth comes from a trial hole on your plot, not from a map.
The Windsor estates set their own limits on what can be added
Appendix 2 of the Windsor Neighbourhood Plan contains seven named area design guides: Alma Road South, Bolton Avenue and Bolton Crescent, Clewer Corridor, Dedworth Drive, Gardner Cottages, Illingworth and the Laing Estate. The guide describes the Dedworth Drive neighbourhood as 1950s public housing laid out on Garden City principles, with semi-detached houses on spacious plots. The Illingworth estate is a loop off St Leonards Hill built around the 1970s, with some 80 houses all originally constructed as two storey houses with open front gardens.
Two further constraints sit outside the planning system. The deeds of individual Laing Estate properties include covenants intended to retain the estate's original design features, so the title can matter as much as the policy. Bolton Avenue and Bolton Crescent form part of the Boltons Estate, originally built in late Victorian and early Edwardian times on very large plots that have gradually been infilled. The design guide records that the Bourne Ditch runs behind Bolton Crescent and passes under Bolton Avenue, in a culvert in places and open in others. Capital works on the Bourne Ditch at Windsor were undertaken during the early 1990s to raise the standard of protection for adjacent homes to a 1 per cent annual probability flood.
- Side-return and rear extensions
- First fix and second fix plumbing and electrics throughout
- Bi-fold, sliding and French door openings to the garden
- Single and double storey extensions
- Insulation, airtightness and thermal detailing to regulations
Trusted House Extensions in Windsor
An extension carries separate approvals with a build hanging off them, and we run all of it from one point of contact. We appoint and coordinate the structural engineer and build to the calculations issued, and we never produce those calculations. We coordinate the building control application and the inspections with the Royal Borough of Windsor and Maidenhead. We also tell you at the start which validation documents an application on your property is likely to need, so that time sits in front of the build rather than inside it. GS Renovation Ltd is fully insured, registered in England, company number 14611254, and rated 4.9 from 31 reviews on MyBuilder. Quotations are free and without obligation.
- Photographs kept at each stage for your records
- Itemised pricing, so you can see what every stage costs
- Clean, tidy and reliable tradespeople
- Brick and mortar matched to the existing elevation
- Permitted development checked before you pay for a planning application
- Scaffolding and site set-up planned for tight London plots