House Extensions Windsor

Whether it is a single storey kitchen extension, a wraparound or a two storey addition, a house extension in Windsor 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.

For Windsor, on London-derived ranges a single-storey rear extension typically starts from £40,000, or £2,000 to £2,800 per m²; Berkshire work generally runs below that. GS Renovation Ltd, a building contractor based in Potters Bar, does the work in Windsor: the structural engineer we appoint designs the structure, we build it, and building control signs it off.

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.

Housing in Windsor

Windsor counts roughly 16,960 homes for Council Tax; 22% are older than 1919, 8% interwar and 68% post-war, with 34% flats and 66% houses. A rear extension on a terrace in Windsor digs foundations close to both neighbours, so notices to both are normal, and 30% of homes here are terraced.

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, and it is the part that shapes a design. The Windsor Neighbourhood Plan has been part of the development plan since 29 June 2021 and covers the majority of the residential areas of Windsor, excluding the town centre, the Castle, Home Park and the riverside around Alexandra Gardens. Old Windsor has its own plan, adopted on 17 December 2019. For a kitchen extension across the back of a house or a wraparound, the council reads the proposal against any area design guide in the plan that covers your street as well as the Borough Local Plan, so we check which applies before drawings are commissioned. The build itself then follows a set order: foundations to the depth the engineer sets from a trial hole, the shell made weathertight, and only then the old rear wall opened on steel, so the house stays usable for most of the programme.

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.

Kitchen extensions in Windsor

48% of the houses in Windsor are semi-detached or detached, which usually leaves room for a kitchen extension across the full rear width, with a steel beam over the opening into the house. The fit-out is priced on its own page, kitchen renovation in Windsor; the build figures are in the table above.

Side return extensions in Windsor

30% of homes in Windsor are terraced houses and 22% of all homes were built before 1919; side returns are found on the older terraces, beside the back addition. A side extension is not permitted development in a conservation area, and elsewhere is limited to one storey, 4 metres high and half the house's width, so many go to Royal Borough of Windsor and Maidenhead as an application. Our side return extension page covers the build in detail, with build rates of £2,400 to £3,200 per m².

Double storey extensions in Windsor

Under permitted development a two-storey rear extension can go no more than 3 metres beyond the original rear wall and must stay at least 7 metres from the rear boundary, and in a conservation area it is not permitted development at all. Many gardens are too short for the 7 metre rule, so a double storey extension in Windsor often needs planning permission from Royal Borough of Windsor and Maidenhead. With 32% of homes here semi-detached or detached, the side of the house is often an option too. Our double storey extension page explains the build, at £1,800 to £2,600 per m² across both floors.

• Side-return and rear extensions
• First fix and second fix plumbing and electrics throughout
• Bi-fold, sliding and French door openings to the garden

VOA Council Tax stock of properties, March 2025, for all 5 statistical areas in Windsor. Counts rounded by the VOA.

If you are weighing up house extensions in Windsor, it may help to read RSJ and steel beam installation in Windsor, house extensions in Slough, double storey extensions, party walls and extensions, house extensions across London and Party Wall Act Explained.

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.

• Programme planned around party wall notice periods before excavation near a neighbour
• Clean, tidy and reliable tradespeople
• Permitted development checked before you pay for a planning application

Checked by Abdelaaziz Saaid, founder and director of GS Renovation Ltd (Companies House no. 14611254). Structural design is by the consulting engineer appointed to your project.