House Extensions in Windsor
Rear extensions, side returns, and wraparound builds on Victorian and Edwardian properties across Windsor SL4. Free written estimate. Building control coordination included.
AA Contractor carries out house extensions in Windsor from initial groundworks through to building control sign-off, covering structural calculations, trades coordination, and finishes under one contract.
Windsor’s housing stock shapes every extension project here. The Victorian terraces on St Leonard’s Road, Frances Road, and Arthur Road carry solid brick walls, narrow side passages, and original suspended timber floors. The Edwardian semis across Clewer and in Old Windsor sit on plots wide enough for a side return but tight enough to require careful structural assessment before a single slab of concrete is poured.
Newer properties in Dedworth and on the estates off Maidenhead Road bring their own set of considerations. No two projects are the same, and the planning environment in Windsor adds a layer of complexity that does not apply in most other Berkshire towns. We work through that complexity every day.
Types of House Extension in Windsor
Rear Extension
Extends the ground floor out into the back garden, typically creating a larger kitchen, an open-plan kitchen-dining space, or an additional reception room.
Under permitted development, an attached house can build a single storey rear extension up to 3 metres deep without a planning application. The Larger Home Extension Scheme (Prior Approval) allows up to 6 metres. In Windsor, conservation areas and Green Belt restrictions apply on top of these national limits.
Typical cost: £65,000–£90,000 for 20m² (excl. VAT & fees)Side Return Extension
Fills the narrow alleyway alongside Victorian and Edwardian terraces. On streets like Arthur Road and Frances Road near Windsor town centre, these passages are 1.2–1.8 metres wide and unused. A side return extension widens the back of the house by 15–20m².
Almost always requires a party wall agreement with the adjoining neighbour under the Party Wall Act 1996. We serve the required notices and manage the process.
Typical cost: £55,000–£75,000 for 12–15m² (excl. VAT & fees)Wraparound Extension
Combines a rear extension and a side return into a single L-shaped structure, giving the maximum possible ground floor space from one project.
The combined footprint almost always exceeds permitted development limits, so a full planning application to the Royal Borough of Windsor and Maidenhead is standard. Structural calculations are required at the corner junction where loads redistribute through new steelwork.
Typical cost: £95,000–£135,000 for 20–25m² (excl. VAT & fees)Double Storey Extension
Builds on two floors simultaneously. Because groundworks, foundations, and scaffolding are shared between both levels, the cost per square metre is lower than two separate single storey builds. Typically adds one or two bedrooms and a bathroom above.
Planning permission is almost always required. RBWM assesses overlooking, daylight loss, and massing as part of the application.
Typical cost: £120,000–£160,000 for 35–40m² (excl. VAT & fees)Planning Permission for House Extensions in Windsor
Planning rules for house extensions in Windsor have two layers: the national permitted development framework that applies across England, and the local restrictions imposed by the Royal Borough of Windsor and Maidenhead on top of it.
What permitted development allows
- Attached houses: rear extension up to 3m deep (or up to 6m under Prior Approval)
- Detached houses: up to 4m deep (or up to 8m under Prior Approval)
- Maximum height of 4m for single storey extensions
- Materials must match the existing house
- No extension forward of the principal elevation
- Extensions must not cover more than 50% of the garden
- Rights are removed for listed buildings entirely
Windsor-specific restrictions
- 27 conservation areas across the Royal Borough (Windsor Town Centre, Eton, Clewer Village, Inner Windsor, Trinity Place)
- Metropolitan Green Belt covers significant parts of the borough outside built-up areas
- RBWM approved only 68.4% of applications in 2025 — one of the lowest rates in England
- Article 4 Directions remove permitted development rights in certain parishes
- Listed buildings require Listed Building Consent in addition to planning permission
- Conservation areas restrict side extensions even within PD dimensions
For properties in or adjacent to a conservation area, or where permitted development status is uncertain, a pre-application enquiry to RBWM before committing to a design saves time and money. We recommend this route for any project on SL4 properties within the Windsor Town Centre or Clewer Village conservation areas.
The Lawful Development Certificate
A Lawful Development Certificate (LDC) is written confirmation from RBWM that a project was lawful under permitted development rules. It is not required before work starts. But solicitors ask for it when a homeowner comes to sell or remortgage a property that has been extended, because the LDC is the only formal document that proves the work did not need planning permission.
Applications go to RBWM and are determined within the standard 8-week statutory period. We can advise on whether your project qualifies before any work begins.
How We Manage a House Extension in Windsor
Every project starts with a site visit and a free written estimate. That estimate sets out the extension type, footprint, structural requirements, materials, programme, and trades sequence. We do not give verbal estimates over the phone without seeing the property.
Written scope and estimate
After a site visit, we produce a written scope of works covering the extension type, footprint dimensions, structural requirements, materials specification, programme of works, and expected timeline. This is not a verbal estimate on a scrap of paper. It is a document you can read, question, and compare.
Structural assessment
Most extensions on Windsor’s older housing stock require a structural assessment before design drawings are produced. We arrange this with a structural engineer, whose calculations determine the foundation type, beam sizes where load-bearing walls are altered, and structural details at roof junctions.
Properties in Dedworth and Oakley Green sit on clay soils that shrink and swell with seasonal moisture changes. We give particular attention to foundation depth and type on these sites.
Planning and building regulations
For permitted development projects, we confirm the position in writing. For projects requiring Prior Approval or a full planning application to RBWM, we manage the process. Separately, we submit a building regulations application to RBWM building control for all projects. If the project affects a shared wall, we serve the party wall notice and manage the process through to a party wall award where needed.
Groundworks and foundations
Groundworks begin with setting out, excavation, and foundation pouring. We coordinate drain runs, service diversions, and the damp proof course at this stage. Building control carries out a foundation inspection before the concrete is covered.
Structure, roof, and external finishes
Brickwork rises from the foundation to wall plate level. Where internal load-bearing walls are removed to create an open-plan layout, steel beams (RSJs) are installed and propped until the structural engineer confirms the load path is intact. Roofing follows, then windows and doors, including bifold doors where specified. External facing brick or render matches the existing house as closely as the available material allows.
Internal works and building control sign-off
First-fix electrics and plumbing run through the new structure. We fit the insulation. Plastering and skimming follow. Second-fix electrics, plumbing, and joinery complete the space. Building control carries out a final inspection and issues a completion certificate. We give the completion certificate to the homeowner for their records.
House Extension Costs in Windsor
Windsor falls within the London and South East pricing band for residential construction. National average figures carry a 20–40% uplift when applied to this area.
| Extension type | Typical footprint | Cost range — standard spec | Excl. VAT & fees |
|---|---|---|---|
| Rear extension — single storey | 20m² | £65,000 – £90,000 | ✓ |
| Side return extension | 12–15m² | £55,000 – £75,000 | ✓ |
| Wraparound extension | 20–25m² | £95,000 – £135,000 | ✓ |
| Double storey rear extension | 35–40m² | £120,000 – £160,000 | ✓ |
Figures are estimates based on current South East build rates. They include groundworks, foundations, structure, roofing, windows and doors, first and second fix, plastering, and decoration. They exclude VAT at 20%, architectural drawings, planning fees, structural engineer fees, and party wall surveyor costs.
Extending Period Properties in Windsor SL4
SL4 has a higher concentration of pre-1919 housing stock than most Berkshire postcodes. The Victorian terraces built between roughly 1870 and 1905 cluster around the streets within walking distance of Windsor and Eton Central station: St Leonard’s Road, Frances Road, Arthur Road, and St Mark’s Road. The Edwardian semis that followed spread into the streets west of Windsor town centre and into the earlier parts of Clewer. These properties share several characteristics that directly affect extension design and build.
Solid brick walls
Most pre-1919 houses in Windsor have 9-inch solid brick external walls with no insulation cavity. The junction between old and new must be detailed carefully to avoid cold bridging and damp penetration.
Suspended timber floors
Victorian and Edwardian houses typically have timber joists over a void rather than a concrete slab. The junction where the original timber floor meets the new insulated slab requires careful design to avoid differential settlement.
Narrow side passages
The side passage on a Victorian terrace in SL4 is typically 1.0–1.8 metres wide. A structural engineer’s assessment of the existing rear corner is standard practice before a side return extension is designed.
Original ceiling heights
Pre-1919 houses in Windsor typically have ground floor ceiling heights of 2.7–3.1 metres. This works in favour of extensions that need to match existing internal ceiling levels without raising the external eaves height.
Conservation area implications for Windsor SL4
Properties within Windsor Town Centre, Eton, Clewer Village, Inner Windsor, and Trinity Place conservation areas sit under RBWM’s conservation area design guidance. Extensions on rear elevations visible from a public area, including some rear gardens that back onto public paths, require materials and design treatment that respects the character of the area. Matching brick bond, mortar colour, and window reveals matter to RBWM’s conservation officers. We work within these requirements as part of every project on a conservation area property.
House Extension FAQs for Windsor Homeowners
Most single storey rear extensions in Windsor do not need a planning application. Permitted development rights allow attached houses to extend up to 3 metres deep and detached houses up to 4 metres, provided the extension meets the national criteria on height, materials, and boundary setback.
The Larger Home Extension Scheme extends these limits to 6 metres for attached houses and 8 metres for detached, subject to neighbour consultation and Prior Approval from RBWM.
Planning permission is required for extensions that exceed these limits, any extension on a listed building, extensions in Windsor’s conservation areas that affect a visible elevation, and projects where an Article 4 Direction has removed permitted development rights. A Lawful Development Certificate from RBWM confirms in writing that a project was lawful under permitted development, and is worth obtaining before any property sale or remortgage.
A single storey rear extension in Windsor costs between £65,000 and £90,000 for a 20m² footprint at standard specification, based on current South East build rates.
A side return extension runs from £55,000 to £75,000. A wraparound extension costs from £95,000 to £135,000. A double storey rear extension typically costs £120,000 to £160,000 for a 35–40m² combined footprint.
These are build costs only. Add VAT at 20%, architectural fees of £2,500 to £6,000, a planning fee of £258 for householder applications, party wall surveyor costs where applicable, and a 10–15% contingency for unforeseen site conditions.
Extensions in Windsor’s conservation areas are possible but require planning permission regardless of size or position. RBWM requires that materials, window profiles, mortar colour, and roofline treatment are sympathetic to the character of the area.
Windsor Town Centre, Eton, Clewer Village, Inner Windsor, and Trinity Place are among the 27 conservation areas in the Royal Borough where these rules apply. Extensions on rear elevations not visible from any public area may be treated more flexibly, but RBWM assesses this case by case.
Permitted development rights allow most attached houses in SL4 to build a single storey rear extension up to 3 metres deep without a planning application. Detached houses can extend up to 4 metres.
The Larger Home Extension Scheme allows up to 6 metres for attached and 8 metres for detached properties, subject to Prior Approval from RBWM and a 42-day neighbour consultation period. These rights do not apply to listed buildings, properties in conservation areas where extensions affect a visible elevation, or where an Article 4 Direction has removed them. The Royal Borough of Windsor and Maidenhead is the local planning authority for all applications in SL4.
A single storey rear extension in Windsor takes 10 to 16 weeks on site from groundworks to completion. Pre-build stages typically add 8 to 14 weeks before work begins on site.
A wraparound or double storey extension takes 18 to 26 weeks on site. Pre-build stages cover structural assessment, drawings, and planning or building regulations approval. AA Contractor provides a programme at the start of every project so homeowners know what to expect at each stage.
A party wall agreement is required when an extension involves work on or near a shared wall, or when foundations are excavated within 3 metres of a neighbouring property’s structure.
This applies to most side return and rear extensions on Victorian terraces and Edwardian semis in Windsor, where the houses sit in close proximity along streets like St Leonard’s Road and Arthur Road. The Party Wall Act 1996 requires that the written notice is served on all adjoining owners before work begins. If a neighbour does not respond or does not consent, a party wall award is prepared by a surveyor.
An extension that adds a bedroom or creates a larger kitchen-dining space reliably adds value in Windsor’s property market.
Windsor carries strong underlying demand. Its proximity to London Waterloo via Windsor and Eton Central and Windsor and Eton Riverside stations, the Royal estates, and well-regarded local schools mean properties consistently attract buyers willing to pay for well-used space. An extension that brings a two-bedroom terrace on St Mark’s Road or St Leonard’s Road to three bedrooms is typically worth more to the market than it costs to build at current South East rates.
All house extensions in Windsor require building regulations approval, regardless of whether planning permission is needed. Building regulations cover structural design, foundation specification, insulation values, ventilation, drainage, fire safety, and electrical installation.
AA Contractor submits a full plans application to RBWM building control before work begins. Inspections take place at foundation stage, damp proof course stage, structural frame completion, and final completion. RBWM building control issues a completion certificate on satisfactory final inspection. This certificate is needed when the property is sold or remortgaged.
Why Windsor Homeowners Work with AA Contractor
Residential building work across Berkshire, including period terraces and semis across Windsor SL4
Local to Windsor, Slough, Reading, Maidenhead, and the surrounding Berkshire area
Every project starts with a site visit and a written scope of works. No verbal estimates without seeing the property
Written scope of works on every project — a document setting out the footprint, structural approach, materials, programme, and trades sequence
Building control coordination handled directly — we prepare and submit the full plans application, manage inspections, and provide the completion certificate
Planning advice included — we advise on the planning route, manage RBWM applications, and serve party wall notices where needed
Fully insured — public liability and employers’ liability insurance in place for every project
Structural assessment arranged — we work with a structural engineer on every project that involves load-bearing walls or complex foundations
Period property experience — 20 years working on Victorian and Edwardian housing stock across SL4 and the wider Berkshire area
Get a Free Written Estimate for Your Windsor House Extension
To discuss a house extension in Windsor, call us or use the contact form. We carry out free site visits across SL4 and the surrounding Berkshire area, with no obligation to proceed.
Free site visit • Written scope of works • No obligation • Covering Windsor SL4 & Berkshire