Windows in London Conservation Areas 2025: Planning Rules, Double Glazing, Secondary Glazing, and Timber Sash Requirements

**The national position — what is permitted development for windows?**:

Under the Town and Country Planning (General Permitted Development) (England) Order 2015 (GPDO), as amended, certain works to existing dwellinghouses are permitted development (PD) — meaning they can proceed without a planning application. For windows and glazing, the relevant PD rights are:

• *Schedule 2, Part 1, Class A (enlargement, improvement or other alteration) — dwellinghouse*:
• •The enlargement, improvement, or other alteration of a dwellinghouse is permitted development provided it does not: create a new dwelling; exceed the volume limits; and (critically for windows) not include a window in any wall or roof slope forming a side elevation of the dwellinghouse that would be within 10.5m of the boundary if above ground floor.
• •Replacing an existing window with a new window of the same or smaller size in the same opening is generally permitted development nationally — the works do not constitute 'enlargement' of the building. However, condition A.2(d) states that the materials used in the exterior must be of a similar appearance to the materials used in the construction of the exterior of the existing house — this condition means that replacing timber sash windows with UPVC sash windows of a similar visual appearance may technically be PD, but replacing timber sash windows with UPVC windows of a significantly different profile and appearance may not be.

• *Windows: what typically IS PD nationally (outside conservation areas)*:
• •Replacing existing windows with new windows of the same size, opening type, and similar appearance
• •Installing window locks; secondary glazing; window film (no change to external appearance)
• •Installing a roof window (Velux style) in a rear roof slope of any size, provided it does not project more than 150mm from the roof plane and does not protrude above the highest part of the roof

• *Windows: what requires planning permission nationally*:
• •New windows in a blank wall (where no window previously existed)
• •New windows in a side elevation within 10.5m of the boundary (obscure-glazed if above ground floor)
• •Roof windows on a principal elevation (any elevation fronting a highway) or where projecting more than 150mm

**Article 4 Directions — how London conservation areas restrict what you can do**:

In London conservation areas, the planning authority (the London Borough) can make an Article 4 Direction under Article 4 of the GPDO. An Article 4 Direction withdraws specified permitted development rights within the designated area, requiring a planning application for works that would otherwise be PD nationally.

*What Article 4 Directions typically restrict in London conservation areas*:

London conservation areas vary in the scope of their Article 4 Directions, but the most common restrictions imposed on dwellinghouses include:

| Restriction | Common in London CAs? | Implication | |---|---|---| | Replacement of windows (material change — e.g. timber to UPVC) | **Very common** | Replacing timber sash windows with UPVC requires planning permission in most London CAs | | Replacement of windows (any replacement, including like-for-like timber) | Varies by Borough | Some London CAs require planning permission for ALL window replacements, even like-for-like timber | | Insertion of new windows in existing elevations | Very common | New window openings in any elevation require planning permission in most London CAs | | Installation of roof windows on principal elevation | Very common | Front-facing (street-facing) rooflights require planning permission in most London CAs | | Cladding or rendering the exterior | Very common | Applying render to an existing brick elevation in a CA almost always requires planning permission | | Changes to boundary walls, gates, or fences | Common | Replacing a brick or stone wall with metal railings, or vice versa, may need permission |

• *Finding out which Article 4 Directions apply to your property*:
• •Check the London Borough's local planning page — most Boroughs list their conservation areas and Article 4 Directions online
• •Search the property address on the Borough's planning portal — Article 4 Directions are noted in the constraint records
• •Contact the Borough's planning department with the property address — they will confirm which Article 4 Directions apply and whether planning permission is needed for your specific works

*The consequence of a mistake*: carrying out works that require planning permission (due to an Article 4 Direction) without permission is a breach of planning control. The Borough can issue an enforcement notice requiring you to remove the unauthorised works (the non-compliant windows) and restore the original. Timber sash windows removed and replaced with UPVC under an Article 4 Direction may need to be removed and the original style reinstated at the homeowner's cost.

**Listed buildings — stricter rules than conservation areas**:

• For listed buildings, the rules are significantly stricter than for conservation areas:
• •**All works that affect the character of a listed building** (including window replacement — even like-for-like) require Listed Building Consent from the London Borough
• •UPVC windows are almost universally refused for listed buildings — conservation officers typically require timber windows (single-glazed or slimline double-glazed) in the original pattern
• •Secondary glazing (installed on the inside of the original window, which stays in place externally) is the most commonly approved solution for adding thermal and acoustic performance to listed building windows without requiring LBC for the external fabric
• •The Listed Building Consent application must be made separately from any planning application — a separate process with a separate decision

**What conservation officers in London typically require for window replacements**:

Where planning permission IS required for window replacement in a London conservation area, the conservation officer will assess the application against the Council's conservation area appraisal and management plan. The general principle across London Boroughs is:

*The key tests that conservation officers apply to window applications in London CAs*:

1. **Material** (frame material): timber is the historically appropriate material for Victorian and Edwardian London terraces — original windows were painted softwood (commonly Scottish redwood / Scots pine or Baltic pine in Victorian construction). Conservation officers in inner London conservation areas almost universally refuse UPVC (unplasticised polyvinyl chloride) frames on front elevations, because: - UPVC profiles are significantly bulkier than timber profiles — the wider sight lines change the proportional relationship between glass and frame that is characteristic of Victorian windows - UPVC frames are not repairable — they are replaced rather than maintained; life expectancy is 20–30 years vs. 50–100+ years for maintained timber - UPVC windows have a distinctive sheen and an inability to be painted that conflicts with the material character of conservation area terraces - Most London conservation area appraisals explicitly cite traditional joinery (timber) as a positive contributor to the character and appearance of the area

2. **Profile** (frame and sash dimensions): the conservation officer typically requires the new windows to match the existing profile — specifically: - Sash width, depth, and moulding profile - Glazing bar dimensions (where glazing bars are present in the existing window) - Meeting rail dimensions - Frame depth and reveal

3. **Glazing pattern**: the conservation officer will typically require the new windows to replicate the existing glazing pattern — the number and configuration of panes, the astragal/glazing bar arrangement. Victorian London sash windows typically have: lower sash 2/2 (two panes per sash; 4 panes total) or 2/4 (6 panes total); upper sash 2/4 or 2/6. Georgian windows often have 6/6 or 12/12 patterns.

4. **Double glazing in conservation areas** — the London position:

*Can you have double-glazed windows in a London conservation area?*

Yes — subject to the following conditions that most London conservation officers apply:

• •**Slimline double-glazed units**: standard double-glazed units (16–20mm cavity; 24mm overall) create a wider sight line than single-glazed windows. In a traditional timber sash, the sash rail depth is designed for single-glazed glass (3–4mm thick). A standard double-glazed unit is 24mm thick — this requires either a deeper sash rail (visible difference from original) or a non-compliant thin-frame approach. *Slimline double-glazed units* (6–8mm cavity; 4–4mm glass; 14–16mm overall) fit within a near-original timber sash rail depth and are accepted by most London conservation officers for timber sash replacements on non-front elevations and increasingly on front elevations too in the most pragmatic Boroughs

• •**Vacuum glazing (Pilkington Spacia; LandVac)**: a newer technology — two panes of glass separated by a microscopic vacuum gap (0.1mm) held by micropillar arrays. Total unit thickness: as little as 6mm (comparable to single glazing). U-value: approximately 1.0–1.2 W/m²K (comparable to standard double glazing). Fitted within a single-glazed timber sash without any modification to the sash dimensions. Most conservation officers accept vacuum glazing as meeting both conservation requirements (same profile) and energy performance requirements

• •**Front elevation vs. rear elevation**: conservation officers are typically stricter about front (street-facing) elevations. On rear elevations (not visible from the street), most London conservation areas permit standard double-glazed units in matching timber frames

**Timber sash window replacement — the conservation area standard for London Victorian terraces**:

*What the conservation officer expects for a Victorian London terrace window replacement in a conservation area*:

| Element | Conservation standard | |---|---| | Frame material | **Painted softwood timber** (engineered timber / modified wood like Accoya accepted by most London CAs; UPVC refused on front elevations) | | Sash type | **Double-hung vertical sliding sash** (for Victorian and Edwardian terraces) — matching existing | | Glazing bars | Match original (remove glazing bars and replace with plain sash only where original had no bars) | | Glazing unit | Slimline DGU (6mm cavity; 14–16mm total); vacuum glazing; or single glazed — match original sight lines | | Meeting rail | Match original meeting rail depth — typically 38–50mm | | Horns | Joinery horns on lower sash (a Victorian feature — most London CAs require horns to be replicated on replacement sashes) | | Colour | Painted white (or original colour if documented) — not stained or bare timber | | Ironmongery | Traditional sash fasteners and sash lifts matching original pattern | | Opening method | As original — do not change from sliding sash to casement without specific conservation officer agreement |

• *Leading suppliers of conservation-area compliant timber sash windows in London (2025)*:
• •Selectaglaze (secondary glazing specialist — for listed buildings)
• •Mumford & Wood (slimline DGU timber sash)
• •Pronto Sash (Accoya timber sash)
• •The Sash Window Workshop
• •Roseview Windows (Urban Collection — conservation-area compliant slim DGU sash)

*Cost of timber sash window replacement in a London conservation area (2025)*:

| Type | Supply cost per window | Supply and install (London) | |---|---|---| | Hardwood sash (standard DGU) | £600–£1,200 | £900–£2,000 | | Accoya or engineered timber (slimline DGU) | £900–£1,600 | £1,300–£2,500 | | Vacuum glazed timber sash (Pilkington Spacia) | £1,200–£2,000 | £1,800–£3,000 | | Like-for-like single-glazed timber sash repair (rather than replacement) | £200–£400 per sash | £350–£700 |

**Secondary glazing — the conservation-area safe alternative to full window replacement**:

• Secondary glazing involves installing a new inner window pane within the existing window reveal, on the interior face of the existing (primary) window. The existing external window is left completely in place — unchanged. Secondary glazing:
• •Does NOT change the external appearance of the building in any way
• •Does NOT require planning permission (in all but the most exceptional cases)
• •Does NOT require Listed Building Consent (LBC) because the external fabric is untouched
• •Provides significant thermal and acoustic performance improvements
• •Is reversible — can be removed without any damage to the existing window

*Secondary glazing performance — thermal and acoustic*:

| Secondary glazing type | Thermal U-value improvement | Acoustic performance improvement | |---|---|---| | **Single-pane secondary glazing** (existing single-glazed primary + secondary single-glazed inner pane; typical cavity 50–100mm) | From ~4.8 W/m²K to ~2.0 W/m²K | 30–35dB Rw (good acoustic improvement from road noise) | | **Double-pane secondary glazing** (existing single-glazed primary + secondary DGU inner pane; larger cavity) | From ~4.8 W/m²K to ~1.4 W/m²K | 35–40dB Rw | | **Vacuum secondary glazing** (Selectaglaze; inner pane of vacuum glass) | From ~4.8 W/m²K to ~1.0 W/m²K | 40–45dB Rw |

*Acoustic performance context for London road-facing terraces*: London's Victorian terraces on busy roads typically experience external noise levels of 65–80dBA. A good acoustic double-glazed window might reduce this by 35–40dB — bringing internal levels to 30–40dBA. Secondary glazing with a 100mm+ cavity and 6.4mm laminated acoustic glass on the inner pane can achieve 40–45dB Rw — providing significantly better acoustic performance than standard double glazing in a normal-width sash frame, because the wider cavity delivers better low-frequency sound attenuation.

• *When secondary glazing is particularly recommended in London*:
• •Listed buildings (LBC required for any change to external window — secondary glazing avoids this)
• •Conservation area properties where the conservation officer has refused (or would likely refuse) full window replacement
• •Victorian terraces on major London roads with high traffic noise — the wider cavity of secondary glazing typically outperforms standard DGU for acoustic attenuation
• •Rentals or investment properties where cost is more sensitive than aesthetics

• *Secondary glazing costs (London 2025)*:
• •Standard aluminium frame secondary glazing (lift-out; horizontal sliding; or hinged): £150–£350 per pane (supply and install)
• •For a standard Victorian 2/2 sash window (two sash openings): £300–£700 per window installed
• •Full house secondary glazing (10 windows; 20 panes): £3,000–£7,000
• •Selectaglaze premium secondary glazing system: £400–£800 per opening (higher quality; better seals; specialist manufacturer)

**Velux rooflights (roof windows) in London conservation areas**:

*When is planning permission required for a rooflight in a conservation area?*

Nationally (outside conservation areas): roof windows (Velux-style) in a rear roof slope are PD if they do not project more than 150mm from the roof plane and do not protrude above the highest point of the roof. Planning permission is required for rooflights on a **principal elevation** (any elevation facing a highway).

• In London conservation areas:
• •**Rear roof slope rooflights**: in most London conservation areas, rear rooflights remain PD (not restricted by Article 4 Direction) — this is the most common case
• •**Front roof slope rooflights** (visible from the street): typically restricted by Article 4 Direction in London conservation areas — planning permission required. Many London conservation areas prohibit front-facing rooflights entirely, particularly where the roofscape is a key characteristic of the conservation area (such as in uniform Victorian terrace roofscapes)
• •**Flat roof rooflights (roof lanterns or flat rooflights on extension roofs)**: flat rooflights on extensions are assessed as part of the extension planning application; generally acceptable on flat roofs that are not visible from the street

• *Conservation officer guidance on rooflights in London CAs*:
• Where rooflights are permitted (on rear slopes in most London CAs), conservation officers typically require:
• •Flush or conservation-cut rooflights (Velux Conservation — sits flush with the roof tile plane rather than projecting 150mm as a standard Velux does). Conservation-cut installation involves cutting the tile/slate to sit around the rooflight rather than over the standard flashing
• •Matching tile/slate pattern maintained around the rooflight
• •Black-painted frames (not white) to blend with the tile/slate background

• *Conservation cut vs. standard Velux installation cost comparison*:
• •Standard Velux GGL/GHL installation: £400–£700 per unit (supply and install on a pitched roof)
• •Conservation-cut installation (flush with tile plane): £600–£1,000 per unit

**Practical guide — what to do as a London conservation area homeowner wanting new windows**:

*Step 1 — Check whether an Article 4 Direction applies to your property*: Contact the Borough's planning department or check their planning portal. Confirm which Article 4 Directions apply and what they restrict at your specific property address (not all properties within a CA boundary are subject to the same restrictions — some are more tightly controlled than others).

*Step 2 — Check whether your property is listed*: Search the National Heritage List for England (NHLE) at historicengland.org.uk. If listed, all external changes require Listed Building Consent — secondary glazing is the safest route.

*Step 3 — If planning permission is needed, speak with the conservation officer before submitting*: Most London Boroughs offer pre-application advice to homeowners at a cost of £150–£400. A 30-minute conversation with the conservation officer will give you a clear indication of what window design they will accept before you invest in a design package. Do not proceed to purchase windows and apply for retrospective permission — conservation officers rarely accept retrospective applications for windows as a satisfactory approach.

*Step 4 — Specify the right product*: For timber sash replacement on a Victorian London terrace in a conservation area: Accoya or engineered timber with slimline DGU (14–16mm) or vacuum glazed unit; matching sash dimensions and glazing pattern; white-painted finish.

*Step 5 — Consider secondary glazing as an alternative if full replacement is refused or not justified*: If the conservation officer refuses full window replacement (particularly on a listed building), or if the existing timber sash windows are structurally sound (worth preserving with refurbishment rather than replacement), secondary glazing provides a planning-safe solution that improves thermal and acoustic performance significantly without external change.