Window Replacement in London: What You Need to Know About Costs, Specs, and Planning

**Why window replacement is a Building Regulations matter**:

Replacing windows in an existing home is a notifiable building work under the Building Regulations Part L (Conservation of Fuel and Power). Any new or replacement window must meet a minimum energy performance standard.

**The Part L U-value requirement for replacement windows**:

Replacement windows must achieve a maximum whole-unit U-value of 1.4 W/m²K. The 'whole-unit' U-value is the weighted average of the frame U-value and the glazed unit U-value across the entire window area (including the frame).

• *What U-value means for glazing*:
• •Single-glazing: approximately 5.0–6.0 W/m²K — far above the 1.4 W/m²K requirement
• •Standard double-glazing (clear float, air-filled cavity): approximately 2.8–3.0 W/m²K — still above the requirement
• •Double-glazing with low-e coating and argon fill: approximately 1.1–1.4 W/m²K — meets the requirement
• •Triple-glazing: approximately 0.6–0.9 W/m²K — significantly exceeds the requirement but at higher cost

*Low-e (low emissivity) coating*: A thin metallic oxide coating applied to one surface of the inner pane of a double-glazed unit that reduces radiant heat loss through the glass. Essential for meeting the 1.4 W/m²K requirement. Standard on most replacement double-glazed units in the UK market.

*Argon fill*: The cavity between panes filled with argon gas (denser than air) rather than dry air — reduces convective heat transfer. Also standard on most quality replacement sealed units.

**FENSA and Building Control notification**:

The replacement window must be either: 1. Installed by a FENSA-registered installer — FENSA is the government-authorised competent person scheme for window and door installation. A FENSA-registered installer can self-certify that the replacement windows meet Part L, issue the homeowner with a FENSA certificate, and notify Building Control on the homeowner's behalf. 2. Notified to Local Authority Building Control before installation, with a completion certificate issued by Building Control after installation.

*Why FENSA matters*: When selling the property, solicitors will ask for evidence that any window replacements were carried out to Building Regulations standards. A FENSA certificate (which FENSA keeps on a central register) is the standard form of evidence. Window replacements without a FENSA certificate or BC notification create a conveyancing issue.

*FENSA registration check*: Homeowners can check whether a proposed installer is FENSA-registered at fensa.org.uk/consumer.

**Energy Rating (Window Energy Ratings)**:

In addition to U-values, windows are sometimes marketed with a Window Energy Rating (WER) — a letter grade (A+, A, B, C, etc.) calculated on a formula that takes into account U-value, solar heat gain, and air leakage. Part L compliance requires the U-value, not the WER — but an 'A-rated' or 'A+-rated' window typically also meets the U-value requirement.

**When planning permission is required for window replacement**:

For most houses, window replacement is permitted development — no planning application is required. However, in Conservation Areas and for listed buildings, planning rules restrict what types of replacement windows are acceptable:

**Conservation Areas**:

In a Conservation Area, window replacements are subject to planning control where they would materially affect the character or appearance of the building. In practice:

• •Replacing a timber sash window with a uPVC sash of similar proportions: typically acceptable without planning permission
• •Replacing a timber sash window with a uPVC casement (different opening style): typically requires planning permission as it changes the appearance
• •Replacing original timber windows with any window that changes the glazing bar profile, frame thickness, or division pattern: requires planning permission

Many London LPAs have issued supplementary planning documents on window replacement in Conservation Areas — these documents specify what materials, profiles, and glazing bar configurations are acceptable. Check the LPA's planning policy pages before specifying replacement windows in a Conservation Area.

*Article 4 Directions*: Some London Conservation Areas have Article 4 Directions specifically removing PD rights for window replacement, meaning all window changes require planning permission. Check the LPA's planning portal.

**Listed buildings**:

For listed buildings, every element of the original fabric has significance. Replacing original windows in a listed building requires Listed Building Consent (LBC), regardless of the window material or style chosen. LBC applications for window replacement must include detailed specifications and may require a heritage impact assessment.

*Replacement window materials for listed buildings*: The standard requirement for listed buildings is either retention of the original windows (with draught-proofing and secondary glazing) or like-for-like replacement in timber with matching profiles and glazing bars. uPVC is rarely acceptable in a listed building context.

**Secondary glazing as an alternative**:

Where planning rules prevent replacement of original windows (listed buildings, Conservation Areas where original timber sashes must be retained), secondary glazing is a common alternative. Secondary glazing is an internal panel fitted inside the reveal of the original window — it creates an internal double-glazing effect without replacing the original window.

*Secondary glazing performance*: Typically achieves a combined U-value (original window + secondary glazing) of 1.5–2.0 W/m²K — better than single glazing, slightly below the Part L 1.4 W/m²K replacement window standard, but acceptable where the primary window cannot be replaced.

*Secondary glazing cost*: £300–£600 per window (supply and fit), compared with £500–£1,500 for full replacement of a comparable timber sash window.

**Permitted development for windows — key exclusions**:

• •Flats and maisonettes: window replacement in flats is NOT permitted development and requires planning permission in some cases (check LPA)
• •Front elevation windows in Conservation Areas: typically require planning permission where PD rights have been restricted
• •Windows that enlarge the opening or move the window to a new position: these are alterations to the structure and require planning permission

**The three main window frame materials**:

**uPVC (unplasticised PVC)**:

*Characteristics*: Low maintenance (no painting), thermally broken frame (good energy performance), moderate to good acoustic performance, wide range of hardware and finishes. *Suitable for*: Non-Conservation Area properties where matching the character of the area is not a planning issue; modern or post-war buildings; rental properties where maintenance cost is a priority. *Not suitable for*: Conservation Area properties where matching original timber windows is required by planning policy; high-end new builds where the frame profiles are too bulky; properties where architectural character matters. *Cost range (supply and fit)*: £300–£800 per window (casement); £500–£1,200 per window (sash — vertical sliding or tilt-and-turn).

**Aluminium (thermally broken)**:

*Characteristics*: Very slender frame profiles (smaller sight lines than uPVC, closer to original timber in appearance); can be powder-coated in any RAL colour; excellent long-term durability; thermally broken frames meet Part L. *Suitable for*: Modern extensions, new build, Conservation Area applications where a contemporary but slender profile is required; high-end residential where appearance matters. *Not suitable for*: Listed building contexts where timber is required; budget-constrained projects. *Cost range*: £600–£1,500 per window (casement); £900–£2,500 per window (large sliding/folding).

**Softwood or hardwood timber**:

*Characteristics*: Authentic appearance matching original Victorian/Edwardian timber sashes; can be repaired and repainted; planning-compliant in Conservation Areas; thinner frame profiles possible than uPVC; good thermal performance with double-glazed units. *Suitable for*: Conservation Area properties where planning requires matching original timber; listed buildings; any property where maintaining architectural character is a priority. *Maintenance*: Timber windows require repainting every 5–10 years — ongoing maintenance cost compared with uPVC. Softwood frames (Redwood, Scots Pine) require more maintenance than hardwood (Accoya, Iroko, Oak). *Cost range*: £700–£2,000 per window (casement); £800–£2,500 per window (traditional box sash with weights).

**Typical total cost for window replacement in a London terrace**:

A standard 3-bed Victorian terrace in London typically has 10–14 windows (including front sashes, rear casements, bedroom windows, and landing window). Total cost for full window replacement:

| Frame type | Total for 12 windows (supply, fit, FENSA) | |---|---| | uPVC sash (to match existing) | £8,000–£15,000 | | Aluminium thermally broken | £12,000–£22,000 | | Timber softwood sash | £12,000–£25,000 | | Timber hardwood (Accoya) sash | £18,000–£35,000 |

*Note*: These are ranges — significantly influenced by window size, opening mechanism, hardware specification, and whether structural lintels need replacement.

**Window glazing specifications for different requirements**:

• •*Standard double glazing (4mm float / 20mm argon / 4mm low-e)*: U-value approximately 1.1–1.3 W/m²K. Meets Part L. The standard specification for most domestic replacement windows.
• •*Acoustic glazing (6.4mm laminated / 16mm cavity / 4mm float)*: U-value approximately 1.8–2.0 W/m²K (slightly above Part L). A secondary acoustic pane (6.4mm laminate) with an air cavity improves Rw (sound reduction index) from typical 32 dB to 38–42 dB. Relevant for properties near main roads, railway lines, or flight paths in London.
• •*Triple glazing (4mm / 14mm / 4mm / 14mm / 4mm)*: U-value approximately 0.6–0.8 W/m²K. Significantly better energy performance. The additional weight (approximately 40–50% heavier than double glazing) requires heavier frames and stronger hinges. Cost premium over double glazing: 20–35%. Mainstream in Scandinavia; increasingly specified in UK Passivhaus and low-energy projects.