Building Regulations for Windows in London Extensions: Part L, Part N, Part F, and Part K

**Part L 2021 whole-window U-value target for extensions in England**:

Building Regulations Approved Document L (Conservation of fuel and power), Volume 1 for dwellinghouses (the 2021 edition, in force for England from 15 June 2022), sets the following limiting fabric values for extensions:

| Element | Maximum U-value (limiting fabric standard) | |---|---| | Walls | 0.18 W/m²K | | Roof | 0.18 W/m²K | | Floor | 0.13 W/m²K | | Windows, rooflights, and glazed doors | 1.4 W/m²K |

The 1.4 W/m²K U-value is a **whole-window U-value** (Uw) — that is, the combined thermal transmittance of the glass pane AND the window frame, calculated as a weighted average. This is not the centre-pane U-value (Ug) of the glass alone (which is typically 0.6–1.0 W/m²K for modern triple-glazed or double-glazed low-E units) — the frame contributes a higher U-value than the glass in most window constructions, which brings the whole-window Uw up.

**What achieves 1.4 W/m²K Uw in practice?**

| Window specification | Typical Uw | |---|---| | Single glazed (pre-1980s) | 4.5–5.5 W/m²K | | Double glazed (non-low-E, air-fill) | 2.8–3.2 W/m²K | | Double glazed, low-E, argon-fill (standard Part L 2013 spec) | 1.4–1.8 W/m²K | | Double glazed, low-E, argon-fill, thermally broken frame (aluminium) | 1.3–1.6 W/m²K | | Double glazed, low-E, argon-fill, uPVC or timber frame | 1.2–1.5 W/m²K | | Triple glazed, low-E, krypton/argon-fill, thermally broken frame | 0.7–1.0 W/m²K |

*Minimum compliant specification for a London extension (Part L 2021)*: double-glazed, low-E coated glass, argon-filled sealed unit, in a thermally-broken aluminium, uPVC, or timber frame — achieving a whole-window Uw of 1.4 W/m²K or better. Confirm the Uw with the window supplier at the specification stage; manufacturers should be able to confirm the Uw for any window configuration.

**Window Energy Rating (WER) and BFRC certification**:

A widely used alternative to specifying Uw directly is to specify windows by Window Energy Rating (WER) — a rating scheme developed by the British Fenestration Rating Council (BFRC). The WER expresses window energy performance on a scale from A++ (best) to G (worst), taking into account not just the heat loss through the window (U-value) but also the solar gain through the glass and the air infiltration through the window frame (the net energy balance).

*Part L 2021 and WER*: Part L 2021 does not mandate a specific WER for extensions — it sets the Uw limit of 1.4 W/m²K. However, specifying a minimum WER-B or WER-A rated window is generally accepted by Building Control as evidence of compliance. A WER-C rated window typically achieves Uw ≈ 1.4–1.6 W/m²K; WER-B ≈ 1.2–1.4 W/m²K; WER-A ≈ 1.0–1.2 W/m²K.

**Replacement windows — Part L requirements for replacing existing windows**:

Where existing windows are replaced (not as part of a new extension, but as a standalone replacement project), Part L 2021 requires that the replacement windows meet the Uw ≤ 1.4 W/m²K (or better) standard — that is, single-glazed or old double-glazed windows cannot be replaced with another single-glazed window or pre-standard double-glazed window. Replacement windows must be registered on FENSA (Fenestration Self Assessment), or the homeowner must notify Building Control and obtain a completion certificate.

**Rooflights in extensions — Part L 2021 requirements**:

| Rooflight type | Typical Uw / performance | |---|---| | Part L 2021 maximum | 1.4 W/m²K (same as windows) | | Standard flat glass double-glazed rooflight | 1.1–1.3 W/m²K (compliant) | | Polycarbonate rooflight | 1.2–2.8 W/m²K depending on thickness and system — check for compliance | | Lantern rooflight (triple glazed) | 0.9–1.2 W/m²K (compliant; premium specification) | | Velux flat roof window (GGL range) | 1.0–1.4 W/m²K depending on glazing specification (T = 1.4; U4 = 1.4; U6 = 1.1) |

**Percentage of glazing limits in extensions**:

• There is a limit on the total area of glazing (windows + rooflights + glazed doors) in an extension relative to the extension floor area. Under Part L 2021 for dwellinghouse extensions:
• •The total area of glazing (windows + rooflights) in the extension should not exceed 25% of the extension floor area PLUS the area of any existing openings that are being blocked up as part of the extension project
• •Rooflights specifically: no more than 25% of the floor area of the roof in rooflights

*The practical implication*: a 20m² single-storey rear kitchen extension can have a maximum of 5m² of glazing (windows + rooflights combined) before triggering additional Part L compliance calculations. Many London kitchen extensions aim for large bifold doors + roof lantern or multiple rooflights — which can easily exceed 25%. Where glazing exceeds 25% of floor area, the extension must demonstrate compliance through a SAP/whole-dwelling energy calculation rather than simply meeting the elemental U-value targets — the calculation demonstrates that the overall energy performance of the extended dwelling still meets the target. This calculation is typically provided by the architect or a SAP assessor.

**Building Regulations Part N — safety glazing in critical locations**:

Approved Document N (Glazing — Safety in relation to impact, opening and cleaning) sets out the requirement for safety glazing in 'critical locations' — areas where the Building Regulations consider there to be a significant risk of a person falling or walking into glazing. In these locations, glazing must be of a type that, if it breaks, breaks safely and reduces the risk of serious laceration injury.

**Critical location definitions under Part N**:

Critical locations in a dwelling are defined in two categories:

• *Category A — critical locations in walls and partitions*:
• •Any glazing that has its lowest point less than 800mm above finished floor level in a building used as, or within, a dwelling
• •That is also: in a wall, partition, or door; and is wider than 250mm and taller than 250mm; and is within a zone up to 1,500mm above floor level

• *Critical location diagram for walls*:
• The critical zone is the area of glazing that is within 300mm of a door frame (and up to 1,500mm high), or within the band 0–800mm above floor level in any wall or partition. In plain terms:
• •Any glass panel in a wall or partition that extends from the floor to less than 800mm is a critical location
• •Any glass panel within 300mm of a door is a critical location up to 1,500mm high (because a person pushing the door could overreach and hit the adjacent glass)

• *Category B — critical locations in doors*:
• •Glass in a door: any glazed panel in a door or door sidelight where the lowest point of the glazing is less than 1,500mm above the floor (virtually all glazed panels in doors and door sidelights)

**Safety glass specifications — what satisfies Part N in critical locations**:

In critical locations, glazing must either: 1. **Break safely**: safety glass (toughened glass; laminated glass) — when it breaks, it breaks into small, relatively harmless pieces (toughened) or holds together (laminated) rather than shattering into large sharp shards 2. **Be robust enough not to break**: glass that is thick enough and of a specification that it will withstand impact without breaking (less commonly applicable in practice) 3. **Resist penetration**: a glazing configuration that, if the outer pane breaks, does not allow penetration — typically a laminated unit

*Types of safety glass accepted under Part N*:

| Glass type | Breakage pattern | Part N critical location | Typical use | |---|---|---|---| | Toughened glass (thermally toughened; BS EN 12150) | Shatters into small rounded fragments (dice) | Yes — Class 1 to EN 12600 | Most common safety glass in extensions; bifold door panels; frameless glazing; low-level panels | | Laminated glass (two panes with PVB interlayer; BS EN ISO 12543) | Holds together on breakage; PVB interlayer retains the fragments | Yes — typically Class 2 to EN 12600 | Overhead glazing (rooflights; skylights; glass walkways); security; rooflights above headheight | | Wired glass (glass with embedded wire mesh) | Holds together on breakage | No longer accepted as safety glass under BS EN 12150 | NOT accepted as safety glass for new installations; sometimes seen in old fire-rated glazing |

*Glass safety marking*: safety glass must be permanently marked (etched or printed) on each pane with: the glass type; the standard; and the classification. The marking is typically a small logo and letter/number in a corner of the glass. Building Control will check for safety glass markings during inspection of glazed extension panels in critical locations.

*Practical critical location check for a London kitchen extension*: 1. All glazed bifold/sliding door panels — critical location (door glass) — must be toughened or laminated safety glass 2. Any fixed glazed side panels beside the bifold doors within 300mm of the door frame and below 1,500mm — critical location (within 300mm of door) — must be toughened or laminated safety glass 3. Any floor-level glazing (glass floor panels; glass balustrades) — critical location — must be laminated (holds together if broken) 4. Any glazing at window sill height below 800mm — critical location — must be safety glass 5. Glass rooflight panels: overhead glazing — strong recommendation for laminated glass (if a rooflight breaks, a laminated panel holds together rather than raining shards into the room)

**Building Regulations Part F — trickle ventilation in new windows**:

Approved Document F (Ventilation) 2021 requires that all new windows installed in extensions (and in replacement window schemes) include trickle vents — small background ventilators in the window head that allow a controlled low flow of fresh air into the room when the window is closed. Part F trickle vent requirements for dwellinghouses:

*Equivalent area (EA) requirements per Approved Document F Volume 1 (2021)*:

| Room type | Minimum trickle vent EA | |---|---| | Habitable room (living room; dining room; bedroom; study) | 2,500 mm² EA per window | | Kitchen | 2,500 mm² EA | | Bathroom | 2,500 mm² EA | | Utility room | 2,500 mm² EA | | Wet room (en-suite without window) | Not applicable (no window) |

• *What is equivalent area (EA)?*: EA is a standardised measure of the airflow capacity of a vent. A 2,500mm² EA trickle vent is approximately 30mm × 84mm of open area — a small slot in the window head. Trickle vents should be:
• •In the window head (top of the frame) — not in the mid-frame or sill
• •Openable by the occupant from the inside
• •Draught-protected (have an internal baffle or brush to prevent cold draughts when fully open)
• •Compliant with BS EN 13141-1 for the specified EA

*Trickle vents and conservation areas / aesthetics*: trickle vents in the window head are visible on the external face of the window frame. In conservation areas or listed buildings where the window appearance is important, low-profile trickle vents are available that sit flush with the window profile. In some listed building cases, through-wall background ventilators (positioned adjacent to the window) may be an alternative to trickle vents in the window — agree with Building Control and the conservation officer before specifying.

**Building Regulations Part K — protection from falling through openable windows**:

Approved Document K (Protection from falling, collision and impact) sets out requirements for guarding at windows in buildings used as dwellings. The key requirement relevant to extensions is:

• *Opening restrictors at windows above floor level*:
• •Where a window can be opened to provide an unguarded opening (that is, there is no window guard/balustrade preventing a person from falling through the opening), and the window sill is 800mm or less above the floor, AND the distance from the opening to the ground level below is 600mm or more — the window opening must be restricted
• •The restriction means that the window cannot be opened beyond a width sufficient to allow a small child to fall through: typically a maximum 100mm opening gap is the accepted standard for child safety opening restriction at upper-floor windows
• •The restrictor must be releasable from the inside (for fire escape — this links to Part B escape window requirements — see `fire-safety-extension-guide`) but not easily operable by a small child

*Practical Part K implications for London extension windows*:

| Location | Part K requirement | |---|---| | Ground-floor extension windows | No opening restrictor required (less than 600mm fall distance in most cases) | | First-floor extension windows (where sill below 800mm) | Opening restrictor required if opening could allow child to fall | | Rooflights in extension (flat roof with access) | Guarding required if the rooflight provides access to a fall | | Juliet balcony glazing | Must meet balustrade standards (height; loading) per Part K |

*Child safety opening restrictor specification*: a simple handle-operated restrictor (like a window restrictor arm; cable restrictor; chain restrictor) that limits the opening gap to 100mm in the child-safe mode and is releasable by an adult (typically by pressing a button or using a coin) for cleaning and emergency escape. Cost: approximately £15–40 per restrictor. Must be installed on all sashes of a casement or tilt-and-turn window that could provide an unguarded opening at first-floor level with a sill below 800mm above floor level.

**Window frame material options for London extensions — uPVC vs aluminium vs timber**:

• *uPVC windows*:
• •Most affordable frame material; typical whole-window Uw: 1.2–1.6 W/m²K (easily meets Part L 2021 with double-low-E/argon-fill glazing)
• •Maintenance-free; will not rot, rust, or require repainting
• •Limited colour options compared with aluminium (standard: white; cream; grey; anthracite grey — most popular for contemporary London extensions; mahogany woodgrain)
• •Typical lifespan: 20–35 years before degradation of seals and profile appearance
• •Planning in conservation areas: uPVC windows are rarely approved on the principal (street-facing) elevation in London conservation areas — the profiles are considered too different from traditional timber sash profiles
• •Cost for a standard London terrace extension (3 windows + bifold doors): uPVC windows approximately £3,000–6,000 supply only

• *Aluminium windows (thermally broken)*:
• •Most popular frame material for contemporary London extensions in 2025 — slim sight lines; wide colour choice (powder-coated in any RAL colour); sharp modern aesthetic
• •Thermally broken aluminium: must have a thermal break (a continuous strip of low-conductivity material, typically polyamide, separating the inner and outer aluminium profiles) — non-thermally-broken aluminium does not meet Part L 2021 and is not acceptable in extensions
• •Typical whole-window Uw: 1.2–1.5 W/m²K with double low-E argon-fill glazing (thermally broken frame)
• •Slim profile available: aluminium sliding doors and sliding windows can be specified with 50–80mm frame widths — significantly slimmer than uPVC (typically 90–120mm)
• •Cost for a standard London terrace extension: thermally broken aluminium windows and bifold/sliding doors approximately £6,000–15,000 supply only (dependent on size and system)

• *Timber windows*:
• •Traditional appearance; appropriate for conservation areas and listed buildings where timber sash windows are required
• •Excellent thermal performance with triple-glazed units
• •Requires maintenance (painting or oiling every 5–7 years)
• •Hardwood (oak; meranti; sapele) or softwood (redwood pine; Accoya — modified softwood) options
• •Cost: timber sash windows for a London terrace extension: £800–1,500 per window (supply only)

**Planning requirements for windows in London conservation areas**:

*Article 4 Directions and conservation area restrictions on window replacement*:

Many London Borough conservation areas have Article 4 Directions that remove the permitted development right to replace windows on principal elevations. This means that replacing a timber sash window on the front elevation of a Victorian terrace in a conservation area (even a like-for-like replacement) may require planning permission.

• *Common conservation area planning policies for windows in London*:
• •Front elevation (principal elevation): original or period-appropriate timber sash windows are typically required to be retained or like-for-like replaced; uPVC replacement sash windows are almost universally refused in conservation areas; aluminium sliding sash profile windows are sometimes accepted but case-specific
• •Rear elevation extensions: more flexibility is typically allowed — aluminium casement or fixed-frame windows in contemporary extension design are usually acceptable on rear elevations in conservation areas; prior discussion with the conservation officer is advisable
• •Secondary glazing: in conservation areas where the original sash windows on front elevations must be retained but the client wants improved thermal performance, secondary glazing (an inner frame with a second pane of glass fitted on the inside face of the original window) is the accepted approach. Secondary glazing achieves whole-system Uw of approximately 1.8–2.5 W/m²K (not as good as new double-glazed windows, but Building Control typically accepts secondary glazing as a reasonable measure in listed and conservation area contexts where replacement is not appropriate)
• •Listed buildings: replacement windows in a listed building require Listed Building Consent. Original windows must typically be retained and repaired in preference to replacement. Any new window (including in a new extension) on a listed building should be specified and agreed with the conservation officer and the LPA before ordering