Joinery Specification for London Extensions and Refurbishments: Windows, Doors, Stairs and Fitted Items

**Window types for London extensions**:

The most common window types used in London residential extensions are:

• *1. uPVC windows*:
• uPVC (unplasticised polyvinyl chloride) windows are the most commonly installed window type for London extensions where cost is the primary driver. They are:
• •Lowest cost of the main material options
• •Good thermal performance (modern uPVC frames with multi-chamber profile achieve frame U-values of 1.2–1.4 W/m²K)
• •Low maintenance (do not require painting)
• •Less aesthetically refined than timber or aluminium for premium projects
• •Available in white (standard), grey, anthracite, and other RAL colours (coloured uPVC is more expensive; light grey and anthracite grey are popular for London extensions)
• •Not suitable for conservation areas or listed buildings where the LPA requires timber frames

• *2. Aluminium windows (powder-coated aluminium)*:
• Aluminium windows are increasingly the preferred specification for quality London residential extensions:
• •Slimmer sightlines than uPVC (the aluminium extrusion is structurally stronger for its cross-section, so the visible frame elements can be narrower — typically 55–75mm visible frame face vs. 90–120mm for uPVC)
• •Higher-quality aesthetic — particularly with the slim-profile thermally broken aluminium systems from manufacturers such as Smart Alusion, Velfac, Schüco, Origin, and Reynaers
• •Better long-term durability and colour retention (powder-coat finish is more stable than coloured uPVC over time)
• •Thermally broken (required for Part L compliance — U-value ≤ 1.6 W/m²K for the window overall)
• •Higher cost than uPVC: typically 40–80% more per window unit
• •Suitable for conservation area applications where aluminium is accepted (some LPAs accept slim aluminium as an alternative to timber in conservation areas; check before specifying)

• *3. Timber windows*:
• Timber windows are required in some conservation area applications and for listed building work. Also preferred for some high-specification and vernacular-style projects.
• •Most appropriate for conservation areas and listed buildings
• •Highest quality aesthetic (particularly for sash windows, which should always be timber for listed building and many conservation area situations)
• •Hardwood (Accoya, sapele, iroko) is durable and requires painting every 5–7 years; softwood requires more frequent repainting
• •More expensive than aluminium for equivalent performance
• •Good thermal performance available with slim double-glazed units and thermally broken frames

**Glazing specification for London extensions**:

*Double glazing (standard)*: Thermally broken double-glazed unit, typically 28mm overall thickness (4–20–4 with argon fill). U-value of the glass unit: 1.1 W/m²K with low-E coating; overall window U-value (glass + frame): typically 1.4–1.6 W/m²K. This is the minimum acceptable standard for Part L compliance (overall window U-value ≤ 1.6 W/m²K).

*Triple glazing (premium)*: Increasingly specified for high-performance projects and north-facing windows where heat loss is a greater concern. Typical triple-glazed unit: 44mm overall (4–16–4–16–4 with argon fill); U-value of glass unit: 0.5–0.7 W/m²K; overall window U-value typically 0.8–1.2 W/m²K. Higher cost (typically 30–50% more per unit than equivalent double glazing) with diminishing marginal performance return compared to a well-insulated triple-glazed unit. In London's relatively mild climate, triple glazing payback period on energy savings is long.

*Solar control glazing*: Where south- or west-facing extensions have large glazed areas, solar control glass (low solar gain coating) should be specified to prevent overheating in summer. Building Regulations Part O (Overheating) now requires overheating analysis for many residential extensions — large south-facing glazing without solar control is a common Part O compliance failure.

**Bi-fold and sliding doors for London extensions**:

Bi-fold doors and large sliding doors (also called 'lift and slide' or 'patio slide' doors) are ubiquitous in London rear extensions — they provide the large opening between indoor and outdoor that is one of the most desirable features of a modern extension.

• *Bi-fold doors*:
• •Panel widths typically 700–1,000mm; total opening widths from 2.1m to 7.2m (or more, with wider sets)
• •Bottom-track or top-hung (top-hung eliminates the bottom track trip hazard but requires more structural support above)
• •Thermally broken aluminium is the standard material for quality bi-fold doors
• •U-value overall: typically 1.4–1.6 W/m²K
• •Cost: typical 3.0m wide 3-panel bi-fold in thermally broken aluminium: £2,500–£4,500 supply; larger widths proportionally more
• •Lead time: 4–12 weeks from order to delivery — must be ordered early in the programme
• •Threshold: critical detail (see design review guide) — confirm level threshold vs. stepped threshold, threshold drainage, and weather seal type

• *Sliding doors (lift and slide, patio slide)*:
• •Increasingly popular over bi-fold in quality London extensions — the glass panel area is larger relative to the frame area (fewer frame elements); they don't require internal or external space for the panels to fold into
• •Typical widths: 2-panel slide (one fixed, one sliding): 2.1–6.0m; multi-panel slide: up to 12m+
• •Higher quality aesthetic (typically) than bi-fold for premium projects
• •Manufacturers: Schüco, Reynaers, Origin (premium); Eurocell (mid-market)
• •Cost: typically 10–30% more than equivalent bi-fold configuration

**Rooflights and roof lanterns**:

*Fixed rooflights (e.g., Velux FCM, Fakro FF, or flat-glass roof window)*: Fixed rooflights flush-mounted in a flat or low-pitch roof extension. Standard double-glazed thermally broken units. Velux and Fakro are the market-standard brands with wide availability.

*Roof lanterns*: A roof lantern (also called a skylight lantern) is a glazed pyramidal or ridge structure that sits above the flat roof level, providing overhead natural light and a sense of volume. Popular for single-storey rear extensions where the ceiling height is relatively low. Typical sizes: 1.0×1.0m to 2.5×3.0m. Costs: standard aluminium roof lantern (Korniche, Ultrasky brands) £800–£2,500 supply; premium (Velux modular skylight, Glazing Vision) £3,000–£8,000+ supply. Roof lantern installation requires a structural upstand and waterproof junction — allow £500–£1,500 for structural upstand and waterproofing in addition to the lantern supply cost.

**Internal doors for London extensions and refurbishments**:

Internal doors are one of the most visible finish elements of a refurbishment — and a common area where clients either underspecify (hollow-core flush doors that feel cheap) or overspecify (heavy solid hardwood doors in a mid-market property).

• *Standard (hollow core flush doors)*:
• •MDF-faced hollow-core construction; typically 44mm thick; available in painted white (primed) or oak veneer
• •Cost: £50–£120 supply; standard liners/frame set £80–£150; ironmongery separate
• •Suitable for: internal rooms (bedrooms, bathrooms, utility) where durability and acoustic performance are not a priority
• •NOT suitable for: entrance doors; fire doors (must be FD30 solid-core leaf)

• *Solid core flush doors*:
• •Solid core (typically fire-grade solid core) MDF or hardwood-faced; typically 44mm thick
• •Better acoustic performance and durability than hollow-core
• •Required for fire doors (FD30 — 44mm solid core leaf with intumescent strips; FD30S with cold smoke brush)
• •Cost: solid core FD30 door £80–£200 supply; with frame/liner £120–£250
• •Suitable for: loft conversion escape route doors (all required to be FD30); HMO bedrooms; anywhere requiring acoustic separation or fire performance

• *Panel doors (shaker, 4-panel, 2-panel)*:
• •MDF or solid timber panel construction; contemporary shaker style (flat recessed panel) is the most popular in London residential refurbishments
• •Available painted (primed) or prefinished (factory painted or stained)
• •Cost: standard MDF shaker door £90–£180 supply; solid timber £150–£350 supply
• •Suitable for: all internal rooms; provides a higher-quality aesthetic than plain flush doors at modest additional cost

**Fire door requirements for London residential projects**:

• Fire door (FD30 / FD30S) requirements in UK residential construction:
• •New loft conversions: all doors on the protected staircase must be FD30 (fire resistant for 30 minutes) — this includes the room doors on each landing level
• •Houses extended to 3 storeys: similar rules apply — the staircase becomes a protected route and all doors opening onto it must be FD30
• •HMOs: all bedroom doors and kitchen door (opening onto the escape route) must be FD30S (FD30 with cold smoke strip)
• •Any house with an attached garage: the door between garage and house must be FD30

FD30 fire doors must: be a certified 44mm solid-core leaf; have intumescent strips in the frame rebates; have a self-closer (overhead door closer); have a maximum 3mm gap at sides and top and 8mm at the bottom (before intumescent strip compression). FD30S additionally requires a cold smoke brush seal.

**Staircases for London loft conversions**:

The staircase for a loft conversion is often the most challenging and most expensive joinery item in the project — particularly in London Victorian terraces where the available floor area at first-floor level for a new staircase opening is constrained.

• *Standard straight-flight staircase*:
• •The cheapest and most straightforward option where the floor plan allows
• •Standard softwood (pine) riser-and-tread closed staircase with softwood newels and MDF handrail: £2,500–£5,000 supply and install
• •Oak or ash closed staircase with hardwood newels and handrail: £4,500–£9,000 supply and install
• •Design requirements: maximum 42° pitch; minimum 800mm clear width; 2.0m headroom at pitch line; minimum 1.9m at lowest obstruction

• *Quarter-turn and half-turn staircases*:
• •Required where there is insufficient run for a straight flight — a quarter-turn (L-shaped) or half-turn (U-shaped with an intermediate landing) staircase uses a shorter horizontal run by incorporating a turn
• •Typically 20–40% more expensive than a straight flight
• •Quarter-turn oak staircase: typically £6,000–£12,000 supply and install

• *Space-saving staircases (alternating tread, paddle stair)*:
• •Used only where the available plan simply cannot accommodate a conventional staircase at 42° or less
• •An alternating tread stair (where left and right treads alternate so that each full tread is only half the width — allowing a steeper effective pitch while the actual tread depth is compliant at each individual foot placement) can fit a much tighter floor plan
• •Alternating tread stairs are permitted under Part K of Building Regulations only for access to a single room in a loft and only where a conventional stair is not possible
• •Cost: typically £3,000–£8,000 supply and install

**Glass balustrades and open-tread staircases**:

Glass panel balustrades are increasingly specified in London loft conversions and contemporary refurbishments — they maintain sightlines and visual connection through the staircase and landing and are especially effective in constrainted Victorian terrace plans where an open-tread stair with glass balustrade makes the staircase feel lighter and less space-consuming.

• •Toughened safety glass (6.4mm toughened glass with PVB interlayer — laminated safety glass) is required by Part K for balustrades up to 1000mm high
• •Frameless glass balustrade (glass panels fixed at base only, with minimal or no top rail): most architectural aesthetic but highest cost — typically £400–£700/m run supply and install
• •Semi-frameless (glass panels in aluminium channel at base + top rail): £250–£400/m run
• •Standard timber balustrade (spindles, handrail, newels): £180–£300/m run

**Fitted joinery (wardrobes, storage, shelving)**:

• Fitted joinery — wardrobes, bookshelves, alcove cupboards — is a client-supplied item under standard RCB contract terms. RCB can recommend and coordinate fitted joinery suppliers or subcontractors, but the cost is not included in a standard extension or loft conversion price unless specifically requested and separately quoted. Typical fitted joinery costs:
• •Standard fitted wardrobe (sliding door, laminate interior): £800–£2,500 depending on size
• •Fitted alcove shelving and cupboard unit (carpentry only): £500–£1,500
• •Bespoke fitted library / media unit (MDF, painted): £1,500–£5,000+

**Why joinery lead times matter for London extension programmes**:

Joinery items — particularly external windows, bi-fold or sliding doors, and staircases — have the longest lead times of any element in a London residential extension. Getting the joinery procurement programme wrong is one of the most common causes of programme delays on London extension projects. The key joinery items and their typical lead times (2025):

| Item | Typical lead time from order | |---|---| | uPVC windows (standard sizes) | 2–4 weeks | | Aluminium windows (standard range) | 4–8 weeks | | Aluminium windows (bespoke/non-standard) | 8–14 weeks | | Timber windows (softwood, standard sizes) | 4–8 weeks | | Timber windows (bespoke hardwood) | 8–16 weeks | | Bi-fold doors (standard) | 4–8 weeks | | Bi-fold doors (large format / bespoke) | 8–14 weeks | | Lift-and-slide doors | 8–16 weeks | | Roof lanterns (standard sizes, stock) | 2–4 weeks | | Roof lanterns (bespoke) | 6–12 weeks | | Velux rooflights (standard sizes) | 1–2 weeks | | Standard softwood staircase | 2–4 weeks | | Bespoke oak staircase | 6–12 weeks | | Glass balustrade panels (bespoke) | 4–8 weeks |

**The critical implication for the construction programme**:

For most London rear extension programmes where the on-site construction period is 10–16 weeks, this means that bi-fold doors and bespoke windows must be ordered BEFORE the construction programme starts — not after the frame is built. The contractor cannot afford to wait for windows to arrive before progressing the roof and weatherproofing. The sequence is:

1. Finalise window and door specification (sizes confirmed from structural drawings) 2. Place window/door order: Week 0 (construction start) 3. Begin construction: Week 0–4 (foundations, frame) 4. Structural frame complete: Week 4–6 (extension frame ready to receive windows) 5. Windows/bi-fold doors delivered: Week 4–8 (timed to arrive when frame is ready)

If the windows are ordered in Week 4 when the frame is up, and they have an 8-week lead time, the programme is delayed to Week 12 before the extension can be made weathertight — adding unnecessary time and cost. Early ordering (coordinating with the structural programme) prevents this.

**Who is responsible for the joinery specification in a design-and-build arrangement**:

• In an RCB Design & Build contract, the joinery specification (material, sizes, manufacturer, finish colour, hardware) is agreed with the client during the pre-construction phase — typically as part of the detailed specification meeting held before works start. RCB's pre-construction team will:
• •Present the recommended joinery specification for the project
• •Provide supply-only and supply-and-install costs for each element
• •Order items with long lead times early in the programme
• •Coordinate delivery with the construction programme to avoid delays

The client is responsible for: selecting the specific internal door style and finish; the staircase style and material; second-fix items (door handles, hinges, escutcheons); and fitted joinery (wardrobes, fitted storage). These should be decided and confirmed to RCB before the pre-construction phase is complete, so that any long-lead items (bespoke staircase; bespoke fitted wardrobes) can be ordered in time.