Windows and Doors in London Renovations: Timber Sash, Steel, and Planning Constraints

Windows and external doors are both a technical and a planning decision. They determine how light enters the building, how the facade reads from the street, and how much heat is lost through the building envelope. In most of inner London — where conservation areas and Article 4 Directions restrict permitted development — they also require careful engagement with the local planning authority before any replacement work begins.

This guide covers the principal window and door systems used in London residential renovation, their planning implications, and how to specify them correctly.

The planning context in inner London

Most of inner London's residential streets are within conservation areas. Conservation area status means that any works that would "materially affect the external appearance" of a building require planning permission — including window and door replacement, even where planning permission would not normally be required.

In practice, this means:

• —Replacing like-for-like (e.g. timber sash with timber sash, same profile, same configuration): usually accepted as not materially affecting appearance. Some councils permit this under a prior approval process; others require a full application. Check with the local planning authority before proceeding.
• —Changing window type (e.g. timber sash to aluminium): requires planning permission in a conservation area, and is frequently refused on aesthetic grounds.
• —Changing configuration (e.g. adding a window, enlarging an opening): requires planning permission.
• —Listed buildings: all window and door changes require Listed Building Consent, regardless of conservation area status. LBC applications must demonstrate that changes are reversible and minimally impactful.

The consequence of this planning context is that timber sash windows — the historic original specification for pre-1920 London housing — are the appropriate specification for any principal elevation in a conservation area, listed building, or property where planning authority approval is required.

Timber sash windows

The traditional London window: a double-hung sliding sash in painted softwood, with a six-over-six or four-over-four pane configuration in Victorian and Edwardian properties. The originals, where they survive in sound condition, should be repaired rather than replaced — see the separate guide on timber sash window restoration.

Where replacement is necessary (beyond economic repair), a new timber sash window should match the original in:

• —Profile: the moulding profiles on the sash frame, glazing bars, and box frame should replicate the original. Many joiners work from historic samples. SPAB (Society for the Protection of Ancient Buildings) publishes guidance on appropriate profiles.
• —Glazing bar width: original Victorian glazing bars are typically 18–22mm. Modern 'slim-line' glazing bars in uPVC or aluminium — often 35–40mm — are not appropriate for period elevations and are almost always refused in conservation areas.
• —Glass: cylinder glass (with its characteristic slight distortion) is the historically correct specification for period properties; modern float glass reads differently and is less appropriate in sensitive restorations. For secondary glazing or where thermal performance is the priority, modern glass is acceptable.
• —Double glazing in sash windows: slim double-glazed units (4–8mm spacer bar) can be specified in new timber sash windows, and are increasingly accepted by planning authorities in conservation areas provided the external appearance is preserved. The weight of a double-glazed unit requires a heavier sash and stronger cord/spring balance system — confirm with the joiner.

Cost: replacement timber sash windows are made to order. A standard 900×1500mm double-hung sash in softwood, factory-primed, supply only: £800–£1,800. Fitted and decorated: £1,500–£3,000 per window depending on size and specification.

Steel windows (Crittall-style)

Steel-framed windows with slim sight lines and divided lights are associated with the Arts & Crafts and early Modernist periods (1890s–1940s) and with contemporary industrial-influenced interiors. They have seen a strong resurgence in London renovation over the last decade, specified both in period properties (where they are historically appropriate) and in contemporary rear extensions and basement conversions (where their slim profiles suit modern aesthetic preferences).

Original Crittall: W20 and W40 series Crittall windows, still manufactured in the UK, are the historically correct specification for 1920s–1950s properties and garden rooms of that period. Single-glazed only in the original profile; thermally poor but appropriate for conservation-sensitive work.

Contemporary slim-profile steel: manufacturers including Crittall, Forster, Optimum (aluminium with steel visual aesthetic), and Thermalsash produce double or triple-glazed steel-look windows with significantly better thermal performance than originals. These are increasingly specified in rear extensions where the planning authority has accepted a contemporary design language.

Planning: steel windows on principal elevations of listed buildings require LBC. In conservation areas, like-for-like replacement of existing steel windows with matching new steel is generally acceptable. New steel windows in existing openings on principal elevations require pre-application advice.

Cost: steel windows are significantly more expensive than timber or uPVC. A 1,200×1,800mm Crittall double-glazed unit: £1,500–£3,500 supply; installation adds £500–£1,200 per opening.

Aluminium windows

Powder-coated aluminium windows are the dominant specification in contemporary rear extensions and basement conversions in London. They offer:

• —Slim sightlines (comparable to steel, better than timber)
• —Minimal maintenance (no repainting)
• —Wide range of colours (RAL and BS colour coding)
• —Good thermal performance in thermally broken systems

Thermal break: the critical specification for aluminium windows. A thermally broken aluminium frame includes a layer of polyamide separating the inner and outer aluminium elements, preventing direct conduction of cold from outside to inside. Non-thermally-broken aluminium frames are significantly colder and will show condensation at the frame in winter. Always specify thermally broken systems in habitable spaces.

Planning: aluminium windows on principal elevations in conservation areas are almost universally refused. On rear elevations of properties in conservation areas, contemporary materials are more commonly accepted if the design is of high quality. Always seek pre-application advice before specifying aluminium on any conservation area elevation visible from a public vantage point.

External doors

External doors are subject to the same planning constraints as windows on principal elevations. For Victorian and Edwardian properties:

• —Front doors: the original four- or six-panel painted timber door is the appropriate specification in a conservation area. Replacement front doors should replicate the original configuration, including any fanlight, sidelight glazing, and ironmongery position. Contemporary composite doors with moulded panels are not appropriate for period terraces in conservation areas and are generally refused.

• —Rear and side doors: more flexibility in specification. Aluminium or timber folding-slide or bi-fold doors are commonly accepted on rear elevations; the planning constraint is primarily on the principal (street) elevation.

• —Bi-fold vs sliding doors: in a rear extension opening to a garden, bi-fold doors open fully to create a seamless indoor-outdoor transition but reduce the unobstructed opening width by approximately 15% (the stacked door panels). Sliding doors (lift-and-slide or minimal-frame sliding) provide a wider unobstructed opening when open and a cleaner sightline when closed. For large openings (over 3m), sliding is generally the better functional specification.

Thermal performance

Building Regulations Part L requires replacement windows to meet a minimum U-value of 1.6 W/m²K (whole window) in existing dwellings. High-performance double-glazed systems achieve 1.2–1.4 W/m²K; triple-glazed systems reach 0.8–1.0 W/m²K.

For a well-insulated renovation targeting good thermal performance, specify double-glazed units as a minimum throughout. Triple glazing adds cost and weight without proportionate benefit in a London climate — double glazing with a low-e coating and argon fill is the standard quality specification and the point of diminishing returns.