What requires Listed Building Consent
Any change to a listed building's windows requires Listed Building Consent (LBC) under section 16(2) of the Planning (Listed Buildings and Conservation Areas) Act 1990. 'Any change' includes: (1) Replacement of an existing window with a new window of any description, even like-for-like; (2) Installation of double-glazed or slim DG sealed units in original frames (technically a 'fabric alteration'); (3) Installation of secondary glazing (an internal-mounted glazing system); (4) Changes to glazing bars, glazing patterns, or window proportions; (5) Removal or replacement of original ironmongery (sash locks, fitches, lifts). LBC application process: applicant or agent submits an LBC application to the local planning authority's conservation team; application fee is currently £0 (LBC applications are free of charge in England); supporting documents include existing drawings, proposed drawings, photographs, a heritage statement justifying the proposed change, and details of any sash or section profile being replicated. Decision timeline: 8 weeks (statutory) but commonly 10–14 weeks in busy London boroughs (Westminster, Kensington and Chelsea, Camden); 8 weeks delegated decision for minor changes. Appeals: refused LBC can be appealed to the Planning Inspectorate within 6 months.
What is typically approved by listed building grade
Conservation officer approval patterns vary by listed grade. Grade I (highest national interest, 2.5% of listed buildings): replacement only with exact replica of original — historic glass (cylinder glass, hand-blown) where possible; bespoke joinery to match original section profile to within 3mm; original ironmongery restored or replaced with matching reclaimed period equivalent. Slim DG is generally refused on Grade I — conservation officers consider it a material change to the historic fabric. Secondary glazing (Selectaglaze, Granada): approved if discreetly designed and not visible from primary view; preferred over slim DG. Cost: £4,000–£12,000 per Grade I window. Grade II* (5.5% of listed buildings): like-for-like replacement preferred; slim DG sometimes approved for windows not visible from primary view, depending on conservation officer judgement; secondary glazing routinely approved. Cost: £3,000–£8,000 per Grade II* window. Grade II (92% of listed buildings): like-for-like replacement easily approved; slim DG (11mm Slimlite or 14mm Histoglass) increasingly approved on Grade II — conservation officers focus on the surface appearance and slim sightline match. Selectaglaze secondary glazing routinely approved. Cost: £2,200–£4,500 per Grade II window for sash replacement; slim DG retrofitting (existing frame) £1,200–£2,500.
Slim double glazing on listed buildings — the modern approach
Slim profile double-glazed sealed units have transformed the listed building window debate over the past 15 years. Major slim DG manufacturers: Slimlite (Scottish; 11mm overall, edge-spacing 4mm); Histoglass (Sussex; 14mm overall, edge-spacing 6mm; UK heritage market leader); Cylindrical Glass Specialists (Cornwall; bespoke slim DG with restored cylinder glass outer pane); Pilkington Spacia (vacuum-insulated, 6.2mm overall — the slimmest sealed unit on the market, U-value 1.0 W/m²K). Conservation officer acceptance: Slimlite 11mm and Histoglass 14mm are now broadly accepted for Grade II listed building windows in most London boroughs. Pilkington Spacia 6.2mm is increasingly accepted on Grade II*; rare on Grade I. Performance: slim DG with original timber sash gives U-value 1.6–1.9 W/m²K (vs 4.8–5.2 for original single glaze); Pilkington Spacia achieves 1.1 W/m²K. Visual: from outside, slim DG is virtually indistinguishable from single glaze when the original putty profile is restored over the sealed unit. From inside, slight refraction may be visible at the spacer edge. Application process: an LBC application proposing slim DG should include a sample slim DG unit photograph, comparison photos of similar approved buildings, and a heritage statement justifying the energy improvement.
Secondary glazing — the most consent-friendly upgrade
Secondary glazing (an internal-mounted secondary window inboard of the existing primary window) is the most consent-friendly thermal and acoustic upgrade for listed building windows in London. Leading suppliers: Selectaglaze (Hertfordshire; UK market leader, 70+ years; conservation-spec horizontal sliders, vertical sliders, lift-outs); Granada Secondary Glazing (Sheffield; budget-mid spec); MitchellandDickinson (heritage); Roseview Windows (sash specialists). 2026 cost per window: Selectaglaze Series 10 horizontal slider 900×1500mm £550–£900 supplied and installed; vertical slider for sash heritage match £750–£1,200; lift-out for fixed light £450–£700. Performance: combined original sash + Selectaglaze secondary U-value 1.5–1.8 W/m²K (similar to slim DG-only); acoustic STC 38–46 dB (significantly better than any primary glazing alone — 8–14 dB more attenuation than single-pane glass). Whole-house Selectaglaze installation (12–16 windows on a Grade II Georgian terrace): £8,500–£18,000 total. LBC consent: secondary glazing on Grade I, II* and II listed buildings is routinely approved — conservation officers consider it the least intrusive thermal upgrade because the original primary window is unchanged. The application can typically be made as a delegated decision and approved in 6–8 weeks. Maintenance: Selectaglaze warranted 10 years on hardware, 20 years on the frame; the primary listed sash retains original specification.
