Repair vs replacement decision framework
Per-window assessment by joinery surveyor (Ventrolla, Sash Window Workshop, Hicks Joinery, Mumford & Wood — typically £180–£280 per house survey covering all windows + report). Repair viable when: (1) frame (boxing + outer cill) sound or has localised decay <25% of length; (2) sashes structurally sound or splice-repairable; (3) glazing bars intact or repairable; (4) cords + weights functional or replaceable; (5) profile original + matchable. Repair scope: deboxing (remove sash + parting bead + staff bead, full inspection); splice repair (cut out decay, scarf-joint new matching timber, glue + dowel); cord replacement (waxed cotton or terylene); weight rebalance (cast iron weights — add/remove for slim-DG weight change); brush draught-strip (Ventrolla or Stormguard system) installed to all 4 sides of each sash; slim-DG retrofit (Histoglass, Slimlite, Pilkington Spacia, 4–11mm units) routed into existing rebate or shallow rebate enlargement. Cost £950–£1,650 per sash unit + scaffold £18–£32/m². Programme 1–2 weeks per house of 8–14 windows. Replacement required when: (1) frame >40% decayed or wholly rotten; (2) sashes structurally unsound; (3) historic uPVC/aluminium installed in place of original, needing reversion; (4) glazing bar pattern wrong + LPA requires reinstate. Replacement scope: full new timber sash (or casement) box + sashes + cords + weights + ironmongery + cill, manufactured by tier 1/2 joinery workshop to LBC/CA approved spec, slim-DG or single-glazed, factory-finished water-based microporous paint. £1,400–£2,800 per unit supplied + fitted depending on size + spec. Programme 14–20 weeks lead + 1–2 weeks site install per 8–14 windows. See [[heritage-joinery-cost-london]].
LBC + CA + grant + thermal + acoustic gain
LBC default position: Historic England + SPAB technical guidance — repair where viable, replace only where original beyond economic repair. ~70–80% of LBC Grade II window cases approved as repair. Conservation officer + joinery surveyor agreement window-by-window. Submission: photo + assessment per window + repair vs replace recommendation + spec for either route. CA: less prescriptive than LBC but Article 4 directions often impose timber + matching profile + draught-strip-acceptable. Refusal common for uPVC. Thermal performance: original single-glazed sash U-value 4.8–5.4 W/m²K; repair + draught-strip + slim-DG U-value 1.6–2.2 W/m²K (close to modern double-glazing 1.4 W/m²K). Annual heating saving £140–£280 per house with full sash repair + slim-DG upgrade vs original single-glazed. Acoustic: original single 25–28 dB; repair + slim-DG 33–38 dB; replacement double-glazed 38–42 dB; secondary glazing (Selectaglaze) 42–52 dB added on top. Grants + VAT: 0% VAT on LBC qualifying repairs on Grade I/II*; 5% VAT on energy-saving materials (slim-DG retrofit qualifies). Listed Places of Worship grant scheme for ecclesiastical (rare residential). Builder's All-Risks insurance reduction ~5–8% on draught-strip + slim-DG retrofit (improved thermal envelope). Sale value uplift: full house sash overhaul + slim-DG retrofit typically +£18,000–£35,000 on £1.2M Victorian terrace (Knight Frank period property research 2025). Programme + cost example: Victorian semi 12 sash windows full repair + slim-DG: £14,500–£19,800 + £2,400 scaffold = £16,900–£22,200 vs full replacement £18,400–£33,600 + £2,400 scaffold = £20,800–£36,000. 25–55% saving + heritage authenticity. Builderr default: repair-first survey on every period house, replacement only where SE/joinery surveyor confirms uneconomic to repair.
