Highgate Edwardian circular-economy retrofit

Brief

1908 Edwardian end-of-terrace N6 Highgate — 156m² over 3 storeys, Highgate Conservation Area, Article 4 Direction restricting front-elevation alteration. Owners (climate scientist + furniture designer + 2 children) brief: full house renovation including new 18m² side return, demonstrating circular-economy + low-carbon principles end-to-end — Whole-Life Carbon Assessment (WLCA) per RICS PS 2023 with quantified embodied carbon reduction target ≥35% vs conventional spec; reclaimed materials wherever feasible (brick, timber, fittings); natural insulation (Steico wood-fibre, sheep wool) replacing all petroleum-based foams; lime plaster + natural paints throughout heritage rooms; GGBS-replacement concrete (CEM III/A) for foundations; FSC/PEFC structural timber; design-for-disassembly (mechanical fix, no glue) for kitchen + joinery. Budget £295,000 (£1,890/m²). Side return planning + LBC-equivalent CA consent. Children's air-quality + breathable-fabric major driver alongside carbon.

Challenge

Six compounding constraints. (1) Conservation Area + Article 4 — front elevation untouchable (no PV on front roof; original sash retained); side + rear conditioned (heritage materials match). Planning + Heritage Statement by IHBC consultant (Heritage Collective). (2) Embodied carbon target — pre-design WLCA showed conventional spec 442 kgCO2e/m²; 35% reduction target = ≤287 kgCO2e/m². Required material substitution at every line of bill of quantities. (3) Reclaimed brick supply — needed 8,400 bricks matching London yellow stock; LASSCO Three Mills had 5,200 in stock; Cawarden Reclaim sourced additional 3,200 with 8-week lead — ordering at concept stage to lock supply. (4) Wood-fibre insulation U-value — Steico Flex 140mm achieves U-value 0.20 W/m²K vs PIR 100mm at 0.18; design accommodated 40mm extra wall thickness internally (lost 1.2m² floor area). (5) Lime plaster slow set + skilled labour — 5 lime plasterers in London competent for full-house spec; book 12 weeks ahead; 30-day cure between coats added 6 weeks vs gypsum programme. (6) GGBS concrete + winter pour — CEM III/A foundation pour week 3 in 4°C ambient required Sika ViscoCrete admixture + thermal blankets for 7-day cure. Project ran October–March winter programme.

Solution

Pre-design + WLCA + consents 16 weeks. WLCA by Targeting Zero (RICS PS 2023 method, One Click LCA tool) — conventional spec baseline 442 kgCO2e/m² × 174m² post-extension = 76,900 kgCO2e A1–A5; target spec 287 kgCO2e/m² × 174m² = 49,940 kgCO2e (–35%). Planning + Heritage Statement submitted week 8; approved week 16 zero amendments. CE Statement prepared voluntarily — quantified retention (94% existing structure retained), waste hierarchy targets (88% diversion-from-landfill via 5-bin sort), reclaimed quantities (8,400 bricks + 84m timber joinery + 12 doors + 6 fireplaces). Build 22 weeks. Foundations weeks 1–3: trench foundation CEM III/A concrete (50% GGBS replacement) — 6.8m³ at 0.45 kgCO2e/kg vs CEM I 0.95 = 17,200 kgCO2e saved on substructure alone; winter pour with Sika ViscoCrete + 5-day insulated cure. Side return walls weeks 4–7: reclaimed London yellow stock brick (8,400 bricks at 0.005 kgCO2e/brick vs new 0.21 = 1,700 kgCO2e saved); NHL 3.5 lime mortar (carbonating — net-negative over 5 years); 100mm Steico Flex wood-fibre cavity insulation. Roof weeks 8–10: existing roof retained + 200mm sheep wool insulation between/over rafters (Black Mountain UK, 95% recycled fibre); reclaimed Welsh slate via SalvoWeb. Internal weeks 11–22: Steico Flex 80mm IWI to external walls of heritage rooms (preserving sash reveals + cornices); lime plaster to heritage rooms (NHL 2 base + finish by SPAB-affiliated plasterer); gypsum to side-return + kitchen + bathrooms for speed; Edward Bulmer natural paint heritage rooms; Little Greene Intelligent low-VOC modern rooms; reclaimed pitch pine flooring (MASS Camberwell); 4 reclaimed Victorian cast-iron fireplaces (LASSCO Brunswick House + Drummonds); deVOL mechanical-fix kitchen (bolt-on plinths + screw-down worktop — design-for-disassembly); reclaimed brass ironmongery (Willow & Stone); ASHP 7kW Vaillant aroTHERM Plus replacing gas boiler (BUS £7,500 grant); MVHR Zehnder ComfoAir; 3.2kWp Sunology PV rear roof only (Article 4 restricts front). Final WLCA week 22: measured 271 kgCO2e/m² × 174m² = 47,150 kgCO2e — 38.7% below conventional baseline, exceeded 35% target.

Outcome

WLCA verified 271 kgCO2e/m² A1–A5 — 38.7% below conventional baseline (29,750 kgCO2e A1–A5 saved). CE Statement metrics: 94% structure retention; 88% waste diversion from landfill; 8,400 reclaimed bricks + 84m reclaimed timber + 12 reclaimed doors + 6 reclaimed fireplaces; 100% natural insulation (no PIR/PUR); 100% lime plaster + natural paint heritage rooms; 100% FSC/PEFC structural timber. Operational: EPC E (51) → B (84); heating cost £2,400/year → £580/year (£1,820 saved annually); CO2 in operation 4,200 → 480 kgCO2e/year. Property valuation pre £1.18M → post £1.62M (£440k uplift on £295k spend net of £7,500 BUS grant = 152% gross ROI). Featured: AECB CarbonLite Retrofit Awards 2026 finalist; LETI Retrofit Showcase 2026; Passive House Plus magazine November 2026 issue ('Circular Edwardian' cover feature); RIBA London Awards 2027 shortlist (Sustainability Award category). Builderr first formal WLCA-quantified circular-economy case study + first reclaimed-brick + lime-plaster + sheep-wool + GGBS-concrete combined retrofit. Haringey borough now 2 portfolio case studies; Highgate N6 first project. Featured Targeting Zero + RICS WLCA Best Practice 2027 case database.