1968 detached deep retrofit — EnerPHit, EPC G→A+, net energy positive
Harrow · 1968 detached · HA7 Stanmore · 28-week build
Brief
1968 4-bed detached HA7 Stanmore — 198m² over 2 storeys + integral garage, 480m² plot, original solid-brick + ventilated cavity construction (poorly insulated), original metal-frame Crittall single-glazing failed, asbestos roof tiles end-of-life, original oil-fired heating decommissioned. EPC G (28) — among bottom 5% of London housing stock. Owners (PhD academic + sustainability consultant + 2 young children) brief: certified EnerPHit retrofit (Passivhaus retrofit standard) — net energy positive — EPC A+ target — all-electric, GSHP, MVHR, comprehensive fabric upgrade including external wall insulation, triple glazing, airtightness 1.0 m³/h/m² @50Pa target, 8.4 kWp PV array + 13.5 kWh battery. Budget £348,000. PAS 2035 + EnerPHit dual certification. 60-week total programme.
Challenge
Six compounding constraints. (1) EnerPHit certification ambition: airtightness 1.0 m³/h/m² @50Pa, U-values <0.15 W/m²K on opaque elements, triple glazing Uw <0.85, MVHR heat recovery >82%, certified Passivhaus designer required. Achieving on existing single-leaf brick 1968 fabric requires comprehensive EWI 180mm Kingspan K15 + airtightness membrane + replacement windows + replacement floor build-up + new airtight roof. (2) Asbestos roof tiles end-of-life requiring HSE-licensed removal £14,500. (3) GSHP feasibility: front garden 280m² adequate for 2× 110m vertical boreholes; Environment Agency notification required. (4) Crittall single-glazing replacement: 21 windows + 3 doors — Internorm HF410 timber-alu triple-glazed in matching profiles — £58,500 supply + £18,500 install. (5) Heat loss calculation: pre-retrofit 18.8 kW; post-retrofit 4.2 kW design temp — GSHP sized 12kW; radiators all replaced (eliminated) for full UFH retrofit. (6) PAS 2035 + EnerPHit dual certification with retrofit coordinator + Passivhaus designer (Architype, BRE-certified) from RIBA Stage 1.
Solution
Pre-design + consents 32 weeks. EnerPHit feasibility; PAS 2035 coordinator engaged; Passivhaus designer Architype Architects appointed; MCS heat loss + GSHP feasibility; Environment Agency borehole notification; Building Regulations submission with EnerPHit junction details; BUS + ECO4 + Smart Export Guarantee applications. Build Phase 1 weeks 1–14 fabric upgrade. Asbestos roof tile removal HSE-licensed weeks 1–4 (£14,500); new clay tile roof + 300mm Kingspan K7 in joist void + 150mm K12 above ceiling (warm roof). EWI: 180mm Kingspan K15 to all walls + brick slip finish matching original London Stock + airtightness membrane Pro Clima Intello Plus continuous around envelope. Floor build-up: lifted slab + 200mm Kingspan TF70 + 80mm liquid screed with UFH + airtightness membrane joined to wall membrane. Windows: 21 windows + 3 doors Internorm HF410 triple-glazed; passive-house-grade install tapes; intermediate airtightness test week 12 measured 1.2 m³/h/m² — additional taping at 3 weak junctions; retest week 14 0.92 m³/h/m². Build Phase 2 weeks 15–28 services. GSHP: 2× 110m vertical boreholes drilled in front garden (4-day specialist rig); Mitsubishi Ecodan ground-source 12kW + 300L pre-plumbed cylinder; flow 35°C UFH throughout (full UFH retrofit ground + first floor — no radiators retained); Heatmiser NeoHub multi-zone control 9 zones. MVHR: Zehnder ComfoAir Q600 sized for EnerPHit; semi-rigid 90mm radial ducting through new dropped ceiling zone; commissioned at 96 L/s; measured heat recovery 91% (EnerPHit 82% target exceeded). PV array: 8.4 kWp Solitek monocrystalline (24× 350W) on south-facing rear roof; 13.5 kWh Tesla Powerwall 3 battery; SolarEdge optimisers per panel; Smart Export Guarantee 15p/kWh. Final airtightness test week 28: 0.88 m³/h/m² @50Pa (exceeds EnerPHit 1.0). EnerPHit certification submitted BRE; certified post-construction week 32.
Outcome
Certified EnerPHit retrofit — BRE certification issued week 32. EPC G (28) → A+ (102) — among top 0.1% of UK existing housing stock; A+ band rare in retrofit. Heating bill: pre-retrofit £4,800/year (oil + electric); post-retrofit £450/year all-electric net of PV export earnings (post-retrofit electricity consumption 2,800 kWh/year; PV generation 7,400 kWh/year; battery self-consumption 88%; net export ~3,500 kWh × 15p = £525 SEG payment annually). Net energy positive: £75/year. Net carbon: pre-retrofit 12,800 kgCO2/year; post-retrofit ~0 kgCO2/year (net-zero in operation). MVHR measured CO2 average 480 ppm; humidity 42–55% stable; airtightness 0.88 m³/h/m² @50Pa. GSHP SCOP measured 4.42 (predicted 4.2 exceeded). Building Control completion week 29; PAS 2035 + EnerPHit dual certification complete week 32. Property valuation: pre-renovation £1.4M; post-renovation £1.92M (£520,000 uplift on £348,000 net spend after grants £14,800 — 156% gross ROI). Featured in Passive House Plus magazine cover July 2026; submitted to AECB Carbonlite Awards; case study material for Passivhaus Trust + Retrofit Academy. Builderr first EnerPHit certified project. Harrow borough portfolio now 3 case studies (Pinner, Hampstead Garden Suburb-adjacent, Stanmore).
Spec
Project specification.
Gallery
Inside the build.
"We bought a 1968 detached with EPC G — among the worst-performing houses in London — and asked Builderr to deliver an EnerPHit certified retrofit. No one we spoke to in Harrow had attempted EnerPHit before. Builderr engaged Architype as Passivhaus designer, BRE as certifier, MCS for ground-source design, and committed to the airtightness target from week one. The result: 0.88 m³/h/m² airtightness against a 1.0 target, EPC G to A+, net energy positive operation with the PV + battery. Heating bills are essentially zero — the SEG export covers the import. The house is silent, stable, completely free of draughts and condensation. The indoor air quality measurement of 480 ppm CO2 is materially better than the schools the children attend. £348k spend (£334k net of grants), £520k value uplift, BRE EnerPHit certification, and a Passive House Plus cover. Worth every week of the 32-week pre-design phase."
— Prof. Eleanor and James Whitfield, Stanmore HA7
Builderr vs other London builders.
The construction industry has a wide distribution of operators. Here's what changes between a directly-employed, fixed-scope outfit and the alternatives.
| Criterion | Builderr | Typical London builder | Cowboy outfit |
|---|---|---|---|
| Labour model | Directly employed team (PAYE) | Mixed subcontract gangs | Day-rate cash labour |
| Pricing | Fixed-scope itemised quote | Estimate + provisional sums | Verbal price + variations |
| Design & engineering | In-house architect + SE | Outsourced, separate billing | Builder draws on the back of an envelope |
| Planning + LDC handled | Yes — included in price | Often charged extra | Builder asks you to apply |
| Party wall surveyors | Instructed by us | Your responsibility | Skipped (illegal) |
| Building control | Plans + site inspections booked by us | Building Notice route | Not registered |
| Project management | Dedicated PM, weekly photo updates | Foreman doubles up | Owner-manager juggles 5 jobs |
| Payment schedule | Stage payments against signed-off milestones | Weekly invoices | Cash up front |
| Insurance | £10M PL + 10yr structural warranty | £2–5M PL only | No documented cover |
| Snags at handover | <3 typical | 20–30 typical | Walk-off mid-job common |
| Variation creep | 0% — fixed scope | +15–25% over original quote | +40%+ regularly |
Save £69,600–£156,600 on a whole house.
Industry data (FMB, RICS, Which? Trusted Trader 2024) shows the average London construction project overruns by 18–22% on cost and 25–35% on time. Fixed-scope contracts with a single accountable team eliminate that variance. The savings above assume a typical project at £348,000.
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