3.4–5.2 kWp in-roof arrays across 240 dwellings on the University of Cambridge Eddington development. Factory pre-fit on panelised roof cassettes saved 4 weeks of programme. Per-plot installed cost £4,380 — comfortably below the £4,350 FHS premium benchmark when ASHP is included.
The brief
Phase 4 of Eddington — the University of Cambridge's low-carbon urban extension on the city's north-west edge — required all 240 plots delivered to FHS-equivalent specification ahead of the 24 March 2027 regulatory deadline. The masterplan, originally consented in 2013 with sustainability principles baked into the design code, sets exemplary energy standards for every phase. By Phase 4 (commencement Q1 2026) the developer needed a procurement model with per-plot pricing locked at land-bid stage, a delivery model that protected the existing roofing programme, and an SAP/HEM compliance pack accepted by the project Approved Inspector without need for variation submissions.
House types and system specification
Three core house types were delivered across the 240 plots: 120 × 3-bed semis (42.5 m² ground floor, 3.4 kWp arrays), 80 × 3-bed detached (50 m² ground floor, 4.0 kWp), and 40 × 4-bed detached (65 m² ground floor, 5.2 kWp). All arrays specified Q CELLS Q.PEAK DUO ML-G11+ 425W panels with SolarEdge HD-Wave inverters and per-panel optimisers. Paired with 5 kWh GivEnergy All-In-One AC-coupled batteries and 6 kW Mitsubishi Ecodan ASHPs. Roof pitches standardised at 35° south-facing on the dominant house types; 12 corner plots required 25° east-west split arrays with additional inverters.
Procurement and pricing structure
Per-plot pricing locked at land-bid stage in Q4 2024, with an inflation cap of 3% to delivery window end (Q4 2026). Average installed cost £4,380/plot, comprising £2,950 for solar (panels, in-roof mounting, inverter, monitoring, MCS, warranty), £1,180 for ASHP heating package (heat pump, hot water cylinder, commissioning), and £250 for the battery storage. Below the £4,350 average FHS premium reported in the Government's Impact Assessment due to bulk-procurement of panels (£0.32/W against the £0.42/W spot market price), factory pre-fit efficiency, and amortised SAP/HEM modelling across the 240 dwellings.
Factory pre-fit and delivery model
In-roof systems factory pre-fitted onto panelised roof cassettes off-site at our partner's pre-fabrication facility in Peterborough. Cassettes delivered just-in-time to the roofing stage (programmed 8 weeks ahead of each plot's ready-for-roof date). On-site final electrical connection averaged 2 hours per plot — completed by a 2-person installation team who moved between plots within the same week. Total site labour: 480 person-hours across the 240 plots, compared to the ~1,800 hours typical for site-built installations.
Compliance and Building Control
Full SAP 10.3 modelling delivered for every plot under the FHS transitional period, with parallel HEM models prepared as evidence for the Approved Inspector's pathway preference. Every plot demonstrated DER ≤ TER comfortably (typical margin 12-15%). EPC scores across the development averaged 92 (top of band A), with two plots achieving 95+. Approved Inspector sign-off achieved within 14 days of completion on every plot — no Building Control queries required system variations.
Result and lessons learned
All 240 plots handed over on programme. Buyer-side: 86% expected annual electricity bill reduction at 2026 tariffs (Octopus Intelligent Flux tariff modelled). NHBC accepted the warranty stack without query. Lessons: (1) east-west split arrays on the corner plots needed 2 inverters not 1 — adjusted for Phase 5 pricing. (2) Air permeability target of 3 m³/(h·m²) was hit on 233 of 240 plots; 7 plots needed minor remedial sealing post-airtightness-test. (3) Per-plot pricing model worked well; both parties prefer it over schedule-of-rates contracts on future phases. Phase 5 (a further 180 plots) commenced procurement Q2 2026 at locked pricing.
