Sources, citation manifest and calculator coefficients

Every numeric claim on retrofitcost.co.uk resolves through a margin note to one of the entries below. There is no proprietary cost database: the figures are pulled from published primary sources (Ofgem, DESNZ, BSI, gov.uk, Passivhaus Trust, MCS, GSHPA, Solar Energy UK and the named installers' own publisher pages). Where a vendor is quote-only, no inferred rate is published; the page records the publisher URL and the surveyor's note from that page.

{
  "ofgem_q4_2026": { "gas_p_per_kWh": 7.97, "elec_p_per_kWh": 26.32, "gas_standing_p_per_day": 29.68, "elec_standing_p_per_day": 54.83, "src": "ofgem-q4-2026" },
  "fuel_switch_model": {
    "ashp_spf": { "median": 2.78, "iqr": [2.55, 3.05], "sample": 428, "src": "eoh-hp-performance" },
    "boiler_efficiency": { "mid": 0.85, "band": [0.76, 0.92], "src": "eoh-hp-performance",
      "note": "the trial did not monitor boiler efficiency; the band is the one its hybrid matrix spans, and 0.85 is this file's mid-band assumption" }
  },
  "bus_grant_GBP": { "ashp": 7500, "gshp": 7500, "biomass": 5000, "src": "gov-bus" },
  "measures": {
    "ewi":     { "low": 110, "high": 180, "unit": "m2", "src": "desnz-domestic-cost" },
    "iwi":     { "low":  75, "high": 130, "unit": "m2", "src": "desnz-domestic-cost" },
    "cwi":     { "low":  18, "high":  28, "unit": "m2", "src": "gov-gbis" },
    "loft":    { "low":  12, "high":  22, "unit": "m2", "src": "gov-gbis" },
    "floor":   { "low":  38, "high":  75, "unit": "m2", "src": "passivhaus-trust" },
    "glazing": { "low": 12000, "high": 22000, "unit": "house", "src": "passivhaus-trust" },
    "mvhr":    { "low":  5500, "high": 11000, "unit": "house", "src": "passivhaus-trust" },
    "piv":     { "low":   450, "high":   950, "unit": "house", "src": "approved-doc-f" },
    "ashp":    { "low":  8000, "high": 14000, "unit": "house", "src": "gov-bus" },
    "gshp":    { "low": 18000, "high": 35000, "unit": "house", "src": "gshp-association" },
    "rads":    { "low":  1800, "high":  4500, "unit": "house", "src": "mcs" },
    "solar_per_kWp":   { "low": 1450, "high": 2200, "src": "solar-energy-uk" },
    "battery_per_kWh": { "low":  700, "high": 1050, "src": "solar-energy-uk" },
    "controls":{ "low":   350, "high":   950, "unit": "house", "src": "desnz-domestic-cost" }
  },
  "pas_2035_fees_GBP": {
    "assessor":     { "low":  400, "high":  800 },
    "coordinator":  { "low": 1500, "high": 3500 },
    "designer":     { "low":  800, "high": 3000 },
    "monitoring":   { "low":  250, "high":  800 }
  }
}

The full citation manifest is in /src/data/sources.json. Each entry carries publisher, URL, retrieval date and (where applicable) the specific numeric value pulled. The next scheduled review is 25 November 2026, when Ofgem announces the January to March 2027 cap.

If any figure here is wrong, write to oliver at digitalsignet dot com with the publisher URL that contradicts it. The page will be amended with a dated revision note.