Sources, citation manifest and calculator coefficients
Source-of-record manifest for retrofitcost.co.uk: every primary URL behind every numeric claim, with retrieval dates and the calculator coefficient mapping exposed as JSON.
How this file is sourced
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.
Calculator coefficients (JSON)
{
"ofgem_q3_2026": { "gas_p_per_kWh": 7.33, "elec_p_per_kWh": 26.11, "gas_standing_p_per_day": 29.04, "src": "ofgem-q3-2026" },
"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 }
}
}Citation manifest
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 26 September 2026 (Q4 cap publication).
Corrections
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.