TrustMark, the government-endorsed quality scheme for energy efficiency retrofits, notified the Department for Energy Security and Net Zero (DESNZ) of suspected fraud in April 2024, according to a National Audit Office (NAO) report published on 14 October 20251. The NAO describes the issue as some retrofit businesses overclaiming for work undertaken, and states that undetected fraud in the schemes means fewer consumers may benefit overall1.
The report covers the Energy Company Obligation (ECO), which obliges energy suppliers to fund the installation of energy efficiency measures such as insulation in homes, and which the NAO says is intended to tackle fuel poverty and reduce carbon emissions in Great Britain1. Two schemes are running: ECO4, from April 2022 to March 2026, and the Great British Insulation Scheme (GBIS), which has broader eligibility and runs from March 2023 to March 20261. The government expects the schemes together to deliver £280 million in annual energy bill savings for households1.
Ofgem reports that 243,900 homes have been upgraded under ECO4 to the end of March 2025, and 60,600 under GBIS1. Across both schemes combined there have been 28,000 installations of external wall insulation, 3% of all measures installed, and 45,200 installations of internal wall insulation, 5%1.
The fraud notification was one of two issues TrustMark raised with DESNZ during 2024. In October 2024 it notified the department of high levels of external wall insulation installations that were non-compliant with the relevant quality standard, and the following month it highlighted issues with internal wall insulation1. The NAO says non-compliance covers a wide range of severity, from major issues that pose immediate risks to the health and safety of the household to minor issues such as missing paperwork1.
"in April 2024, TrustMark notified DESNZ of suspected fraud, whereby some retrofit businesses were overclaiming for work undertaken"
The NAO's conclusions state that there have been clear failures in the design and set-up of ECO4 and GBIS and their consumer protection and quality assurance system, which have led to widespread issues with the quality of installations and suspected fraud1. It adds that DESNZ and Ofgem responded quickly once aware, but that the current system left DESNZ with few levers and limited information1.
The report is based primarily on information held by DESNZ and Ofgem, which the NAO audits, with support from TrustMark and UKAS, which as private sector companies the NAO does not audit1. The NAO has not inspected the homes affected or gathered evidence directly from affected households or retrofit businesses, and it does not comment on ongoing investigations into suspected fraud1.
Why it matters for households
ECO-funded work is paid for through energy supplier obligations rather than directly by the householder, so the money available for insulation measures is finite. The NAO's position is that fraud that goes undetected means fewer consumers may benefit overall1, which bears on how far the scheme stretches across eligible homes.
The quality issues sit alongside that. Wall insulation installed to a standard that is not met can range from paperwork gaps to problems posing immediate risks to health and safety1. For a household, the practical question is whether the measure fitted actually performs as intended, since the point of the work is a warmer, cheaper-to-heat home and reduced reliance on bought-in energy. The NAO notes that the relevant businesses have obligations to remediate affected homes1, but it does not set out how many homes are affected, and that figure has not been reported.
What happens next
ECO4 and GBIS both run to March 20261. The NAO states that DESNZ faces two tasks: ensuring the relevant businesses meet their obligations to remediate all affected homes as quickly as possible, and reforming the system so that this cannot happen again1. The report does not set out a timetable for either, and no remediation deadlines have been reported.