Welsh Government confirms Part O overheating changes for existing homes

The Welsh Government has published its response to the consultation on Part O of the Building Regulations in Wales, confirming plans to apply overheating requirements to certain works on existing dwellings.

The document, titled "Building Regulations changes to Part O – Overheating: summary of responses and Welsh Government response", appears on the Welsh Government's Part L 2025 review page, which was last updated on 5 November 2026. The consultation ran from 26 August 2025 to 17 November 2025 and sought views on plans to improve energy efficiency standards in new and existing homes and buildings.

According to the Home Builders Federation (HBF), which summarised the response for members, the changes cover replacement windows and doors in highly glazed flats, loft conversions that create habitable rooms, and highly glazed extensions. HBF said that where glazing in a flat exceeds 15% of the total floor area, solar control measures will be required, with a maximum target g-value of 0.40.

HBF said the Welsh Government also intends to issue further overheating guidance for loft conversions and extensions, setting out how ventilation and heat release should be assessed. The changes are intended as a proportionate first step rather than an extension of Part O to all refurbishment work on existing homes, HBF said, and the regulations and a revised Approved Document O are expected in 2027, with no implementation date yet confirmed.

Background

Part O was inserted into the Building Regulations in Wales by the 2022 amendment regulations, which came into force on 23 November 2022, to require overheating mitigation. Welsh Ministers approved statutory guidance to support compliance, published as Approved Document O.

SME viability

HBF's 6 November newsletter also carried its 2026/27 State of Play report, produced with Close Brothers Property Finance and VELUX, which found that 63% of SME home builders surveyed had delayed progressing a site or put work on hold.

Neil Jefferson, Chief Executive of the Home Builders Federation, said: "While Government has introduced a number of measures aimed at supporting home building, the reality for SME developers remains challenging."

A total of 213 respondents completed the online survey between 2 June and 2 July 2026, with 59% developing 50 units or fewer a year. No responses were received from Northern Ireland.

HBF said 59% of respondents had cut the number of new sites acquired and 41% had made redundancies or reduced their contracted workforce. Some 95% said buyer affordability was constraining demand for new build homes.

Planning delays remained the leading challenge for the seventh consecutive edition, cited by 88% as a major barrier, HBF said, with cumulative viability challenges rising from 64% last year to 82%.

Barrier cited by SME builders	Share of respondents
Planning delays	88%
Cumulative viability challenges	82%
Lack of Government support for first-time buyers	72%
Supply and cost of materials	60%

Phil Hooper, Chief Executive Officer of Close Brothers Property Finance, said: "SME housebuilders tell us the numbers are no longer stacking up".

HBF estimated that taxes, regulatory compliance and higher construction costs have added £76,000 to the cost of building a new home since 2020, which it said was equivalent to more than 20% of the average value of a new-build home.

Levy and first-time buyers

The report calls for the Building Safety Levy, introduced on 1 October, to be repealed. Earlier HBF research with Quantum Development Finance found 91% of SME builders in England said the levy would make developments financially unviable, and 36% had already delayed, redesigned or cancelled schemes in anticipation of it. HBF attributed £2,320 of the £76,000 cost increase to the levy.

On the Your First Home scheme, HBF said home builders taking part will pay a fee on a sliding scale starting at 1% of the sale price.

The Welsh Government's Part O regulations and revised Approved Document O are expected in 2027, according to HBF.