Wales has its own 2026 editions of Approved Document L (energy and greenhouse-gas emissions) and Approved Document F (ventilation). The Welsh Government states that the new guidance takes effect on March 4, 2027, subject to transition provisions for individual buildings. These dates apply in Wales; they are not England’s dates and do not establish which edition governs an ADU or conversion.
Check the work and its dates
The official Welsh Government circular WGC 006/2026 says the documents apply only to buildings and building work in Wales. It describes transition conditions tied to a relevant building notice, initial notice, full-plans deposit, and whether work on the individual building is regarded as commenced before the stated deadline. Where a site contains several buildings, the transition is assessed building by building.
The later Welsh Government circular WGC 007/2026, published July 22, 2026, draws attention to regulations correcting technical issues and omissions. Its accessible summary does not enumerate the corrections, so check the current Welsh regulations and affected guidance before relying on an edition or provision. This circular alone does not show whether a correction changes a particular project’s requirements.
Before relying on an edition, ask the building-control body to confirm the project classification, building-control route, notification or application history, and commencement evidence. A project should not be treated as covered by the old guidance merely because design or procurement began before the effective date.
Conversions can raise a distinct energy question
The 2026 Welsh Approved Document L describes conversion or a change of energy status as turning previously unheated space, such as a garage or loft, into heated space. An extension or conversion that increases habitable volume may trigger consequential energy-efficiency improvements, subject to technical, functional, and economic feasibility. The guidance describes a 15-year payback test for the economic part of that assessment. Whether a particular garage conversion, basement unit, or attached ADU meets those categories depends on the actual work and the applicable document; the summary is not a project calculation.
The 2026 amendment regulations (WGC 003/2026) and the current Approved Document L, Volume 1 provide the controlling detail. WGC 003/2026 says that, from March 4, 2027, Part L will require on-site renewable electricity generation when a dwelling or a building containing a dwelling is erected, subject to specified limitations. It also changes the dwelling energy-performance metric to energy use intensity. The circular describes transitional treatment for work already under way; a secondary dwelling’s classification and whether a limitation or transition applies still depend on the building and its records. This guide does not reproduce the complete technical method. Approved Document F separately covers dwelling ventilation.
Keep the nations and approvals separate
Welsh building regulations and guidance do not determine planning permission, utility capacity, fire review, or the legal classification of a proposed secondary dwelling. England, Scotland, and Northern Ireland use separate documents, authorities, and transition tests. See the UK building-control route guide.
For an individual project, retain the relevant notice or application, commencement records, project description, code edition, energy and ventilation calculations, and inspection evidence. Ask the Welsh building-control body which requirements and transition provisions apply before work starts or a compliance conclusion is published.