For a secondary dwelling or another building project in Wales, building-control approval has a commencement threshold as well as a start-on-site notice. Welsh Government guidance describes rules that took effect on 1 July 2026. They apply to building-control approvals under the Welsh regime; they do not determine planning permission, classify an ADU proposal, or establish that a design complies with technical requirements.
Separate the site-start notice from regulatory commencement
For a building that is not higher-risk, the dutyholder must notify the building-control body at least two working days before the intended start on site. For a higher-risk building, the notice period is five working days. A separate commencement notice is due no more than five working days after the work reaches the regulatory commencement threshold.
The threshold depends on the building and work. For a complex building, the guidance refers to completion of the supporting foundations and lowest floor structure. For other construction or a horizontal extension, it refers to the sub-surface structure, foundations, any basement, and ground-floor structure. For other work, commencement is tied to the point when building-control input is needed for enforcement purposes. Ask the building-control body which definition applies before scheduling the notices; visible work on site and regulatory commencement are not always the same event.
The authority may reject a commencement notice within four weeks, with reasons, if it is not satisfied that the work has met the required threshold. Keep evidence of the actual work completed and of both notices with the project record.
Track the three-year lapse period
The approval automatically ceases to have effect if the applicable commencement threshold is not reached within three years. Where one approval covers more than one building, lapse can occur separately for each building. Work commencing on one building therefore does not necessarily keep approval alive for another.
The 1 July 2026 start date has transitional provisions for plans deposited before that date. Check the transition rule, the approval date, building class, and relevant threshold with the building-control body. Do not assume that every project has a fresh three-year period beginning on 1 July 2026.
These rules govern approval validity and notices. They do not set a planning decision time or a total project duration. For the wider nation-by-nation route, see UK secondary-dwelling planning and building routes.