In short
- Under Approved Document L, renovating a wall includes adding a new layer such as render or cladding, or replacing an existing layer by stripping back to the substrate.
- If you renovate more than 50% of a wall element's surface, or more than 25% of the whole building envelope, the wall should be upgraded to a U-value of 0.30 W/m²K where technically, functionally and economically feasible.
- The economic test is usually a simple payback of 15 years or less.
- This work is notifiable: building control, through the council or an approved inspector, needs to be involved.
What counts as renovation
Approved Document L, which covers conservation of fuel and power, treats a wall as a thermal element. Renovation of a thermal element includes providing a new layer, such as cladding or rendering the outside, and replacing an existing layer, such as stripping render back to the brick and re-rendering.
Simple repairs, patching and repainting are not renovation in this sense.
The thresholds
The requirement to upgrade applies when the renovation covers:
- more than 50 per cent of the surface area of the individual wall element; or
- more than 25 per cent of the total external envelope of the building.
When triggered, the whole element should be upgraded to achieve a U-value of 0.30 W/m²K, provided this is technically, functionally and economically feasible. If not, the best practicable standard should be achieved.
The feasibility test
Economic feasibility is usually judged on a simple payback of no more than 15 years. For a solid wall being fully re-rendered, adding external wall insulation often passes. For a cavity wall, the practical upgrade may be to fill the cavity. Where insulation would reduce a room or path below a usable width, or cause moisture risk on a historic wall, it may not be technically or functionally feasible.
Historic and traditional buildings
Approved Document L recognises that listed buildings, buildings in conservation areas and traditionally constructed buildings may need a different approach, so that energy upgrades do not harm character or cause damp. Historic England publishes guidance on this. See rendering period and solid-wall homes.
What to do in practice
- Ask your contractor whether your project is notifiable and how they will handle building control.
- If full re-rendering is planned, get advice on the insulation option and its payback.
- Keep the completion certificate. Solicitors ask for it when you sell.
Questions people ask
Do I need building control approval to re-render my house?
If you are stripping and re-rendering, or adding a new render layer, over more than 50% of a wall or 25% of the building envelope, the work is likely to be notifiable. Patch repairs are not.
Do I have to add insulation when I re-render?
You should upgrade to a U-value of 0.30 W/m²K where technically, functionally and economically feasible. Building control will advise on your situation.
Who handles building control?
Either your local authority's building control service or a registered building control approver, often arranged by the contractor.