Quick answer: if you renovate more than 50% of the surface of a wall, roof or floor — or more than 25% of the whole envelope — and that element is worse than the threshold U-value in Approved Document L Table 4.3, it should be upgraded to the improved value: roof 0.16, cavity wall 0.55, wall with internal or external insulation 0.30, floor 0.25 W/m²K. If that is not technically or functionally feasible, or will not pay back within 15 years, upgrade to the best that is — generally no worse than 0.70 — and record why.
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Tick "work to existing elements" in the tool. Extensions and renovations in one report.
Table 4.3 — existing elements in an existing dwelling
| Element | Threshold — worse than this and it must be upgraded | Improved — the value to reach |
|---|---|---|
| Roof | 0.35 | 0.16 |
| Wall — cavity insulation | 0.70 | 0.55 |
| Wall — internal or external insulation | 0.70 | 0.30 |
| Floor | 0.70 | 0.25 |
Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments (England), Table 4.3. Values in W/m²K.
What counts as renovation
Providing a new thermal layer, or replacing an existing one. In practice that is: stripping and re-covering a roof, applying external render or cladding, dry-lining internally, filling a cavity, or re-screeding a floor with new insulation. Decorating, repointing and patch repairs are not. The test is how much of that individual element you are re-forming — over half of it, and the upgrade duty bites.
Three situations, three rules
- Renovating. Table 4.3 as above, with the feasibility test. Under the 50% trigger there is no upgrade duty, but the element must not end up worse than it started.
- Replacing the whole element. It becomes a new element: it must meet the Table 4.2 limiting values — roof 0.15, wall 0.18, floor 0.18 — and be no worse than what it replaced.
- Retained elements joining the heated envelope. Loft conversions, garage conversions, a change of energy status: the retained walls, floors and roofs are treated as renovated elements and go to the Table 4.3 improved values, whatever proportion is being worked on.
The payback test, worked
The economic feasibility test is a simple payback of 15 years or less. Take a solid wall of 60 m² at roughly 2.10 W/m²K, externally insulated to the improved value of 0.30 at a cost of about £12,000:
= (2.10 − 0.30) × 60 × 2000 × 24 ÷ 1000 ÷ 0.9 × £0.065 ≈ 5,760 kWh, about £374 a year. Simple payback ≈ £12,000 ÷ £374 = 32.1 years — beyond the 15-year test, so the duty is to upgrade to the best value that is feasible, with the reasoning recorded, rather than to hit 0.30 regardless of cost.
Those inputs are the tool's defaults and they are indicative — degree-days, fuel price and system efficiency all vary, and mains gas at 6.5 p/kWh gives a very different answer from electricity. Building Control may want your own figures. RenoMonkey shows the estimate and lets you change all three.
Why it is worth catching early
Almost all of this is cheap on a drawing and expensive on site. A warm-deck flat roof designed in from the start costs a little more insulation; the same roof discovered at inspection means stripping a finished covering. The same goes for a wall that has already been rendered. If the job also involves an extension, both halves are checked together — and both appear in the same report.
Frequently asked questions
When does re-roofing or re-rendering trigger an insulation upgrade?
When you renovate more than 50% of the surface area of that element — or more than 25% of the whole building envelope, which catches everything. "Renovating" means providing a new layer or replacing an existing one: a new roof covering, new render or cladding, internal dry-lining, or filling a cavity.
What U-value does the upgraded element have to reach?
If the existing element is worse than the threshold value in Table 4.3, it should be upgraded to the improved value in the same table — roof 0.16, cavity wall 0.55, wall with internal or external insulation 0.30, floor 0.25 W/m²K. If the existing element is already at or better than the threshold, no upgrade is required; it just must not end up worse than it was.
What if the improved value is not achievable?
Then you upgrade to the best value that is technically and functionally feasible and pays back within 15 years. An upgraded element should generally not be worse than 0.7 W/m²K. The reason has to be recorded — a room that would lose too much floor area to internal insulation, a wall that cannot take the weight, a payback that runs past the test — because Building Control will ask why the improved value was not met.
Does replacing a flat roof covering count?
Yes, and it is the most common one. Stripping and re-covering a flat roof is renovating the whole element, so a roof worse than 0.35 W/m²K should be brought up to 0.16 — which usually means a warm-deck build-up rather than a like-for-like recover. It is much cheaper to design in than to be told at inspection.
What about a loft or garage conversion?
Those elements are retained but become part of the heated envelope, so they are treated as renovated elements whatever percentage is being worked on: the garage wall, floor and roof all have to reach the Table 4.3 improved values where they are worse than the threshold.
Is decorating or repointing caught?
No. Renovation means a new or replacement thermal layer. Repointing, painting, patch repairs and replacing a covering like for like on a small area are not renovation of a thermal element. The trigger is the amount of the element you are actually re-forming.
Next: re-roofing in detail · garage and loft conversions · replacement windows · U-values for the new parts · work out an upgraded build-up free · the dated reference tables.