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Check a renovated thermal element against Part L
Renovated or replaced, RenoMonkey runs the Table 4.3 test, the 50% rule and the payback justification for the build-up you are actually proposing.
Free to check. Building Control-ready PDF £35 +VAT (£42 inc VAT).
Quick answer: renovate more than 50% of a wall, roof or floor — or more than 25% of the whole envelope — and if that element is worse than its Table 4.3 threshold 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 feasible or will not pay back within 15 years, upgrade to the best value that does, and record why. Not sure what the existing element achieves? RdSAP’s age-band defaults are the assessor’s fallback and a reasonable first estimate.
Why this catches people
The rule is not aimed at renovation as an activity. It exists because the moment somebody has a roof covering off or a scaffold up, the cost of adding insulation collapses — and if the opportunity is missed, the next one is thirty years away. So Part L attaches a duty to the moment rather than to the intention.
The practical consequence is that the duty usually arrives as a surprise, mid-job, from a building control officer, at the point where it is most expensive to satisfy. Everything below is cheap on a drawing and dear on a scaffold.
What counts as renovating a thermal element
Renovation, in Part L’s sense, means providing a new thermal layer, or replacing an existing one. In practice that is:
- stripping a roof and re-covering it
- applying new external render or cladding
- dry-lining a wall internally
- filling a cavity
- re-screeding a floor
It is not redecorating, repointing, or patching a small area. The distinction is whether you are re-forming the element, not whether you are spending money on it.
Then the size test. The duty bites when you renovate more than 50% of the surface area of that individual element — one elevation, one roof slope, one floor, assessed on its own rather than averaged across the house. A second test catches larger projects: work affecting more than 25% of the total building envelope brings every element in the envelope into scope. Renovating thermal elements works through the triggers in detail, and re-roofing covers the most common case on its own.
The values, and how to read them
| Existing element | Upgrade if worse than | Improve to |
|---|---|---|
| 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 |
All figures W/m²K, from Approved Document L Volume 1, 2021 edition, Table 4.3.
Read the first column as a test, not a target. An element already better than its threshold is left alone — a roof at 0.25 W/m²K being re-covered has no upgrade duty attached to it, because it is already better than 0.35. It is only when the existing element is worse than the threshold that the second column becomes the aim. And whatever happens, an element should not be left worse than 0.70 W/m²K, and never worse than it started.
Knowing which side of the threshold an existing element sits on means calculating it rather than guessing, which is what U-Monkey is for — and the same tool will tell you what a proposed upgrade actually achieves.
The 15-year payback escape
This is the clause that makes solid-walled and stone houses workable, and it is also the clause most often waved at building control without a number behind it.
If reaching the improved value is not technically feasible (the wall cannot carry it), not functionally feasible (the room becomes unusable, or the detail creates a problem), or not economically feasible — a simple payback longer than 15 years — then the duty changes. It becomes: upgrade to the best value that is feasible, and record the reasoning.
The economic test is arithmetic:
annual saving = (existing U − target U) × area × degree-days × 24 ÷ 1000 ÷ system efficiency × fuel price
simple payback = cost ÷ annual saving
Every input is arguable, and that is the point — degree-days, pence per kWh and boiler or heat-pump efficiency all move the answer, and mains gas gives a very different result from electricity. Building control may reasonably ask for your figures rather than accept a default. What they should not accept is the conclusion without the working. RenoMonkey runs the calculation, lets you change all three inputs, and prints the justification.
Three cases that are not renovation
Replacing an element outright. A wall taken down and rebuilt, or a new floor, is a new element, not a renovated one. It takes the Table 4.2 limiting values — wall 0.18, roof 0.15, floor 0.18 W/m²K — and must be no worse than what it replaced. The 50% trigger and the payback escape do not enter into it.
Retained elements joining the heated envelope. A garage conversion, a loft conversion, or any change of energy status brings walls, floors and roofs into the heated envelope that were previously outside it. These are treated as renovated elements and go to the Table 4.3 improved values whatever proportion is being worked on — the 50% trigger does not apply, and this is the case people most often assume they have escaped. Garage and loft conversions covers it.
Controlled fittings. Replacement windows and doors take 1.4 W/m²K or the equivalent energy rating, each on its own — they are not area-weighted in an existing dwelling. Where a building’s character genuinely requires the original windows, Approved Document L accepts a centre-pane U-value of 1.2 or better, or single glazing with low-emissivity secondary glazing, as a recorded case. Replacement windows and doors has the detail.
When there is an extension too
Plenty of jobs are both — an extension across the back and a re-roof over the rest, or a garage conversion that also gains a new side wall. The two halves are assessed against different tables, but they belong in one submission rather than two, and the extension side has its own rules. Building regulations for an extension sets those out; if the work is a conversion of a building to a dwelling rather than an alteration to one, SAP calculations for conversions is the route.
What changes on 24 March 2027
Every figure on this page is from Approved Document L Volume 1, 2021 edition incorporating the 2023 amendments, which governs work in England up to 24 March 2027. Work starting on or after that date falls under the 2026 edition published alongside the Future Homes Standard.
For renovation work the date that matters is when the work starts, so a job planned over that boundary is worth pinning down early.
The bottom line
Three questions, in order. Am I re-forming more than half of this element? Is it currently worse than its threshold? And if so, does the improvement pay back within fifteen years? Only if the first two are yes does the third one matter — and the third is a calculation, not an opinion. Work all three out before the scaffold goes up and the answer is cheap; work them out after the covering is back on and it is not.
Common questions
Do building regulations make you insulate when re-roofing?
They can. Stripping and re-covering more than half of a roof slope counts as renovating a thermal element. If the existing roof is worse than 0.35 W/m²K it should be improved to 0.16, unless that is not technically or functionally feasible or will not pay back within 15 years — in which case the duty is to reach the best value that is feasible, with the reasoning recorded.
What is the 50% rule in Part L?
Renovating more than 50% of the surface area of an individual thermal element — one elevation, one roof slope, one floor — triggers the upgrade duty for that element. A separate test catches work affecting more than 25% of the total building envelope, at which point every element in the envelope is caught.
What counts as renovating a thermal element?
Providing a new thermal layer, or replacing an existing one. In practice: stripping and re-covering a roof, applying new render or cladding, dry-lining internally, filling a cavity, or re-screeding a floor. Redecorating, repointing and patch repairs are not renovation, and neither is work to less than half the element.
What U-values does Part L want for renovated elements?
Approved Document L Table 4.3 sets a threshold and an improved value for each: roof, upgrade if worse than 0.35 to 0.16; cavity wall filled with insulation, worse than 0.70 to 0.55; wall with internal or external insulation, worse than 0.70 to 0.30; floor, worse than 0.70 to 0.25 W/m²K. Read it as a test, not a target — an element already better than its threshold is left alone.
What is the 15-year payback rule?
The economic feasibility escape. If the improvement would not achieve a simple payback of 15 years or less, the duty becomes to upgrade to the best value that does pay back, rather than to hit the improved value regardless of cost. It is a reasoned position that has to be shown with figures — the improvement in U-value, the area, degree-days, system efficiency and fuel price — not simply claimed.
Does a garage conversion have to meet Part L?
Yes, and more strictly than a renovation. A retained element that becomes part of the heated envelope — the garage wall, the ceiling that becomes a floor in a loft conversion — is treated as a renovated element whatever proportion of it you are working on. The 50% trigger does not apply; the upgrade duty is automatic.
What U-value do replacement windows need?
Replacement windows and doors are controlled fittings and take 1.4 W/m²K, or the equivalent window or doorset energy rating. Where a building's character genuinely requires the original windows, Approved Document L accepts a centre-pane U-value of 1.2 or better, or single glazing with low-emissivity secondary glazing behind it — recorded as a reasoned case rather than assumed.