Building Regulations for Rendering: The 25% Rule and When Building Control Needs to Know
Last updated: 13 October 2026
By Darren , Owner, Ace Rendering Specialists. Rendering homes across Kent and South East London since 1999.
Building regulations for rendering apply in England when a re-render covers more than 50% of one external wall. They also apply when the work renovates more than 25% of the whole building envelope. Above either threshold, the wall's insulation must be upgraded where feasible. The work must also be notified to Building Control.
Picture two quotes for the same house. One prices render only. The second mentions insulation and Building Control, and costs far more. This guide shows you which side of the threshold your job falls on. You will also see what a compliant quote should contain.
Most house rendering projects on older homes replace render that is cracked or blown. Replacing a layer is exactly what triggers the rule. Everything here applies to England as of October 2026. Wales, Scotland and Northern Ireland have separate rules.
Do you need Building Regulations approval to re-render a house?
Yes, in many cases. In England, Building Regulations apply when new or replacement render covers more than 50% of one external wall. The rules also apply when the work covers more than 25% of the building envelope. Small patch repairs and repainting are not covered. Garden walls are not covered either.
Work through five questions in order. A "no" at any step ends the check.
Is the wall part of the heated house? If no, the rule does not apply.
Is render being added or replaced? If no, the rule does not apply.
Is the area over 50% of that wall, or over 25% of the envelope? If no, no upgrade is required.
Does the wall miss the target U-value? If no, no extra insulation is expected.
Is the insulation upgrade feasible? If no, agree the best feasible U-value with Building Control.
Five "yes" answers mean you insulate the wall and notify Building Control.
Planning permission is a separate question about appearance. Check that with your council's planning team, especially in a conservation area.
What is the 25% rule for rendering?
The 25% rule is the common name for the Building Regulations requirement on renovating thermal elements. Regulation 23 applies when more than 25% of a building's envelope is renovated. Regulation 23 also applies when more than 50% of one wall is renovated. The whole wall must then be thermally upgraded where feasible.
Where does the law sit? Regulation 23(1) of the Building Regulations 2010 sets two tests. One test is a renovation covering more than 50% of an individual element's surface area. The other test is a "major renovation". Part 6 of the same regulations defines that term. A major renovation is one where more than 25% of the building envelope's surface area is renovated.
Confusion comes from official guidance. The Planning Portal says the regulations would normally apply where 25 per cent or more of an external wall is re-rendered. Treat a quarter of one wall as your cue to ask Building Control. Expect an upgrade requirement over half a wall, or over a quarter of the envelope.
Your building envelope is the outer shell of the heated house. Count the external walls, ground floor, roof, windows and doors together.
What counts as a thermal element?
A thermal element is a wall, floor or roof that separates a heated part of a building from the outside. Windows, doors and rooflights are not thermal elements. An external house wall is a thermal element. A garden wall, boundary wall or unheated garage wall is not a thermal element.
That definition comes from regulation 2(3) of the Building Regulations 2010. Walls next to the ground or an unheated space also count.
Thermal element
Not a thermal element
External wall of the house
Garden or boundary wall
Wall between the house and an unheated integral garage
Wall of a detached, unheated garage
Gable wall of a heated loft room
Party wall shared with a heated neighbour
Edge cases need a ruling. Ask Building Control about parapets, chimney stacks and conservatory dwarf walls.
What counts as renovation, and are repairs exempt?
Renovation of a thermal element means adding a new layer or replacing an existing layer. Rendering a bare wall counts as renovation. Hacking off old render and applying new render also counts. Repainting and decorative finishes are not renovation. Patch repairs below the 50% and 25% thresholds do not trigger an insulation upgrade.
Regulation 2 of the Building Regulations 2010 gives that definition and excludes decorative finishes. Approved Document L is the government's guidance on meeting the energy rules. Paragraph 11.2 of the 2021 edition lists rendering or cladding the external surface as renovation.
Job
Renovation?
Hack off old render and re-render
Yes
Render over bare brick
Yes
Fit external wall insulation
Yes
Repaint with masonry paint
No, a decorative finish
Patch repair under both thresholds
Yes, but no upgrade is required
Patches add up. Several render repairs on one elevation are measured together.
Thin coatings sit in a grey area. Building Control decides whether a product is a new layer or a decorative finish. Ask before you order an exterior spray wall coating or a CorkSol spray cork render for a large area.
How do you work out whether a rendering job is over 50% or 25%?
Measure the rendered area against two totals. First, compare the rendered area with the full area of that elevation. More than 50% triggers the insulation requirement. Second, compare all renovated areas with the whole building envelope. More than 25% of the envelope is a major renovation, which also triggers the requirement.
For external render, the element is the elevation the wall sits in. Approved Document L says so at paragraph 11.4 of the 2021 edition. You do not add all your walls together for the 50% test.
Here is the arithmetic for an example two-storey semi. The front and rear elevations are 30 m² each. The side elevation is 40 m². Add a 48 m² ground floor and a 48 m² ceiling. The envelope totals 196 m².
Job
Share of the elevation
Share of the envelope
Rule triggered?
12 m² patch on the front
40%
6%
No
Whole side wall, 40 m²
100%
20%
Yes, over 50% of the element
14 m² on each of three elevations
47%, 47% and 35%
21%
No, but close on both tests
Front, rear and side, 100 m²
100% each
51%
Yes, on both tests
Notice the second row. One whole side wall is only 20% of the envelope. That job still triggers the rule, because it covers 100% of its elevation. On most houses the 50% test bites first.
Openings can change the result. Ask Building Control how they treat windows and doors in the measurement.
What U-value does a re-rendered wall have to reach?
Re-rendered walls that trigger the thermal element rule should reach a U-value of 0.30 W/m²K. That target applies to walls insulated internally or externally. Walls suitable for cavity insulation should reach 0.55 W/m²K. Where the target is not feasible, the upgraded wall should generally be no worse than 0.70 W/m²K.
U-value is the rate of heat flow through a wall, measured in W/m²K. Lower numbers mean less heat loss.
Wall and insulation type
Target U-value (W/m²K)
Wall suitable for cavity insulation
0.55
Wall with internal or external insulation
0.30
General backstop where the target is not feasible
0.70
These figures come from Table 4.3 of Approved Document L, Volume 1, 2021 edition incorporating 2023 amendments. The 0.30 figure is the goal, not the bottom of a range. According to Tom Carruthers, Technical Manager at Mike Wye, in August 2026, "Table 4.3 is not a general permitted U-value range." His guide to Part L calls the improved value the normal target for a renovated element.
Why do walls get this attention? Around a third of the heat lost in an uninsulated home escapes through the walls, according to the Energy Saving Trust.
Insulation thickness comes from the system supplier's U-value calculation. Rules of thumb are not accepted as evidence.
When is insulation not required on a re-render?
Insulation is not required when the re-render stays under the 50% and 25% thresholds. Insulation is also not required when the wall already meets the target U-value. An upgrade can be reduced when the work is not technically, functionally or economically feasible. Listed buildings and conservation areas have special allowances.
Feasibility has three limbs under Regulation 23: technical, functional and economic. Approved Document L sets the economic test at paragraph 4.13. An upgrade that cannot pay back within 15 years is not economically feasible. You then upgrade to the best U-value that does pay back within 15 years.
Functional limits are common with external insulation. Examples include a wall on the pavement line, shallow eaves and narrow side access.
Any lesser standard is agreed with Building Control and written down. Neither you nor your contractor can simply declare the job exempt.
Does the rule apply to listed buildings and conservation areas?
Listed buildings and homes in conservation areas need not comply fully. The allowance applies where compliance would unacceptably alter the home's character or appearance. Older walls built with lime mortar or lime render need extra care. Approved Document L says to improve them only where that will not cause long-term damage.
What does Building Control expect, and who notifies them?
Qualifying re-renders must be notified to Building Control before work starts. Notification is usually made through a building notice or a full plans application. The owner is ultimately responsible, although the contractor often submits the paperwork. Building Control checks the insulation specification and issues a completion certificate at the end.
Building Control is the service that checks building work against the regulations. You have three routes.
Route
Who notifies
What you receive
Local authority building control
You or your contractor, with a building notice
Completion certificate
Registered building control approver
The private approver tells the council
Final certificate
Competent person scheme
A registered installer self-certifies the insulation work
Certificate of compliance
Expect checks on the measured areas, the wall build-up, the U-value calculation and the product certificates. Fees vary by council, so ask yours for the current charge.
Keep the certificate with your house documents. This guide is general information for England, and Building Control decides each case.
Why do some rendering quotes include insulation and others do not?
Rendering quotes differ because some contractors price only the render. Other contractors also price the insulation upgrade that Building Regulations can require. On a solid-walled house, re-rendering more than half an elevation can mean external wall insulation. A render-only quote for the same job may not comply.
External wall insulation (EWI) is a system of insulation boards fixed to the outside wall and finished with render. External wall insulation adds boards, fixings, extended sills and detailing around windows. That extra scope explains the price gap.
Quote type
What it includes
Meets the rule when
Paperwork to expect
Render only
Removal of old render, basecoat and finish coat
The job is under both thresholds, the wall already meets the target, or Building Control has agreed an upgrade is not feasible
None under the thresholds. Otherwise a building notice and written agreement
Thin thermal render or coating
A thermal coat a few millimetres thick
Building Control accepts the calculated U-value as the best feasible
Building notice and U-value calculation
External wall insulation
Insulation boards, fixings, reinforced basecoat, finish, extended sills and reveals
The system is designed to reach 0.30 W/m²K
Building notice or scheme certificate, U-value calculation, completion certificate
Scaffolding and wall preparation are already paid for on a re-render. That is why the regulation picks this moment to require insulation. Every insulated system still ends with a decorative coat, such as a silicone render finish.
Put these six questions to any renderer before you accept a quote:
What percentage of each elevation, and of the envelope, are you re-rendering?
Does the job trigger Regulation 23, and why?
What is the wall's U-value before and after, and who calculated it?
Who notifies Building Control, and is the fee included?
If insulation is left out, which feasibility reason applies, and has Building Control agreed it?
Which documents will I receive at the end?
What happens if you re-render without telling Building Control?
Re-rendering above the 50% or 25% thresholds without sign-off breaches the Building Regulations in England. The local authority can require the work to be altered or removed. A notice can be served up to 10 years after completion. Missing completion certificates also cause delays when a home is sold.
Section 36 of the Building Act 1984 gives that power. Owners get 28 days to comply. After that, the authority can do the work itself and recover its costs. The Building Safety Act 2022 extended the time limit for a section 36 notice to 10 years.
In reality, problems usually surface at sale. A buyer's solicitor asks for the completion certificate. Without one, you may need a regularisation certificate from the council or an indemnity policy.
Is the rendering rule changing in 2027?
Approved Document L was reissued in a 2026 edition, published on 24 March 2026. The 2026 edition takes effect on 24 March 2027 for most domestic work. Until then, the 2021 edition with 2023 amendments applies. The 50% and 25% triggers come from Regulation 23, which the new edition does not replace.
Will the numbers change? No. Table 3.3 of the 2026 edition of Approved Document L keeps 0.30, 0.55 and 0.70 W/m²K for renovated walls. Only the references move, so Table 4.3 becomes Table 3.3.
Jobs that span the changeover depend on transitional arrangements in Circular Letter 01/2026. Ask Building Control which edition applies to yours.
How Ace Rendering handles Building Regulations on a re-render
Ace Rendering Specialists is a family-run rendering company based in Maidstone, Kent. We have rendered and insulated homes with our own team since 1999. On a re-render, our process is:
Survey.
Quote.
Options.
Notification.
Handover.
You can see finished projects in our rendering and insulation case studies.
Key takeaways
Measure first. Compare the area being rendered with 50% of that elevation and 25% of the whole envelope. Over either line, expect a 0.30 W/m²K target unless Building Control agrees otherwise. Get the notification route and the final certificate confirmed in writing before work starts.
Still choosing a system? Our guide to render types compares the main finishes.
Not sure which side of the threshold your home falls on?
Ace Rendering Specialists offers free surveys across Kent and South East London. We measure each elevation, tell you whether the thresholds are crossed, and quote on that basis. Book a free survey or call 01622 947 167.
Frequently asked questions
Do I need Building Regulations approval to re-render my house?
Often, yes. In England, approval is needed when replacement render covers more than 50% of one external wall. Approval is also needed when the work covers more than 25% of the building envelope. Small patch repairs and repainting are not covered. Garden walls and unheated outbuildings fall outside the rule.
Do I have to add insulation when I re-render?
Insulation must be added when re-rendering covers more than 50% of an external wall in England. The same applies above 25% of the building envelope. No upgrade is needed if the wall already achieves 0.30 W/m²K. The upgrade is only required where it is technically, functionally and economically feasible.
Does a render patch repair need Building Control?
A render patch repair does not need Building Control when it covers 50% or less of that wall. The total renovated area must also be 25% or less of the building envelope. Several patches on one elevation are added together. Measure the combined area before assuming the work is exempt.
Who is responsible for notifying Building Control about rendering work?
The property owner is ultimately responsible for compliance with Building Regulations in England. In practice, the contractor usually submits the building notice. An installer registered with a competent person scheme can self-certify insulation work instead. Confirm in writing who is notifying Building Control before you accept a quote. Keep the completion certificate.
What happens if I re-render without Building Regulations approval?
The local authority can serve a notice requiring non-compliant work to be altered or removed. That notice can be served up to 10 years after the work is completed. Missing certificates also surface when you sell. Buyers' solicitors may ask for a regularisation certificate or indemnity insurance.