A house extension in London must meet Approved Document L, Volume 1: a maximum U-value of 0.15 W/(m²·K) for a new roof, 0.18 for a wall, 0.18 for a floor and 1.4 for a window. The 2026 edition, which carries the Future Homes Standard, takes effect on 24 March 2027 and leaves those four extension figures unchanged.
Part L is a building regulations requirement, checked by a building control body rather than the planning team, and it applies across England. Every London borough works to the same figures. Planning permission, or permitted development, says nothing about whether your walls are insulated well enough.
Key Takeaways
- New extension elements must reach 0.15 (roof), 0.18 (wall), 0.18 (floor) and 1.4 (window) W/(m²·K). Rooflights are 2.2. The 2021 and 2026 editions give the same values.
- Glazing in an extension may not exceed 25% of its floor area, plus the area of any openings the extension removes or covers.
- The Future Homes Standard targets new dwellings. Its renewable electricity requirement does not apply to extensions, but the 2026 document still replaces the 2021 one for extension work from 24 March 2027.
- A building notice, initial notice or full plans application given before 24 March 2027, with work starting before 24 March 2028, keeps the earlier rules for that building.
What U-values must a new extension achieve?
New thermal elements in an extension must not exceed 0.15 W/(m²·K) for the roof, 0.18 for walls, 0.18 for the floor, 2.2 for rooflights and 1.4 for windows (or Window Energy Rating Band B). A U-value measures heat loss through a surface, so a lower number means better insulation. These limits come from Table 3.2 of the 2026 edition.
Doors with more than 60% of their internal face glazed need 1.4 or a Doorset Energy Rating of Band C; other doors need 1.4 or Band B. The 2021 edition's Table 4.2 carries identical figures, which we checked line by line against the 2026 table. A replacement element must meet the table and also be no higher in U-value than the element it replaces (paragraph 3.9).
| Element | Extension or replacement element (Table 3.2) | New dwelling limit (Table 3.1) |
|---|---|---|
| Roof | 0.15 | 0.16 |
| Wall | 0.18 | 0.26 |
| Floor | 0.18 | 0.18 |
| Window | 1.4, or Window Energy Rating Band B | 1.6 |
| Rooflight | 2.2 | 2.2 |
W/(m²·K), from the 2026 edition of Approved Document L, Volume 1.
The right-hand column is the surprise: an extension is held to tighter limits than a new house for roofs, walls and windows. A new dwelling is judged against whole-house targets for primary energy, emissions and fabric efficiency, and the document notes that some elements will need to beat the limiting values to hit them. An extension is judged element by element, so the limits do the work.
Two relaxations appear in the notes. Where insulating a wall to 0.18 would cut a room's internal floor area by more than 5%, a lesser provision may be appropriate. And an extension floor's U-value may be calculated using the exposed perimeter and floor area of the whole enlarged dwelling.
How much glazing can an extension have?
The windows, roof windows, rooflights and doors in an extension must not exceed 25% of the extension's floor area, plus the area of any windows and doors that no longer exist or are no longer exposed because of it. That is paragraph 10.8(d) of the 2026 edition, and paragraph 10.7(d) of the 2021 edition says the same.
A 20m² rear extension therefore allows 5m² of glazing. If it swallows an old kitchen window and back door totalling 3m², the allowance rises to 8m². Bifold and sliding doors use that up quickly, so count before the design is fixed.
Exceeding the limit is not forbidden. The area-weighted method (paragraph 10.9) accepts an extension whose average U-value is no worse than a compliant one of the same size and shape, so more glass can be paid for with better insulation. The SAP method (paragraph 10.10) compares the whole dwelling plus extension against the dwelling plus a notional compliant extension.
What is the Future Homes Standard, and what happens on 24 March 2027?
On 24 March 2027 the Building Regulations etc. (Amendment) (England) Regulations 2026 come into force, together with the 2026 editions of Approved Documents L and F. They add requirement L3, on-site renewable electricity for newly erected dwellings, and retitle Part L as energy and greenhouse gas emissions. Higher-risk building work follows on 24 September 2027.
The date is set in regulation 1 of S.I. 2026/335 and repeated in MHCLG Building Circular 01/2026, which says the aim is homes that need no retrofit to reach zero carbon as the grid decarbonises. The 2021 Approved Document L is withdrawn from the same date, subject to transitional provisions.
Does the Future Homes Standard apply to an extension?
Largely not. Requirement L3 applies when a building is erected, and the 2026 document states that its renewable electricity guidance, paragraphs 5.69 to 5.78, does not apply when a dwelling is extended. The whole-house targets in Sections 1 and 2 are for new dwellings. Extensions are governed by Section 10, in the new edition.
A new-build estate faces a changed standard; a London rear extension faces the same four insulation figures it faced in 2026. What changes is the document your building control body cites. Heating, hot water or on-site generation you provide or extend should comply with Sections 4 and 5 (paragraph 10.11). We did not compare those sections line by line between editions, so if your extension includes new heating, read them in the 2026 edition.
Which edition applies to my project?
The earlier rules continue for a building if a building notice, initial notice or full plans application is given before 24 March 2027 and the building work starts before 24 March 2028. Everything else falls under the 2026 rules. Protection is assessed building by building, not by project or site.
That wording comes from regulation 5 of the 2026 Regulations and paragraph 16 of the circular. Full plans lodged in February 2027 with a start on site that autumn are protected; the same scheme lodged in April 2027 is not. Because extension values are identical, the protection is worth less to an extension than to a new house, but your drawings must still name the correct document.
Which extensions are exempt or treated differently?
A ground-level conservatory or porch is exempt from the energy efficiency requirements if its floor area does not exceed 30m², its glazing meets Part K, any wall, door or window separating it from the house is retained or replaced, and the dwelling's heating does not extend into it. Break any condition and it is treated as an extension.
Carports open on at least two sides, covered yards and covered walkways of up to 30m² at ground level are exempt on the same basis. In London, listed buildings and conservation areas matter too: work need not comply fully where that would unacceptably alter character or appearance, and the building control body should take the conservation officer's advice. A new extension to a historic or traditional dwelling should still comply fully unless matching the host building requires otherwise. Our London borough pages name the Article 4 areas for each borough that has one.
How does Part L show up on your drawings and fee?
Part L compliance appears on construction drawings as the build-up of each new element, its U-value and a glazing schedule. Building regulations drawings from Fixed Fee Plans are £795, fixed. The building control charge is set separately by your council or a registered building control approver. The elemental method needs no energy modelling, only those build-ups and the glazing area.
If the extension opens up the existing house, beams and lintels need engineer's calculations, a separate service from £495. See the building regulations drawings page for stages, the house extensions page for the full sequence and the structural calculations service. Related guides cover Part A, when you need calculations and Part B fire safety for loft conversions.
Frequently asked questions
Will my extension need solar panels from March 2027?
No. Requirement L3 applies when a building is erected, and the 2026 edition says its guidance does not apply when a dwelling is extended.
Do I have to upgrade the rest of my house?
Only what the work touches. Existing fabric that becomes a thermal element because of the extension should meet Table 3.3 (paragraph 10.8(e)), which pairs a threshold with an improved value: 0.16 for a roof, 0.55 for an insulated cavity wall, 0.30 for a wall insulated internally or externally and 0.25 for a floor. If the existing dwelling exceeds 1000m² of total useful floor area, consequential improvements may be required (Section 12).
Which rules do I design to today?
The 2021 edition is in force until 24 March 2027, and the 2026 edition applies from then. A design that meets Table 3.2 meets the extension limits in both.
Sources
All retrieved 10 October 2026.
- Approved Document L, Volume 1, 2026 edition (Tables 3.1 to 3.3; paragraphs 0.9 to 0.16, 3.9, 10.1 to 10.12): https://assets.publishing.service.gov.uk/media/6a9a8da39a177a1decf97e0d/Approved_Document_L__Volume_1_Dwellings.pdf
- Approved Document L (2026) publication page: https://www.gov.uk/government/publications/approved-document-l-2026
- Approved Document L, Volume 1, 2021 edition incorporating 2023 amendments (Tables 4.2 and 4.3; paragraph 10.7): https://assets.publishing.service.gov.uk/media/662a2e3e55e1582b6ca7e592/Approved_Document_L__Conservation_of_fuel_and_power__Volume_1_Dwellings__2021_edition_incorporating_2023_amendments.pdf
- MHCLG Building Circular 01/2026 (commencement, transition, withdrawal of the 2021 edition): https://assets.publishing.service.gov.uk/media/69c2aa1ebd142c66ffe44373/FHBS_Circular.pdf
- S.I. 2026/335, regulations 1 and 5: https://www.legislation.gov.uk/uksi/2026/335/contents/made