Quick answer: On 8 September 2026 MHCLG re-issued the 2026 editions of Approved Document L Volume 1 (dwellings), Volume 2 (buildings other than dwellings) and Approved Document F Volume 1 to correct what Circular 04/2026 calls “minor errors”. The standard itself does not move and neither do the dates: the revised documents take effect on 24 March 2027, or 24 September 2027 for higher-risk building work, under the Circular 01/2026 transitional rules. But if you downloaded the March PDFs, replace them. The corrections touch heat-network sleeving, the Home Energy Model party-wall table, the window U-value convention for existing dwellings, a note to the limiting U-value table, the on-site generation guidance, and flexible ductwork in Part F.
What was published
The circular letter, signed by Keeran Jugdoyal, Deputy Director of MHCLG’s New Build Standards and Performance Division, confirms that the revisions are approved under section 6 of the Building Act 1984, that the revised PDFs are on GOV.UK now, and that hard copies from the publisher will follow. The Approved Document L (2026) page and the Approved Document F (2026) page were both updated the same day.
The revised PDFs are still labelled “2026 edition”, so the quickest way to tell which version you have open is Table 1.2 in Volume 1: if it lists biogas and biomass, you have the September version.
MHCLG also published a separate FAQ on requirement L3 on 8 September — the solar PV requirement. That is a bigger story for housebuilders and we cover it in its own piece.
Volume 1 (dwellings): the six changes
| Where | What changed | Why it matters |
|---|---|---|
| Table 1.2 — low carbon heat sources for a declaration of sleeving capacity | ”Biogas” and “biomass” added alongside “biofuels”; “waste heat” becomes “recovered heat” | Widens what a district heat network can count in a sleeving declaration |
| Paragraph 2.10b — heat network CO₂ and primary energy factors | New wording: account for the performance of the generation system the sleeving is applied to, and pro-rate distribution and storage losses by the sleeved share of heat | Sleeved heat carries a proportionate share of network losses — it is not the plant’s nameplate figure |
| Table 2.1 — HEM condition for party walls | The “unfilled cavity with effective sealing… for a concrete frame construction in a building containing flats” row changes from no heat loss to heat loss | A HEM input for flats; the sealed-but-unfilled concrete-frame party wall no longer gets a free pass |
| Paragraphs 3.4b and 3.4c — window U-values in existing dwellings | Standard window is 1.23m (±25%) wide × 1.48m (–25%) high, not ±25% high | The height tolerance runs downwards only |
| Note 4 to Table 3.1 — limiting U-values | ”(6mm)” inserted after “window unit with standard thickness” | Defines the equivalent unit when thicker glass is needed for other reasons |
| Paragraph 5.68 — on-site electricity generation | A sentence cross-referring other renewable technologies to paragraphs 5.73 and 5.74 is removed | 5.68 applies to all on-site generation; the L3 indicative outputs sit at 5.70 |
Sleeving: two of the six
“Sleeving” is the 2026 edition’s route for a dwelling connected to a district heat network to take credit for low-carbon capacity that the network operator has allocated to it. Under paragraphs 1.8 and 1.9, the operator makes a declaration of sleeving capacity to the building control body, validated by a suitably qualified third party, stating among other things that at least 90% of the heat delivered to the dwelling will come from new or unused capacity of the types listed in Table 1.2, and that any new capacity will be connected within five years of the as-built assessment. Only district networks can be sleeved; communal networks cannot.
The revised Table 1.2 now lists electrically powered heat pumps, recovered heat (including from power stations), geothermal heat, electric boilers, solar thermal, biogas, biomass and biofuels. The two biomass-family additions are the substantive change; “recovered heat” for “waste heat” is a tidy-up.
The new sentences in paragraph 2.10b matter more for the assessor, because they say how the sleeved heat’s factors are built up. The calculation should “take account of the performance of the heat generation system that sleeving is being applied to”, and the “performance of distribution circuits and storage should be applied on a pro-rated basis based on the proportion of heat being supplied under the sleeving arrangements compared to the heat network as a whole”. The existing note is unchanged: a sleeving declaration does not remove the need to calculate the CO₂ emission factor and primary energy factor for the heat.
The HEM party-wall row
Table 2.1 only applies when the approved methodology is the Home Energy Model. For a SAP 10.3 assessment, paragraph 2.12a still sends you to section 3.7 of the SAP specification for party wall U-values, and nothing there has changed. But HEM will arrive, and flats designed now will be assessed in it later. In the March edition an unfilled cavity party wall with effective edge sealing in a concrete-framed block of flats sat in the “no heat loss” column alongside the fully filled cavity. In the revised table it sits with the other unfilled cavities as a heat-loss condition. If the design intent was to rely on the sealed-but-unfilled wall in a HEM assessment, the choice now is to fully fill the cavity or to take the loss.
Windows in existing dwellings
Paragraph 3.4 lists the ways a window U-value may be determined for an existing dwelling: from the actual size and configuration, from a standard window with the actual configuration, from a standard window with a standard configuration, by hot-box test, or from the SAP 10.3 Table 6e defaults. The March edition gave the standard window as 1.23m (±25%) wide × 1.48m (±25%) high. The revision corrects the height to 1.48m (–25%): a standard-size calculation may be for a window up to a quarter narrower or wider, or up to a quarter shorter, than the reference — but not taller. Because a larger glazed proportion usually pulls the whole-window U-value down, an upward height tolerance would in most cases have flattered the declared figure. For new dwellings nothing changes: paragraph 3.2 already requires the U-value to be measured by hot-box or calculated for the actual size and configuration of the window, as we set out in our piece on window and doorset U-values under ADL 2026.
Note 4 to Table 3.1 is the companion correction. Where wind load, safety, security or acoustic performance require thicker glass, the equivalent window unit that has to be shown to meet the limiting U-value is now explicitly one with standard 6mm glazing. The limiting values themselves are unchanged: 0.16 for roofs, 0.26 for walls, 0.18 for floors, 0.20 for party walls, 1.6 for windows and doors, 2.2 for rooflights, and an air permeability no worse than 8.0 m³/(h·m²) at 50Pa (1.57 at 4Pa).
Paragraph 5.68
The sentence removed from paragraph 5.68 said that where another renewable technology is used to generate electricity, the amount produced by all technologies should be equivalent to the output of the standards in paragraphs 5.73 and 5.74. The circular’s explanation is that 5.68 applies to all installation of on-site renewable electricity generation — including replacement systems in existing dwellings under requirement L2 — and that guidance on indicative outputs for requirement L3 belongs, and remains, at paragraph 5.70.
Volume 2 (buildings other than dwellings)
Volume 2 receives the same corrections in its own numbering: Table 1.2 (biogas, biomass, recovered heat), paragraph 2.7b (the pro-rated sleeving sentence), paragraphs 3.2b and 3.2c (the 1.48m (–25%) standard window) and Note 8 to Table 3.1 (6mm standard glazing). Appendix G, the list of documents referred to, is also brought up to date: CIBSE TM39 Building Metering and Monitoring now cites the 2026 edition rather than 2009, and BSRIA BG 49/2024 Commissioning Air Systems replaces BG 49/2015. If your non-domestic commissioning or metering evidence still references the older editions, this is the moment to update the template.
Approved Document F, Volume 1
One change, in paragraph 1.79. The March text allowed a short length of flexible ductwork for the final connection to fan units only in centralised continuous mechanical extract and MVHR systems. The revision deletes that qualification, and the paragraph now reads: “Flexible ductwork should be limited to 200mm in length and used only for final connections to fan unit spigots.” That applies to every system type — intermittent extract fans and decentralised MEV included. The note that follows is untouched: where the fan’s automatic control relies on constant pressure in the ductwork, flexible ducting is not permitted anywhere in the system. Our earlier piece on what Part F 2026 expects by way of ductwork evidence still stands; this simply removes an inconsistency in it.
What to do
Replace the March PDFs with the September ones and check any internal checklist or template that quotes the affected paragraphs — the window convention for existing dwellings, the party-wall condition for HEM, and the sleeving table and factors are the ones most likely to have been copied out. Nothing here reaches back into a Part L 2021 job: the 2021 edition continues to apply to work notified before 24 March 2027 and commenced before 24 March 2028. For a scheme heading for the Future Homes Standard, the numbers in the SAP 10.3 model do not change, but the evidence you present alongside it should now cite the revised text.
Sources: Amendments to Approved Documents L and F, Circular 04/2026 — letter (MHCLG, 8 September 2026) · Approved Document L (2026), revised 8 September 2026 (MHCLG) · Approved Document F (2026), revised 8 September 2026 (MHCLG) · The Future Homes and Buildings Standards: Building Circular 01/2026 — letter (MHCLG, 24 March 2026)