Building regulations for replacement windows in older homes

Building regulations are a separate regime from planning permission and from listed building consent. They are administered by different people, they ask a different question, and a certificate under one is worthless as evidence about the others. For how the three fit together, start with the complete guide to conservation area windows; for the planning side, see do you need planning permission to replace windows in a conservation area?.

This page is deliberately precise about paragraph numbers, because most of the wrong information circulating about heritage windows and Part L consists of real quotations from documents withdrawn in 2022, or from the wrong nation’s guidance.

The statutory chain: why replacing a window is building work

Regulation 2(1) defines the key term — and note what it is:

“controlled service or fitting” means a service or fitting in relation to which Part G, H, J, L or P of Schedule 1 imposes a requirement

Building Regulations 2010, reg.2(1)

Part L imposes a requirement in relation to a window, so a window is a controlled service or fitting. There is no defined term “controlled fitting” anywhere in the Regulations. That phrase is an Approved Document term, and its only definition sits in the body text of Approved Document L Volume 1 at paragraph 10.6. Any page that tells you “the Building Regulations define a controlled fitting as…” is quoting guidance and calling it law.

Regulation 3(1)(b) makes providing one building work: “the provision or extension of a controlled service or fitting in or in connection with a building”. So fitting a replacement window is building work whether or not it needs planning permission.

Regulation 4 sets out what that work must achieve. Three sub-paragraphs matter:

• 4(1)(a): building work “shall be carried out so that … it complies with the applicable requirements contained in Schedule 1”.
• 4(3): on completion the building must comply with Schedule 1 or, where it did not comply before, be “no more unsatisfactory in relation to that requirement than before the work was carried out” — the provision that catches work making an existing shortcoming worse.
• 4(4)–(5), inserted for England by S.I. 2021/1391 with effect from 15 June 2022: where Part L imposes a requirement on the work, the building must on completion also comply with Part F, or be no more unsatisfactory than before. That is the legal reason trickle ventilation became a live argument on window replacements from mid-2022, and it operates independently of anything in Part L.

Note also regulation 4(2), which narrows work described in regulation 3(1)(g), (h) and (i) — where it is not a material alteration — down to Part L, plus regulation 7. A replacement window is regulation 3(1)(b), so 4(2) does not apply to it: Parts B, F, K, L and M are all in play on a window replacement, not just Part L.

The England standard: 1.4 W/(m²·K) or WER Band B

Approved Document L Volume 1: Dwellings, 2021 edition incorporating the 2023 amendments, took effect on 15 June 2022 and is the operative guidance in England today. Paragraph 4.7 says “new fabric elements in existing dwellings should meet the limiting standards in Table 4.2”, and paragraph 10.3 says that for new and replacement windows, roof windows, rooflights and doors, “if the entire unit of that fitting is provided”, the unit should be draught-proofed, meet Table 4.2, and have insulated cavity closers where appropriate.

Four things about that table are routinely got wrong.

It is an either/or. Table 4.2 reads “1.4 or Window Energy Rating Band B minimum”, and paragraph 10.4 confirms that building control bodies “may accept, as evidence of compliance with the standards given in Table 4.2, a Window Energy Rating … from a certification scheme that provides a quality assured process and supporting audit trail”. A window that fails on U-value can still comply on its WER, and vice versa.

Roof windows take the window figure, not the rooflight figure. Note 8 reads “Including roof windows and curtain walling.” Only a rooflight gets 2.2. Manufacturers use both words loosely; the Approved Document does not.

Thicker glass buys no relaxation. Note 7 provides that where thicker glass is needed for wind loading, safety, security or acoustic performance, “an equivalent window unit with standard thickness (6mm) glazing should be shown to meet the required standard”. Specifying laminated or acoustic glass is not a route around 1.4.

The timber concession expired on 14 June 2023. Note 9 allowed timber windows 1.6 W/(m²·K) or WER Band C “until 14 June 2023”, and said in terms that “from 15 June 2023 the full standard of 1.4 W/(m²·K) or Window Energy Rating Band B applies”. Note 13 did the same for timber doors. The concession is still quoted as current on a remarkable number of joinery pages, and in England it has been dead for three years. Wales had its own, differently dated transitional; see below. If you are specifying joinery, read timber windows for conservation areas with 1.4 in mind, not 1.6.

AD L Vol 1 2021, Table 4.2 and notes 7, 8, 9, 13; paras 4.7, 10.3, 10.4 — gov.uk

Ug, Uw and centre-pane: three different numbers

Nothing causes more confusion in heritage glazing than quoting one of these as if it were another. They are not interchangeable.

The gap between the first two columns is large, and it shows up in field data. Historic Scotland’s Technical Paper 9 monitored ten slim-profile and vacuum retrofit systems in category-B-listed Georgian tenements in Edinburgh: in-situ centre-of-pane U-values ran from 1.0 to 2.8 W/(m²·K), while the calculated whole-window figures for the same retrofits were 1.9 to 3.4 for a Georgian six-over-six and 1.4 to 3.0 for a Victorian one-over-one.

Historic Scotland Technical Paper 9 (September 2010), Reports 1 and 3 — Historic Environment Scotland

That is the practical problem the Approved Document has to solve: a slim-sightline timber sash with a period glazing-bar profile can rarely reach 1.4 as a whole window, however good the glass is.

Paragraph 4.10: the character fallback

Here is the answer, verbatim:

If windows or fully glazed external pedestrian doors cannot meet the requirements of Table 4.2 because of the need to maintain the character of the building, either of the following should be met. a. These fittings should not exceed a centre pane U-value of 1.2 W/(m²·K). b. Single glazing should be supplemented with low-emissivity secondary glazing.

AD L Vol 1 2021, para 4.10; “centre pane U-value” defined in Appendix A as “The U-value determined in the central area of the glazing unit, making no allowance for edge spacers or the window frame”

Because 4.10a disregards both spacer and frame, it is achievable where 1.4 is not — but it is not automatic, and the two thin-unit technologies do not perform alike. Vacuum insulated glazing clears 1.2 comfortably: current units publish centre-pane figures of 0.7 W/(m²·K) or better. Slim-profile double-glazed units frequently do not — Histoglass publishes 1.3 to 1.9 W/(m²·K) across its five standard build-ups, and the ten systems Historic Scotland tested carried manufacturers’ centre-pane figures of 1.3 to 2.6. Ask for the declared centre-pane figure of the unit actually being supplied before assuming route (a) is open.

Declared figures: AGC FINEO technical datasheet (U_g 0.7 W/(m²·K), EN 673); Histoglass thin double glazing options (1.3–1.9 across five build-ups); HS Technical Paper 9 Report 1, Table 1 for the ten tested systems

Route (b) is the reason secondary glazing appears in an energy document at all: single glazing plus low-e secondary is a complete compliance answer in its own terms.

Two points that are almost never made. First, in the 2021 edition paragraph 4.10 is not limited to designated heritage assets — the trigger is “the need to maintain the character of the building”, which can in principle be argued for an unlisted house outside a conservation area. Second, the fallback is engaged by the window’s inability to meet Table 4.2, so it presupposes that Part L applies at all. It is a different mechanism from the statutory carve-out, and mixing the two up is the commonest error in this subject.

Regulation 21(3)(a), read word by word

Regulation 21(1)–(2) applies the energy efficiency requirements to work to a building that is roofed, has walls, “uses energy to condition the indoor climate”, and does not fall within a category in paragraph (3). Paragraph (3)(a) reads:

buildings which are— (i) listed in accordance with section 1 of the Planning (Listed Buildings and Conservation Areas) Act 1990; (ii) in a conservation area designated in accordance with section 69 of [that Act]; or (iii) included in the schedule of monuments … where compliance with the energy efficiency requirements would unacceptably alter their character or appearance

Building Regulations 2010, reg.21(3)(a). Amended for Wales only on 4 November 2024 by S.I. 2024/924 reg.50, to refer to the Historic Environment (Wales) Act 2023; the England text still refers to the 1990 and 1979 Acts.

Now follow the definitional chain, because it decides exactly how much this removes. “Energy efficiency requirements” is itself defined in regulation 2(1) as “the requirements of regulations 23, 25A, 25B, 26, 26A, 26B, 26C, 28, 40 and 40A, and Part L of Schedule 1”.

The Approved Document restates the same test for dwellings at paragraphs 0.8 and 0.9. Paragraph 0.8 lists the three categories and says work to them “does not need to comply fully with the energy efficiency requirements where to do so would unacceptably alter the dwelling’s character or appearance”. Paragraph 0.9 is the half that gets left out:

Work to a dwelling in paragraph 0.8 must comply with the energy efficiency requirements where this would not unacceptably alter the dwelling’s character or appearance. The work should comply with standards in this approved document to the extent that it is reasonably practicable.

Under the separate heading “Historic and traditional dwellings”, paragraph 0.10 adds a fabric-risk limb that has nothing to do with designation: energy efficiency “should be improved only if doing so will not cause long-term deterioration of the building’s fabric or fittings”, and this “applies to historic and traditional buildings with a vapour permeable construction that both absorbs moisture and readily allows moisture to evaporate” — wattle and daub, cob, stone, lime render or mortar. Paragraph 0.11 requires extensions to comply fully “unless there is a need to match the external appearance or character of the extension to that of the host building”. Paragraph 0.12 is the one to quote in correspondence: the building control body “should consider the advice of the local authority’s conservation officer”. Paragraph 0.13 points to Historic England’s HEAG014 as second-tier guidance.

AD L Vol 1 2021, paras 0.8–0.13. Historic England, Energy Efficiency and Historic Buildings: Application of Part L (HEAG014, June 2025) — historicengland.org.uk

Three layers of relief, and the two opposite mistakes

Separating these makes the whole subject tractable.

Trade and advice pages then go wrong in both directions, in three recurring ways:

1. “Listed buildings are exempt from building regulations.” No. Only the energy efficiency requirements can go, and only conditionally.
2. “Conservation area status is an automatic Part L exemption.” No. The condition must be satisfied on the facts of this building and this work, and paragraph 0.9 expressly reimposes the duty where character would not be unacceptably altered. Northern Ireland’s guidance states the evidential burden most plainly of anywhere in the UK: the case “must be supported by evidence e.g. by restrictions imposed by the relevant planning authority, advice from the Department for Communities Historic Environment Division, or advice from a qualified conservation specialist.”
3. “It is only a soft relaxation in the Approved Document, so building control can still insist on 1.4.” Also no. Where its condition is met, regulation 21(3)(a) is a statutory disapplication, and a building control body cannot enforce a requirement that does not apply.

NI Technical Booklet F1 (June 2022) para 3.3 — finance-ni.gov.uk

Replacing only the glass: paragraph 10.6

This is the distinction that changes most people’s options, and it has an unusually clean answer:

The term controlled fitting refers to the entire unit of a window, roof window, rooflight or door, including the frame. Replacing glazing, or a window or door in its existing frame is not providing a controlled fitting. Such work does not need to meet the energy efficiency requirements.

AD L Vol 1 2021, para 10.6. Carried forward in substance into the 2026 edition, also at para 10.6.

Read it carefully, though, because it is narrower than it looks. It disapplies the energy efficiency requirements, not the Regulations generally. New glass in a critical location still engages Part K; work affecting an escape window still engages Part B. Whether glazing-only work is building work at all turns on regulation 3: it is not the provision of a controlled fitting, and it is a material alteration only if regulation 3(2)–(3) is engaged, which in an ordinary reglazing job it is not — so no notification arises. Where the work is building work, regulation 4(3)‘s “no more unsatisfactory” test still bites. The planning half of the same distinction is in double glazing in a conservation area.

FENSA, CERTASS and the three routes to compliance

The competent person scheme has a precise statutory basis and a precise statutory limit. Regulation 20(1) applies where the work consists only of work described in column 1 of the Table in Schedule 3, carried out by a person described in column 2. Schedule 3 item 10 is “Installation, as a replacement, of a window, rooflight, roof window or door in an existing dwelling”; the approved persons are registrants of Assure Certification, Blue Flame Certification, CERTASS, Certsure, Fensa Limited under the Fenestration Self-Assessment Scheme, and NAPIT Registration. Item 11 does the same for non-dwellings, excluding load-bearing or structural glass and curtain walling. Note the scope: item 10 is “installation, as a replacement, of a window”, so reglazing is not within it, and a scheme certificate does not purport to cover glazing-only work.

Regulation 20(2) defines exactly what a certificate proves: the authority “are authorised to accept, as evidence that the requirements of regulations 4 and 7 have been satisfied, a certificate to that effect by the person carrying out the work”. Regulation 20(3) requires the installer to give the occupier a copy within 30 days and to notify the building control authority. Regulation 20(5) provides that the certificate “shall be evidence (but not conclusive evidence) that the requirements specified in the certificate have been complied with”.

So using a registered installer is not a legal requirement. Regulation 12(6)(a) says a person is not required to give a building notice or full plans “where the work consists only of work … described in column 1 of the Table in Schedule 3 if the work is to be carried out by a person described in the corresponding entry in column 2”. The scheme is an exemption from notifying, not an obligation to join. That matters in heritage work, because the specialist joiner making windows to a conservation officer’s approved detail may well not be scheme-registered — and the answer is a building notice, not a compromise on the joinery.

Regularisation is the expensive fallback: regulation 18(3) lets the authority require the owner to take reasonable steps “including laying open the unauthorised work for inspection … making tests and taking samples”. On the conveyancing consequences of a missing certificate, see enforcement, time limits and selling the house.

Wales, Scotland and Northern Ireland

The four nations differ on the standard, on the mechanism, and on whether a self-certification scheme exists at all.

Two statutory points. Northern Ireland has a provision on all fours with England’s: regulation 38(2)(a) of the Building Regulations (Northern Ireland) 2012 provides that the energy efficiency requirements “shall not apply to … protected buildings, where compliance with the energy efficiency requirements would unacceptably alter their character or appearance”. Scotland does not have one that we could verify. The Scottish flexibility for listed buildings and conservation areas sits in the Technical Handbook — clause 6.2.7 requires “a flexible approach to improvement” while insisting that “in all cases the ‘do nothing’ approach should not be considered initially” — and not, so far as we found, as a disapplication in the Regulations themselves. We could not find a Scottish equivalent of regulation 21(3)(a), so we do not write that listed buildings in Scotland are exempt. Scotland also sets an absolute floor England does not: “Glazing with a U-value poorer than 3.3 should not be used” (clause 6.2.1).

One currency caveat on Northern Ireland. Technical Booklet F1 dates from June 2022, and the Department of Finance has consulted on amending the Part F guidance. We could not confirm whether a later booklet has been published or brought into operation, so treat WER Band C / 1.6 W/(m²·K) as the position as at the June 2022 edition and check before relying on it.

The Future Homes Standard: published, not in force, and not about your windows

The amending instrument is The Building Regulations etc. (Amendment) (England) Regulations 2026, S.I. 2026/335, published alongside MHCLG Building Circular 01/2026 on 24 March 2026. It renames Part L to “Energy and Greenhouse Gas Emissions”, inserts a new requirement L3 for on-site renewable electricity generation in new dwellings, and makes consequential changes. It comes into force on 24 March 2027, except for higher-risk building work on 24 September 2027, with transitional protection for work notified before 24 March 2027 and commenced before 24 March 2028.

What it does not do matters more. It does not amend regulation 21 — the heritage carve-out survives the Future Homes Standard untouched — and it does not amend Schedule 3. In the 2026 edition of Approved Document L Volume 1, Table 3.2 (the renumbered Table 4.2) still reads “Window — 1.4 or Window Energy Rating Band B minimum”, rooflights are still 2.2, and paragraph 10.6 on reglazing is unchanged. The expired timber transitional notes have simply been deleted.

One change runs against heritage retrofit. The 2021 paragraph 4.10 is triggered by “the need to maintain the character of the building”. The 2026 paragraph 3.11 adds a cross-reference: “because of the need to maintain the character of the building, as set out in paragraph 0.9” — and in the 2026 numbering paragraph 0.9 is the listed / conservation area / scheduled monument list (it is 0.8 in the 2021 edition; the whole heritage block shifts down one). From 24 March 2027 the centre-pane-1.2 route therefore appears tied to designated assets, so an undesignated but architecturally sensitive building will find it harder to rely on. That reading is ours, from the face of the two texts; MHCLG published no explanation in Circular 01/2026 or the consultation response, and we present it as a textual change, not a stated policy intent.

S.I. 2026/335; MHCLG Circular 01/2026; AD L Vol 1 2026, Table 3.2 and paras 3.11, 10.6

What this means in practice

1. Decide first which job you are doing. New glass in existing frames is outside the energy efficiency requirements. New units including frames is a controlled fitting and Part L applies. Nothing else here matters until that is settled.
2. If Part L applies, ask for the right number in writing. Either a whole-window U-value at or below 1.4 W/(m²·K) or a Window Energy Rating of Band B, with the certification scheme named. A centre-pane figure is not an answer to Table 4.2.
3. If the window cannot reach 1.4 without altering the building’s character, say so and use paragraph 4.10. Put the sightline, glazing-bar and rebate constraints in writing, and offer a centre-pane figure at or below 1.2, or single glazing plus low-e secondary glazing.
4. If you are arguing regulation 21(3)(a), build the evidence. Conservation officer correspondence, the conservation area appraisal, a planning refusal or a specialist report is what turns an assertion into a case on the statutory condition. Paragraph 0.12 invites building control to seek the conservation officer’s view.
5. Do not forget Part F. Regulation 4(4)–(5) has applied in England since 15 June 2022, so trickle ventilation is a legitimate question on a whole-window replacement even in a conservation area. It is not disapplied by regulation 21(3)(a).
6. Choose your compliance route deliberately. A registered installer is convenient; a building notice is the answer when the joiner you want is not registered; regularisation is what you are left with if you skipped both.
7. Keep the regimes separate in your paperwork. A FENSA certificate, a planning permission or lawful development certificate, and a listed building consent are three different documents. You may need all three, and holding one is no evidence about the others.

Claims to stop believing

1. “Listed buildings and conservation areas are exempt from building regulations.” Only the energy efficiency requirements, only conditionally.
2. “Timber windows only need 1.6 W/(m²·K) or WER Band C.” England: expired 14 June 2023. Wales: only where notice or full plans were given before 23 November 2023.
3. “You need 1.4 and Band B.” Table 4.2 says “or”; paragraph 10.4 confirms a WER may be accepted as evidence.
4. “Replacing the sealed unit triggers Part L.” Paragraph 10.6 says it is not providing a controlled fitting and does not need to meet the energy efficiency requirements.
5. “You must use a FENSA-registered installer.” Three lawful routes: regulation 20, a regulation 12(2) notification, or regulation 18 regularisation.
6. “Windows are thermal elements, so the 50% rule applies.” “Thermal element” expressly excludes windows, so regulation 23 does not apply to them.
7. “Part L 2026 is already in force.” 24 March 2027 in England for non-higher-risk work, 4 March 2027 in Wales — and the window figures do not change.

Terms of art are defined in the glossary, and every source is listed below and on the sources page. Where we could not verify something — Scotland’s statutory position, and whether Northern Ireland’s June 2022 booklet is still the current edition — we have said so rather than fill the gap.