Window U value regulations for replacements in 2026

1.4 W/m²K. If you are replacing a window in an existing home in England, that one figure sits at the centre of the window U value regulations. It is a ceiling, not a target. On the U-value route, the new window may come in at that figure or below it, but not above it; the alternative route is a Window Energy Rating of band B or better. Plenty of quotes present it as something to be reached. It is closer to a pass mark, set in 2022 in Approved Document L, that most decent units now clear with room to spare.

The confusing part in 2026 is the noise around that number. The Future Homes Standard, a new calculation method for new homes, and figures of 1.2 and 0.8 are all in circulation. Some of this applies to the window in your front bay. Much of it does not. This guide sorts the figures into three groups: the ones that bind a replacement, the ones that belong to new-build design, and the ones borrowed from Passivhaus marketing.

What 1.4 W/m²K actually measures

A U-value is a heat transfer coefficient. It tells you how many watts pass through one square metre of the element for every degree of temperature difference between inside and outside. It is not thermal conductivity, which is a property of the material itself (lambda) and uses a different unit. People confuse the two all the time, and the mix-up matters when a sales sheet quotes the wrong one.

Put numbers on it. At 20 °C indoors and 0 °C outside, a window at 1.4 loses about 28 W per square metre. A typical 1.2 m by 1.2 m casement is roughly 1.44 m², so that is around 40 W leaving through that one opening, continuously, while the cold lasts. At Uw 0.78 the same window loses about 22 W.

Two consequences follow. Lower is better, which is the reverse of most ratings people are used to. And the regulations set a maximum. The question many people type into a search engine, "what is the minimum U-value for a window", has the logic upside down. The minimum standard is a maximum U-value (or, on the other route, a rating of at least band B), and nothing stops you fitting something far better than the limit.

The harder question is which figure a quote is showing you. Four turn up regularly:

Figure	What it covers	Weak spot	Where you meet it
Ug	The centre of the sealed glass unit	Ignores the frame and the edge of the glass, so it flatters the window	Glass specifications, some quotes
Uw	The whole window: glass, frame, spacer and edge effects	Changes with size, how much of the window is frame, and the spacer at the edge of the glass (a warm-edge spacer loses less heat there than an aluminium one)	The figure Part L refers to
Window Energy Rating band	A balance of heat loss, solar gain and air leakage, shown as a letter	A band, not a U-value, so two B-rated windows can behave differently at the edges	Labels from a certification scheme such as BFRC; Building Control may accept a band from such a scheme as the alternative route for replacements
US-style figure	The whole window, in imperial units	Not comparable with UK figures without conversion, and tested under different conditions	Imported product labels, American websites

For a replacement window, Uw is the number that counts. If a quote gives only Ug, you do not yet know whether the window complies.

Window U value regulations for a replacement window, in one table

England's requirements for energy efficiency sit in Approved Document L, Volume 1 for dwellings. The edition in force during 2026 is the 2021 edition incorporating the 2023 amendments; a 2026 edition takes over on 24 March 2027 for most work. You can read the limiting values yourself in Approved Document L on the UK government's publications page. The section on work to existing dwellings is short and repays five minutes of reading before you talk to anyone selling windows.

Situation	Figure that applies	Where it comes from
Replacing a window in an existing home in England	Uw no worse than 1.4 W/m²K, or Window Energy Rating band B as the alternative route	Approved Document L, Volume 1, Table 4.2 and paragraphs 10.3 and 10.4
New window in an extension to an existing home	The same 1.4 limit, or band B, per window, as long as all windows, rooflights and doors together stay within 25% of the extension's floor area, plus the area of existing windows and doors that the extension removes or covers. If not, there are two alternatives: an area-weighted U-value for the extension's elements no worse than that of a compliant extension of the same size and shape, or a SAP comparison of the house with its extension against the house with a notional extension	Approved Document L, Volume 1, paragraphs 10.7 to 10.10
New home	1.2 is the window value in the notional dwelling of the 2021 edition, used to set the target; separately, the windows must not exceed 1.6 as an area-weighted average. Both the overall target and that limiting value have to be met, and meeting the target can require windows well below 1.6	Approved Document L, Volume 1, 2021 edition, Table 1.1 and Table 4.1; SAP now, with the Home Energy Model added in the 2026 edition
Listed building, conservation area	Energy requirements applied as far as reasonably practicable without harming the character of the building	Approved Document L, Volume 1, paragraphs 0.8 to 0.12, plus listed building consent or planning permission where required
Wales, Scotland, Northern Ireland	Separate regulations and guidance documents	The relevant national guidance; do not assume England's figures carry over

Table checked on 9 October 2026 against Approved Document L, Volume 1, 2021 edition incorporating 2023 amendments. The replacement and extension rules were also checked in the 2026 edition as corrected on 8 September 2026; that edition publishes its notional dwelling separately, so the 1.2 figure here is the 2021 value.

That third row explains most of the confusion about 1.2. It is a new-build design value. It is not the limit for replacing a window in the house you already live in.

The fourth row deserves its own reading. Where a building is listed, listed building consent usually decides the outcome before any energy figure comes up, and a like-for-like replacement may not be allowed at all. In a conservation area, ask the council first: an Article 4 Direction can bring window changes under planning control. What conservation officers tend to accept is covered in our guide to listed building window replacement.

So what does 2026 actually change?

On paper, a great deal is moving. In practice, the picture looks different once you narrow it to a single replacement window.

The Future Homes Standard is aimed at new homes. The government consultation published in December 2023 concentrated on new build: low-carbon heating, the fabric specification of the notional dwelling, and replacing SAP with a new calculation engine, the Home Energy Model. Those changes reshape how architects and developers prove a new house complies. They do very little to a homeowner swapping a rotten casement for a new one.

The 2026 edition of Approved Document L takes effect on 24 March 2027 for most building work in England (higher-risk building work follows on 24 September 2027), and it keeps the 1.4 W/m²K limit for a replacement window in an existing English home (Table 3.2), with Window Energy Rating band B still accepted as the alternative route. The text, as corrected in September 2026, is on the UK government's Approved Document L (2026) page. Two practical cautions follow.

• Check which edition applies to your job. Under Circular Letter 01/2026, the new standards do not apply to ordinary building work where a building notice, initial notice or full plans application went in before 24 March 2027, provided the work starts before 24 March 2028; higher-risk buildings have separate dates. For a replacement window the limit is the same either way. Confirm the dates with your installer or Building Control rather than relying on an article, this one included.
• Treat 0.8 as a design choice. A whole-window figure around 0.8 is Passivhaus-grade component territory. It is a sensible target for a very well insulated house. It is not the legal limit for replacing a window in an existing home: the current edition and the 2026 edition both set 1.4 on the U-value route, or band B on the rating route.

If a salesperson tells you a figure is now the law, ask for the paragraph. A real requirement has one.

Why the brochure figure and your window disagree

Uw depends on size. Manufacturers quote it for a reference window set out in the calculation conventions, and real windows are rarely that size. The frame usually performs worse than the glass, so any window with more frame in proportion to its glass does worse than the brochure. A narrow top-light or a small toilet casement can come out noticeably above the headline value. A large fixed pane can come out below it.

This can come as a surprise. Same glass, same profile, three different Uw figures across one house, simply because the openings differ.

Consider two quotes side by side, one listing 1.1 and the other 1.3. The 1.1 turns out to be the centre-pane Ug of the glass unit. The 1.3 is the whole window. The quote that looked worse may well have described the better window; you cannot tell until both state Uw. Nobody lied, exactly; one figure simply answered a question the regulations do not ask.

For reference, the ECO 92 profile system we build at the factory is declared at Uw 0.78 with triple glazing and Uw 0.67 with quadruple glazing; the value for a particular window depends on its size and configuration. The point of quoting them here is the label, not the number: both are whole-window values, and whole-window values are the ones to line up against each other and against 1.4.

One more thing sits outside Uw entirely: the joint between the frame and the wall. A U-value describes the window as a product. How it is packed, sealed and insulated into the opening is a separate question, and a poor installation can undo much of a good figure. Approved Document L asks for two things here: the new unit should be draught-proofed, and insulated cavity closers fitted where appropriate (paragraph 10.3).

If you want to see how size, glazing and opening type move these numbers, the window configurator lets you build a unit and compare options before anyone comes out to measure.

Who signs off a replacement window

A replacement window is a controlled fitting under the Building Regulations, so compliance has to be shown in one of two ways. Either the work is done by an installer registered with a competent person scheme such as FENSA or Certass, schemes through which installers in England and Wales self-certify their own work, or you notify Building Control (the council or a registered building control approver) before the work starts and pay for them to inspect it. Scheme registration covers the installation business and its work; it does not certify the window product. One exception: in a higher-risk building, a block with at least two homes that is 18 metres or seven storeys high, the Building Safety Regulator takes the place of the council and approvers, so check that the installer's scheme covers work on such buildings. The Planning Portal's page on doors and windows sets out both routes.

The certificate matters later, when you sell. A buyer's solicitor will usually ask for it, and missing paperwork for recent windows can hold up a sale while someone arranges retrospective approval.

For windows made outside the UK the same routes apply, and so does the evidence: from the manufacturer you need a declared whole-window U-value in W/m²K, or a Window Energy Rating from a certification scheme, in writing. Your installer or Building Control will want to see it. For a replacement, the regulations accept a value calculated for a standard reference window, 1.23 m wide by 1.48 m high; figures for the sizes you are actually ordering, where the manufacturer can calculate them, tell you more. How a full swap is organised, from survey to fitting, is on our window replacement service page.

Trickle vents: the ventilation clause that comes with it

Replacing windows also brings in Part F, which deals with ventilation. The principle is simple: after the work, the home must be no worse ventilated than before. If the old windows had background ventilators (trickle vents), the new ones need them too, no smaller than the originals and controllable by the occupant or automatically. If they had none, the guidance assumes the new windows will make the house more airtight, so unless it has MVHR you have to show ventilation has not got worse. The usual way is trickle vents in the new windows: at least 8000 mm² equivalent area in each habitable room and the kitchen and 4000 mm² in a bathroom, or, with continuous mechanical extract ventilation, vents in the replacement windows outside wet rooms with at least 4000 mm² in each habitable room. Where those areas are not technically feasible, the vents should come as close to them as is feasible. Another arrangement can work if Building Control accepts it. These are equivalent areas, not free areas, so check which one a vent's data sheet quotes. The detail is in paragraphs 3.14 and 3.15 of Approved Document F, Volume 1, on the UK government's publications page.

This is where a lower U-value and a tighter house can work against each other. Better windows keep more heat in and let less air out by accident, so the deliberate ventilation has to do more of the work. Specify the vents at the same time as the glass, not afterwards.

When a lower number is not worth the money

Clearing 1.4 is easy. Going well below it costs money, weight and sometimes light, and the return depends on the rest of the house.

Run the arithmetic for a modest home with 10 m² of windows. Going from 1.4 to 0.8 cuts heat loss through them by 0.6 W per square metre per degree: about 120 W at a 20 degree difference. That is on the coldest days. Averaged over a heating season, the saving is much smaller. In a solid-wall Victorian terrace with an uninsulated loft, the windows are a modest share of the problem.

Air leakage is a second blind spot. A U-value says nothing about air getting round the frame. Air leaking through a gappy opening carries heat away that no U-value accounts for, so a 0.8 window fitted badly can still leave a room feeling colder than its figure suggests. If a room feels cold near the glass, find out where the air is moving first; our guide to draughty windows covers the test.

Then there is weight. Triple glazing adds a third pane of glass, which matters on tall opening sashes and on older openings that were never designed for heavy units. Depending on the glass specification, it can also trim solar gain and a little daylight. For a fourth pane, the returns shrink further; the numbers are set out in triple vs quadruple glazing.

Where the lower figure does pay is comfort. A better window has a warmer inner surface, so sitting next to it feels less chilly and the risk of condensation on the glass drops. It lowers that risk. It does not replace heating and ventilation.

What we would pick, case by case

Your situation	What we would choose	Why, in one line
Like-for-like swap, cavity-wall semi, gas boiler	Triple glazing if the budget allows; otherwise a good double-glazed window that clears 1.4 at your actual sizes	The limit is easy to meet; comfort near the glass is what you notice day to day
House being prepared for a heat pump	Triple glazing worth pricing, with the window figure set by the heat loss calculation for the whole house	Low flow temperatures reward low heat loss at the edges of the room
Solid-wall terrace, not listed, tight budget	Good double or triple glazing, with money kept back for fitting and sealing	Walls and draughts account for more of the bill than the windows
Listed building or conservation area	Whatever the officer will accept, often repair plus secondary glazing	Consent, where it is needed, comes before any U-value
Extension with large glazed areas	Triple glazing and an early calculation	Total glazed area, not a single window, decides compliance
Quadruple glazing for a replacement	Rarely	Returns fall away after the third pane in most UK homes

Questions people ask about Part L and window figures

My quote shows a U-value of 0.26. Is that good?

Probably not as good as it looks. A figure like 0.26 is almost certainly in imperial units from a US-style label, which converts to roughly 1.48 W/m²K and would sit just above the 1.4 limit. The test conditions also differ, so the conversion is only approximate. Ask the supplier for the whole-window Uw in W/m²K for your sizes before comparing it with anything.

Does the 1.4 limit apply to the glass or to the whole window?

To the whole window. The limit refers to Uw, which includes the frame, the spacer and the edge of the glass unit. A centre-pane glass figure (Ug) will usually be lower, so a quote showing only Ug does not tell you whether the window complies.

Does a new window in an extension have to meet 1.2 or 1.4?

Under the current edition, new windows in an extension to an existing home are generally held to the same 1.4 limit as replacements. The 1.2 figure belongs to new-home design. An extension with a lot of glazing relative to its floor area may need a calculation showing the whole extension performs well enough, so raise it with Building Control early.

Are the figures the same in Wales and Scotland?

Not necessarily. Wales, Scotland and Northern Ireland each have their own regulations and guidance, and the numbers and routes to compliance can differ from England's. Check the national documents for your address rather than applying the English table.

Before you sign an order

• The quote states Uw for the whole window, in W/m²K, not only the Ug of the glass, and says whether it was calculated for the standard reference window or for your actual sizes. Actual-size figures tell you more.
• Every window, including the small ones, comes in at or below 1.4, or meets the rating route your Building Control accepts, with some margin.
• You know who certifies the work: a registered installer's self-certification or a Building Control notice, and who pays for it.
• Background ventilation is settled. Where the old windows had trickle vents, the new vents are at least as large and can be controlled. Where they had none and the house has no MVHR, the new windows carry the equivalent areas Approved Document F gives: 8000 mm² in each habitable room and the kitchen and 4000 mm² in a bathroom, or, with continuous mechanical extract ventilation, vents in the replacement windows outside wet rooms with at least 4000 mm² in each habitable room. Where those areas are not technically feasible, the vents come as close to them as is feasible. Any other arrangement has Building Control's agreement.
• If the house is listed or in a conservation area, you have checked with the council and any consent needed is in hand before anything is ordered.
• If a figure on a specification does not add up, send it to us and we will go through it line by line.

Written by the joinery team at our Kalnciems factory