Larger windows let in more daylight — but they also let in more heat. Since June 2022 every new home in England has had to satisfy Approved Document O, the building regulation that limits summer overheating, at the same time as meeting the daylight expectations set out in BS EN 17037 and BRE BR 209 (2022). For schemes coming forward in 2026, getting that balance right is the difference between a comfortable, compliant home and one that either feels gloomy or bakes every July.
This article explains where daylight ambitions and overheating controls pull against each other, what changed (and what pointedly did not) in the March 2026 Future Homes Standard package, and how to specify glazing that satisfies both a daylight assessment and Part O on the same drawing.
Why daylight and overheating pull in opposite directions
Good internal daylight is largely a function of glazing area, glazing position and the visible light it transmits. Make windows bigger, set their heads higher and use clearer glass, and the daylight factor in the room goes up. The trouble is that the same moves also increase solar gain — the heat that arrives with the sunshine. A south- or west-facing room with generous glazing can be a delight in March and an oven in July.
The lever designers reach for is the glass specification, in particular its g-value (solar factor), which is the fraction of solar energy the glazing lets through. A low g-value cuts unwanted summer heat, but it also reduces useful winter solar gains and dims the daylight reaching the back of the room. Push the g-value too low to win the overheating argument and you can quietly lose the daylight one. That is the core tension this post is about, and it is why daylight design and overheating design should never be done in isolation.
What Part O actually requires
Approved Document O gives guidance on meeting Requirement O1 of Schedule 1 to the Building Regulations: reasonable provision must be made to limit unwanted solar gains in summer and to provide an adequate means of removing heat from the indoor environment. Crucially, mechanical cooling is treated as a last resort — it may only be relied upon where passive measures cannot reasonably do the job.
Compliance can be demonstrated by one of two routes:
- The simplified method. This sets limits on the proportion of glazing by orientation and requires a minimum free area of openable window for purging heat. It is quick, but deliberately conservative, and it can be hard to satisfy on a glazed, single-aspect flat.
- The dynamic thermal modelling method. This follows CIBSE TM59 and simulates the building hour by hour against defined weather data, testing whether bedrooms and living spaces stay within comfort thresholds. It is more work, but it gives credit for shading, thermal mass, cross-ventilation and clever glazing choices that the simplified method ignores.
For a daylight-led design with large windows, the dynamic route is very often the one that lets you keep the glazing you want — because it can demonstrate that shading and ventilation, rather than shrinking the windows, control the heat.
What changed in 2026 — and what did not
The headline of 2026 is the Future Homes Standard, the package of changes set in England on 24 March 2026 and aimed at new homes producing roughly 75–80% less carbon than those built to earlier standards. The fabric and heating provisions of Approved Document L were tightened accordingly, pushing better insulation and low-carbon heating.
What did not change is just as important for daylight designers. No revision to Approved Document O was made alongside the March 2026 package: the 2021 edition, in force since 15 June 2022, continues to apply to all new residential buildings. A separate technical review of Part O was confirmed in April 2026 to take forward issues raised through the earlier call for evidence, but the Government has not committed to a publication date for a revised document. In practice that means the overheating rules you design to in 2026 are the same ones that have applied since 2022 — there is no grace period and no imminent relaxation to wait for.
How BS EN 17037 frames the daylight side
On the daylight side, BS EN 17037 (adopted into BRE BR 209 in 2022) moves the conversation away from a single daylight-factor figure towards target illuminance: a recommendation that a defined fraction of a room’s reference plane achieves a target lux level for a defined proportion of daylight hours, using either climate-based modelling or the daylight-factor approximation. We explain the underlying provision tests in our guide to the BRE 2022 daylight provision tests, and the climate-based methods in our piece on climate-based daylight modelling.
The key point is that BS EN 17037 rewards more and better-placed glazing, while Part O penalises uncontrolled solar gain through that same glazing. Treating them as one design problem — rather than two compliance hoops — is what keeps both happy.
Designing glazing that satisfies both
A handful of moves reliably reconcile generous daylight with Part O:
- Get orientation working for you. North light is steady and cool — it delivers daylight with little overheating risk, so large north-facing windows are usually safe. South glazing benefits most from fixed horizontal shading (overhangs, brises-soleil, deep reveals) that blocks the high summer sun while admitting the lower winter sun and year-round daylight. East and especially west glazing are the hardest to shade because of the low sun angle, so size these more cautiously.
- Choose g-value and light transmittance together. Modern selective coatings give a relatively low g-value while keeping visible light transmittance high — exactly what you want when daylight and overheating both matter. Specify both numbers, not just the U-value.
- Use head height, not just area. A taller window with its head close to the ceiling pushes daylight deeper into the room, so you can often hit a daylight target with less total glass — which also helps Part O.
- Design for purge ventilation. Openable area on more than one aspect, or secure night cooling, lets heat escape and is what the dynamic method credits.
- Model early. Running a TM59 check at concept stage tells you whether your daylight-driven glazing survives Part O before the layout is fixed.
Where this bites in planning, not just building control
The daylight–overheating balance is mostly a building-regulations question, but it overlaps with planning. A scheme that wins consent on neighbour amenity grounds — satisfying the VSC and APSH tests for surrounding properties — still has to deliver acceptable daylight and acceptable summer comfort for its own occupants at building-control stage. This catches out conversions and additions in particular: a glazed rear extension or an office-to-residential conversion can pass on neighbour daylight yet struggle with overheating because of its single aspect and large openings. Coordinating the daylight strategy with Part O from the outset avoids an expensive redesign between planning and construction.
How Fortress Associates can help
Fortress Associates prepares daylight and sunlight reports to BRE BR 209 (2022) and BS EN 17037, and produces Building Regulations drawings across Approved Documents A–S — so the daylight strategy and the Part O response are coordinated rather than colliding. Reports are typically turned around in 4–5 working days, we work nationwide across the UK, and we ask for no advance payment. If you are weighing up window sizes on a new build, extension or conversion, get in touch and we will help you size the glazing to satisfy both the daylight targets and the overheating limits.
Sources & further reading
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