If you are converting a property into a house in multiple occupation (HMO) in 2026, the planning landscape is changing fast. A wave of new Article 4 Directions is removing the permitted development right that previously allowed smaller HMO conversions to proceed without planning permission — and once a full application is needed, councils will almost certainly require a daylight and sunlight assessment alongside it.
Understanding when a daylight report is required, what it needs to cover, and how the BRE 2022 guidelines apply to HMO proposals can save you significant delay and cost on your project.
What is an HMO and why does daylight matter?
A house in multiple occupation is a property let to three or more people who form more than one household and share facilities such as a kitchen or bathroom. HMOs range from student houses and professional flat-shares to larger bedsit blocks.
Because HMO conversions typically subdivide existing rooms and add new windows, or convert outbuildings and basements, they can materially affect the amount of daylight reaching neighbouring properties. If your proposal involves an extension, a new dormer, a raised roof, or any structure that casts a shadow onto an adjacent home, the council's planning officer has grounds to ask for a formal daylight and sunlight assessment prepared in accordance with BRE Report BR 209 (2022).
The 2026 Article 4 Direction wave — why more HMOs now need planning permission
Until recently, changing a house (Use Class C3) to a small HMO (Use Class C4 — up to six people) was permitted development in most of England, meaning no planning application was required. That is rapidly changing.
In early 2026, Spelthorne Borough Council's Article 4 Direction came into force across the whole borough, requiring planning permission for all new C3-to-C4 conversions. Durham County Council confirmed a similar direction taking effect from August 2026. These join dozens of other councils — including many London boroughs — that have already removed the permitted development right in their areas.
What this means in practice: if you are letting a property as an HMO in an Article 4 area, you need planning permission. And planning permission for a proposal that could overshadow or reduce daylight to a neighbouring home will almost always trigger a request for a daylight and sunlight report.
When does a daylight report become a validation requirement?
Councils are not uniform in their validation requirements, but a daylight and sunlight assessment is typically required when your HMO proposal involves one or more of the following:
- A rear extension, side extension, or outbuilding that could shadow neighbouring windows or gardens
- A loft conversion with a dormer or roof extension that adds bulk close to a boundary
- A basement conversion with light wells or external excavations
- A change of use to an HMO on a constrained urban site where neighbouring windows are close
- Any development where the local validation list specifies a daylight impact assessment
It is always worth checking the local validation checklist — available on your council's planning portal — before submitting. Tower Hamlets, for example, specifies daylight and sunlight assessments for proposals likely to affect neighbouring amenity. Many inner-London boroughs take a similar approach given the density of existing residential development.
BRE 2022 metrics: what the numbers mean for your HMO application
The current national standard for daylight and sunlight assessment is BRE Report BR 209 (2022), the third edition of the guidance. It uses three primary metrics when assessing impact on neighbouring properties.
Vertical Sky Component (VSC) measures the proportion of diffuse daylight that reaches a window from the sky. BRE considers a VSC of 27% or above to be well lit. If a proposed development reduces a neighbouring window's VSC to below 27%, or reduces it by more than 20% of its former value, the loss is likely to be noticeable and is treated as an adverse impact.
No-Sky Line (NSL) divides the floor area of a room into the zone that can see the sky through the window and the zone that cannot. BRE's rule of thumb is that if less than 20% of the previously lit floor area loses its sky view as a result of the proposed development, the impact is unlikely to be significant.
Annual Probable Sunlight Hours (APSH) is used to assess sunlight reaching windows that face within 90 degrees of south. The target is for windows to receive at least 25% of annual probable sunlight hours, including at least 5% in the winter months. APSH is particularly relevant for HMO applications where south-facing habitable rooms are affected by an adjoining extension.
For overshadowing of neighbouring gardens and amenity spaces, the report also applies the 21 March equinox test: at least 50% of a garden should receive two or more hours of direct sunlight on 21 March. We explain this test in detail in our post on overshadowing studies and the 21 March equinox.
Internal daylight for HMO residents: BS EN 17037
While most daylight assessments submitted with planning applications focus on impact to neighbouring properties, the BRE 2022 guidance also references BS EN 17037:2018 for assessing the internal environment of new or converted dwellings.
For HMO rooms — particularly basement bedsits, converted lofts, and narrow street-facing rooms — internal daylight can be marginal. BS EN 17037 sets minimum illuminance targets for rooms and uses the Daylight Factor or climate-based Daylight Autonomy calculation to establish whether habitable rooms receive adequate natural light.
Some councils, particularly those with dense existing housing stock, are increasingly asking for internal daylight calculations for proposed HMO rooms to ensure future residents will have adequate amenity. If your scheme involves basement rooms or rear rooms with limited window area, it is worth including an internal assessment in the report proactively. You can read more about this standard in our post on BS EN 17037 explained for UK architects.
Common problems in HMO daylight assessments
There are a few recurring patterns worth knowing about before you design your scheme.
Rear dormer extensions are the most frequent source of daylight objections from neighbours in HMO applications. A full-width rear dormer significantly increases the mass of the roof and can shade windows in the adjacent rear elevation, particularly where the rear garden depth is shallow. Designing the dormer to set in from the boundaries and keeping the eaves height below the ridge line can substantially reduce the impact.
Side returns and infill extensions close a gap between properties that previously allowed oblique sky light into side windows. If the neighbouring property has a kitchen or bathroom window facing the gap, closing it can reduce VSC below the BRE threshold. The solution is often to introduce a rooflight or glazed roof section in the extension rather than a solid lid.
Light wells for basement rooms must be sized to provide adequate VSC to the basement window. A very narrow light well will typically fail the internal daylight test under BS EN 17037. BRE guidance suggests a well width at least equal to its depth for reasonable daylight penetration.
How Fortress Associates can help
At Fortress Associates, we prepare daylight and sunlight reports for HMO conversions, extensions, and change-of-use applications across the UK. Our assessments are prepared in accordance with BRE BR 209 (2022) and BS EN 17037, and include the VSC, NSL, and APSH calculations that councils require at validation stage.
We typically deliver completed reports within four to five working days, and we do not require advance payment — you pay only when the report is ready. Whether you are converting a Victorian terrace into a six-bed HMO in an Article 4 area or seeking approval for a rear extension to an existing let property, we can provide a clear, council-ready assessment. Contact us to discuss your project or visit our services page to learn more about what a daylight report involves.
Sources & further reading
- BRE BR 209 (2022) — Site Layout Planning for Daylight and Sunlight
- National Planning Policy Framework (NPPF) — GOV.UK
- Houses in Multiple Occupation and Planning Restrictions — House of Commons Library
- Tighter Controls for HMOs — Spelthorne Borough Council (2026)
- New Article 4 Measures Confirmed — Durham County Council
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