Daylight & Sunlight Glossary: VSC, NSL, APSH and Key Terms

If you’re preparing, reviewing or defending a planning submission, having a clear daylight and sunlight glossary saves time, avoids misunderstandings and helps everyone talk the same language. This guide explains the core metrics used in UK planning and design — VSC, NSL, APSH, plus related terms from BS EN 17037 — and shows how professionals apply them on real schemes. It follows the 2022 edition of BRE’s Site layout planning for daylight and sunlight and aligns with BS EN 17037.

Vertical Sky Component (VSC)

What it is. VSC measures the fraction of sky visible at a window on an overcast sky (CIE Standard Overcast), expressed as a percentage. It excludes ground and façade reflections - just direct skylight.

How to use it. For an existing neighbour’s window, if the post-scheme VSC is > 27%, there is typically enough skylight for a standard room (with normal glazing area). If it’s < 27% and < 0.80× its former value, the reduction will be noticeable to occupants. Quote both absolute values and ratios, rounded to two decimals.

Why 27%? It’s equivalent to a long, straight obstruction at 25° in section; BRE gives helpful equivalences between obstruction angles, spacing/height ratios and VSC targets.

Area-weighted VSC (method statement)

Where a room has multiple windows on the same elevation lighting the same zone, we report an area-weighted VSC by weighting each window’s VSC by its proportion of the total glazing area to provide a room-level indicator. We do not area-weight across different zones (e.g., through-lounges with > 5 m separation or corner rooms with distinct plan swathes); in those cases we report each window separately and include NSL to describe distribution. Ratios of “Proposed/Existing” are reported to two decimals.

Notes.

Bathrooms, circulation spaces etc. are usually excluded from neighbour impact analysis.

No Sky Line / Daylight Distribution (NSL)

What it is. NSL maps which parts of a room’s working plane (generally 0.85 m above floor in dwellings) can “see” the sky. Areas beyond the no-sky line usually look gloomier and need artificial light more often.

How to use it. If, after development, the lit area is < 0.80× its former value, occupants will notice the change. Report the ratio to two decimals (or the equivalent % loss).

Tip. If layouts are known, you should plot NSL for main rooms (living, dining, kitchen), plus bedrooms though they’re typically less critical.

Annual Probable Sunlight Hours (APSH)

What it is. APSH is the percentage of annual “probable” sunshine hours (allowing for typical cloud cover) that reach a window. It quantifies sunlight, not daylight.

How to use it. For existing rooms that face within 90° of due south, aim for ≥ 25% APSH annually and ≥ 5% in winter (21 Sep–21 Mar). If available sunlight is below those amounts and < 0.80× the former value (annual or winter) and the overall annual loss is > 4% APSH, the reduction is likely to be noticeable.

Scope. You don’t need to assess windows that face wholly within 90° of due north for sunlight.

BS EN 17037 (Interior Daylighting)

BRE’s 2022 guide is intended to be used with BS EN 17037. The Standard offers two approaches for new rooms:

Climate-based targets - achieve ET across ≥ 50% of the reference plane, and ETM across ≥ 95% of the plane, for ≥ 50% of daylight hours; or

Daylight factor targets (UK National Annex equivalents) as an alternative. Typical UK DF minimums often referenced are circa 0.7% bedrooms, 1.1% living rooms, 1.4% kitchens across ≥ 50% of the grid (location-dependent).

Modelling inputs that often matter: realistic glazing diffuse transmittance (≈ 0.68 for clean low-E double glazing), framing & maintenance factors, and practical grid spacing (≤ 0.3 m preferred), with a 0.3 m perimeter band excluded in dwellings.

Obstruction angle (25° rule of thumb)

In a vertical section perpendicular to the affected window wall, measure the angular altitude from the window centre (often 1.6 m above floor for patio doors). If the new mass stays under 25°, diffuse daylight impact is unlikely to be substantial; above 25°, do detailed VSC/NSL checks.

Sunlight on 21 March (amenity test)

For gardens and open spaces, the classic test is: can at least 50% of the area receive two or more hours of sun on 21 March? Use sunpath indicators or equivalent to demonstrate compliance and set the reference point appropriately for awkward garden shapes.

Target illuminance (ET, ETM) & reference plane

In BS EN 17037 terms, ET is the target illuminance met across ≥ 50% of the reference plane, and ETM the target met across ≥ 95% of the plane for ≥ 50% of daylight hours. The reference plane (working plane) is typically 0.85 m above finished floor level in dwellings.

Daylight & Sunlight Glossary - Practical Scenarios

1) Designing a new scheme

Use BS EN 17037 for new rooms (climate-based preferred; DF alternative acceptable per the National Annex). Clearly state reflectances, glazing transmittance, grid banding (exclude 0.3 m from walls in dwellings), and grid spacing (≤ 0.3 m preferred).

When stepping massing or positioning balconies, consider the shading effect of overhangs and mitigation (staggering, stepping back).

For tight streets, align expectations with contextual obstruction angles (e.g., 40° may correspond to ~18% VSC at ground). Use BRE equivalence tables to frame realistic contextual targets.

2) Safeguarding existing neighbours (planning risk)

VSC: flag where < 27% and < 0.80× former value. Explain absolute vs relative change (low baselines create big ratios). Where multiple same-elevation windows light one zone, apply the area-weighted method above; otherwise assess windows individually and use NSL to explain distribution.

NSL: show before/after plans; note if lit area ratio < 0.80.

APSH: assess south-facing rooms; aim for ≥ 25% annual and ≥ 5% winter; check the 4% annual loss and 0.8× ratios.

Balconies above windows amplify relative losses — consider with/without-balcony tests to evidence the true driver of change.

Rights to light ≠ BRE daylight: different methodology and legal test (e.g., 0.2% sky-factor working plane). Meeting BRE guidance is not a guarantee on R2L; obtain specialist advice where relevant.

3) Protecting future development next door

Don’t let today’s proposal “take more than its fair share of light”. Use boundary parameters and a mirror-image approach to set fair, self-consistent targets for adjoining plots; treat extant but unbuilt permissions sensibly (don’t apply 0.8× to a paper scheme).

4) Amenity outside (courtyards, play space, terraces)

Demonstrate the ≥ 2 h on 21 March result (area and percentage), plus a short narrative on seating/play placement to capture sunlight during usable hours. For L-shaped gardens, document how the “centre” has been located for consistency.

Quick reference: daylight & sunlight cheat-sheet

VSC (existing windows): target outcome: ≥ 27%, or if below, ensure not < 0.80× former value; 27% roughly corresponds to a 25° long obstruction in section. Calculate contextual target values in dense urban settings.

NSL (daylight distribution): if post-scheme lit area < 0.80× former value → likely noticeable loss.

APSH (existing rooms within 90° of south): aim for ≥ 25% annual and ≥ 5% winter; beyond 4% annual loss with < 0.80× can be problematic.

Amenity sunlight (21 March): ≥ 50% of area with ≥ 2 h sunlight.

New rooms (BS EN 17037): climate-based ET/ETM across the grid or UK National Annex DF equivalents (e.g., c. 0.7%/1.1%/1.4% for bed/living/kitchen). Use realistic glazing, framing and maintenance factors.

Common pitfalls (and quick fixes)

Reporting “percent losses” without ratios. Always give before/after and the ratio (two decimals). Fix: Add a results column for “Proposed/Existing”.

Misusing area-weighted VSC. Only combine windows lighting the same zone; don’t “average away” a front-zone loss with rear glazing in a through-room. Fix: Keep the room zoning honest; use NSL to evidence distribution.

Testing sunlight on north-facing windows. Skip APSH where windows lie wholly within 90° of due north. Fix: Define façade azimuths once; bind scope to that.

Ignoring balcony effects. Balconies above a window depress both VSC and NSL; relative losses look worse. Fix: Include “without balcony” sensitivity to evidence causation.

Under-specifying BS EN 17037 inputs. Missing reflectances, maintenance factors, or too-coarse grids skew outcomes. Fix: State reflectances; use realistic glazing factors; limit grid spacing to ~0.3 m.

Assuming BRE compliance equals rights-to-light safety. It doesn’t. Different methodology and legal test. Fix: Flag separately; get rights to light advice where relevant.

Amenity maps without an area calculation. A colourful plan isn’t enough. Fix: Quote % area achieving ≥ 2 h on 21 March and note placement of key uses.

Daylight & Sunlight Glossary FAQs

Is BR 209 policy or guidance?

Guidance. Numerical values are advisory and should be applied sensibly and flexibly, balanced with other material considerations.

Do I always test every neighbour window?

No. If the new mass is sufficiently distant — roughly three or more times its height above the window centre — analysis may be unnecessary. Otherwise, perform the full checks.

What if my site already sits in a “tight” urban setting?

Use contextual targets via obstruction-angle/VSC equivalences and a mirror-image method to keep targets self-consistent.

Which is better for new interiors — climate-based or DF?

Climate-based per BS EN 17037 is preferred for realism; DF is an accepted alternative via the UK National Annex and is quicker to compute. State your inputs, especially glazing transmittance and maintenance factors.

Do PV/solar thermal need special sunlight checks?

Yes. Consider PV overshadowing guidance and when to undertake a deeper radiation study (for example where reductions approach material thresholds).

Conclusion - Daylight & Sunlight Glossary

Use this daylight and sunlight glossary as your working reference from feasibility through to planning and discharge of conditions. Anchor neighbour impacts with VSC/NSL/APSH, evidence amenity with the 21 March test, and prove interior quality with BS EN 17037 (climate-based or DF per the UK Annex). Present absolute results and ratios, apply BRE thresholds flexibly but transparently, and explain design-led mitigations. Done well, your daylight and sunlight story is robust, replicable and easy to scan — exactly what busy planners need.

Need Expert Advice?

At Anstey Horne, our specialists surveyors provide independent Daylight & Sunlight Assessment studies that meet BRE 209 (2022) standards and support planning success. If you are preparing a planning application or facing objections, our team can help you navigate the technical and planning complexities with confidence.

For more advice on how we can help support a planning application with daylight & sunlight advice please give us a call. If you would rather we contacted you please fill in our Contact Form and we will be in touch.

For more information on understanding all aspects of our Daylight & Sunlight Glossary see the collection of articles on our blog page.

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