Knowledge Base

Expert guidance, downloadable checklists, and FAQs to help you navigate daylight and sunlight assessments

Key Concepts Explained

When Is a Daylight & Sunlight Report Required?

When planning officers expect a formal study

Local authorities often reference BRE BR209 when judging new builds or extensions. If your project affects nearby windows, rooms or gardens, a study helps avoid objections and redesigns.

1

Key Triggers

New build or large extension close to neighbours
Extra storeys or roof changes (dormers, mansards)
Reduced window-to-window distances
Taller development beside low-rise homes

2

What Planners Consider

VSC (daylight to neighbours)
APSH / WPSH (sunlight to main rooms)
Garden overshadowing (two-hour rule)
Overall proportional impact vs existing

💡

Catch issues early — a quick scoping check often shows simple fixes like small set-backs or roof-angle tweaks.

Daylight & Sunlight for Planning — What Planners Check

Neighbour impact checks using BRE BR209

BR209 offers advisory tests that help planning officers judge proportional change. We explain the metrics clearly and focus on the most sensitive receptors first.

1

How We Assess

Accurate 3D model of existing & proposed
Check worst-affected windows and rooms first
Compare "before vs after" proportional change
Clear diagrams and tabulated results

2

Core Metrics

VSC (daylight reaching a window)
NSL (daylight distribution)
APSH/WPSH (sunlight hours)
Garden tests (two-hour rule)

💡

Ask for a concept-stage review to avoid redesigns and clarify any local policy nuances.

Vertical Sky Component (VSC) — The Planning Metric

What VSC is and why it matters

VSC is the percentage of visible sky a window "sees" at its centre. Obstructions reduce that view and lower VSC.

1

What VSC Is

% of visible sky seen from window centre
Higher/closer mass lowers VSC
Small geometry tweaks can preserve VSC

2

Typical Planning Thresholds

27% VSC — usually adequate
<27% but ≤20% reduction — often acceptable
>20% reduction <27% — potential concern
Linked: APSH (sunlight), WPSH (winter sunlight)

💡

Model early with realistic massing — sloping a roof or chamfering a corner often preserves VSC.

Annual Probable Sunlight Hours (APSH) — The Sunlight Metric

Direct sunlight tests for living spaces

APSH measures how much direct sunlight reaches a window. It complements daylight tests by focusing on solar access over the year and in midwinter (WPSH).

1

Key Points

Counts hours of direct sun at the window centre
Applies to windows within 90° of due south
BRE benchmarks: ≥25% annual, ≥5% winter
WPSH protects midwinter sunlight

2

Linked Metrics

VSC — daylight potential
APSH/WPSH — sunlight availability
Together, they capture ambient & direct light
Reductions beyond limits may need design tweaks

💡

Sloped roofs or modest setbacks can recover sunlight hours. Ask for a rapid APSH/WPSH check alongside your daylight study.

Internal Daylight (CBDM) — Beyond ADF & NSL

EN 17037-aligned, climate-based daylighting

For internal quality we use climate-based daylight modelling (CBDM) instead of legacy ADF/NSL. It reflects real sky/solar conditions and aligns with modern guidance.

1

What We Assess

Space geometry, glazing area & visible transmittance (Tv)
External obstructions, shading & façade design
Surface reflectances, room depth & layout

2

Typical Targets / Outputs

sDA (300/50%), DA & UDI (100–2000) distributions
EN 17037 illuminance recommendations
Visual maps to inform design choices

💡

Model early. We'll balance daylight with overheating, glare and privacy — and advise shading that keeps planning smooth.

Garden Overshadowing & the Two-Hour Rule — Quick Guide

Outdoor amenity checks (BR209)

BRE suggests at least half a garden should receive two hours of direct sunlight on 21 March. If a proposal causes >20% loss of sunlit area vs existing, planners may object.

1

Why It Matters

Neighbour amenity & wellbeing
Planning risk if significantly overshadowed
Simple mitigation often works
Clear visuals help resolve concerns

2

How We Analyse

3D existing & proposed model
Hourly shadows on 21 March
% garden with ≥2 hours sun (before vs after)
Report with diagrams and tables

💡

Mitigate with setbacks, sloped/chamfered roofs and lower boundary walls. We'll flag the simplest options.

Balancing Daylight, Overheating & Glare — The Whole Picture

Why more light isn't always better

Great daylight design balances brightness, comfort and temperature. Excess glazing can increase overheating and glare even if daylight targets are met.

1

Common Conflicts

More glazing → higher solar gains (TM59/Part O)
South/west facades risk overheating without shade
High reflectance interiors can cause glare
Heavy blinds reduce daylight and views

2

Design Strategies

Combine daylight with TM59 overheating analysis
Use external shading: overhangs, fins, light shelves
Optimise window size and transmittance (Tv)
Balance sDA, UDI, glare and cooling load together

💡

Ask about our combined Daylight + Overheating review to find the sweet spot for comfort, compliance and energy use.

BRE vs EN Standards — How Guidance Is Evolving

Understanding BR209 (UK) vs EN 17037 (Europe)

Two main standards are used today. BRE BR209 guides planning decisions on neighbour impact; EN 17037 defines internal daylight quality for occupants.

1

Key Differences

BRE BR209: VSC, NSL, APSH, Garden — planning
EN 17037: illuminance, view/outlook — design
BRE = neighbour impact; EN = occupant experience
Both promote well-lit, context-sensitive design

2

Why It Matters

Authorities rely on BRE for planning decisions
Designers use EN 17037 to tune internal quality
Understanding both provides consistent outcomes
We cross-reference where appropriate

💡

We'll guide when to apply each standard — and how to keep your scheme defensible across planning and design stages.

Design Tips to De-Risk Daylight & Sunlight

Practical moves that keep planners on side

Most daylight issues are solved by simple geometry changes made early. These moves often protect VSC, sunlight and gardens with minimal redesign.

1

Quick Wins

Step back upper storeys near sensitive windows
Chamfer/angle roof edges to clear sightlines
Keep boundary walls lower where possible
Use lighter internal finishes to lift daylight

2

Focus First On

Worst-case receptors (lowest VSC/highest loss)
March-21 garden shadows and % change
Proportional impacts vs existing
Document rationale for any deviations

💡

A 30-minute concept review can save weeks later. Send us your massing and we'll mark up the simplest path to compliance.

Downloads

Download our homeowner checklist, architect’s technical guide, and planning submission pack. Enter your email to receive the file instantly.

Homeowner Checklist

Do you need a Daylight/Sunlight study? A simple yes/no guide for residential projects.

Neighbour impact (VSC/APSH)
Internal daylight prompts
Planning triggers

Architect’s Technical Guide

Pre-submission light review: VSC thresholds, APSH triggers & garden amenity.

VSC ≥27% & 0.8× ratio cues
APSH annual & winter checks
Garden 50%/2hr (21 March)

Planning Submission Pack

Complete list of documents & diagrams typically required by UK LPAs.

VSC/APSH/overshadowing set
Before/after & massing views
Declarations & versioning

Files download instantly after you enter a valid email. We’ll only contact you about daylight/sunlight updates and resources.

Frequently Asked Questions

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