When is a BRE 209 daylight assessment required?

A BRE 209 daylight and sunlight assessment is typically required for larger planning applications, particularly those involving multi-storey development, residential schemes near existing dwellings, or any development where the local planning authority's validation checklist specifies one. Many LPAs in London and other major cities require full BRE 209 assessments for applications above a certain height or within a certain distance of neighbouring habitable windows.

What is the difference between a shadow diagram and a daylight assessment?

A shadow diagram shows the extent of shadows cast at specific times of day and year. A daylight and sunlight assessment goes further: it quantifies the amount of daylight (VSC, NSL) and sunlight (APSH) reaching specific windows and spaces, and compares those figures against BRE 209 guidance thresholds. Many planning applications require both documents.

What does VSC mean in a daylight assessment?

VSC stands for Vertical Sky Component. It measures the proportion of the sky vault visible from the centre of a window, expressed as a percentage. BRE 209 suggests that a VSC of 27% or more indicates adequate skylight. If a proposed development reduces a neighbouring window's VSC by more than 20% of its former value, the impact may be noticeable to occupants.

United Kingdom · Daylight & Sunlight Modelling

Daylight & Sunlight Modelling
Support for UK Planning Teams.

Pilot modelling support for selected UK projects, including VSC, APSH, NSL and shadow-plot outputs. The calculation scope, source geometry, methodology, interpretation responsibility and any independent review requirements are agreed before instruction.

Enquire About August Slots View BRE 209 Reference

18+ Years Experience Current UK Planning Standards VSC / APSH / NSL 48-Hour First Draft

The applicable BR 209 criteria and assessment scope are confirmed before modelling begins

VSC — Vertical Sky Component

27% threshold · less than 20% reduction of former value · skylight adequacy · BRE 209 §2.2

APSH — Annual Probable Sunlight Hours

25% annual sunlight hours · 5% winter minimum · main living rooms facing within 90° of south · BRE 209 §3.2

NSL — No Sky Line

0.8× former value threshold · working plane analysis · proportion of room receiving direct skylight · BRE 209 §2.3

Shadow Diagrams

Overshadowing at equinox and winter solstice · 2-hour sunlight on open space · matched to LPA brief

27%

VSC threshold — adequate skylight under BRE 209 §2.2

25%

APSH minimum — annual probable sunlight hours BRE 209 §3.2

0.8×

NSL threshold — no sky line reduction limit BRE 209 §2.3

VSC

Vertical Sky Component

Measures the proportion of the sky vault visible from the centre of a window, expressed as a percentage of the full unobstructed sky hemisphere.

BRE 209 suggests that a window receiving a VSC of 27% or more receives adequate skylight. If a proposed development reduces a neighbouring window's VSC by more than 20% of its former value (e.g. from 30% to below 24%), the impact may be noticeable to occupants.

Both the absolute VSC value and the percentage reduction are reported. LPAs consider both figures in assessing planning impact.

BRE 209 §2.2

APSH

Annual Probable Sunlight Hours

Measures the number of hours of direct sunlight a window is likely to receive over the year, accounting for typical UK weather patterns.

BRE 209 recommends that main living room windows facing within 90° of south should receive at least 25% of APSH annually, including at least 5% in the winter months (21 September to 21 March).

A reduction of more than 20% of either the annual or winter figure may be considered a significant impact by the LPA.

BRE 209 §3.2

NSL

No Sky Line

The No Sky Line divides the working plane of a room into areas that can and cannot receive direct skylight from the sky. It is assessed at desk height (0.85m) across the full working plane of each room.

BRE 209 suggests that if the area of the working plane receiving direct skylight is reduced to less than 0.8 times its former value, the occupants will notice the reduction in daylight quality.

NSL is often used alongside VSC to provide a more complete picture of daylight impact within rooms.

BRE 209 §2.3

BR 209 thresholds are guidance, not mandatory standards. Local planning authorities apply them with varying degrees of flexibility. The applicable approach is confirmed before commencing every project.

VSC Calculated for All Affected Windows

Vertical Sky Component calculated for every affected neighbouring window identified in the LPA brief. Results reported as absolute VSC value and as a percentage of the former value. Interpretation against BR 209 (2022) 27% threshold and 20% reduction rule.

APSH Analysis

Annual Probable Sunlight Hours calculated for main living room windows facing within 90° of south. Annual and winter (September–March) figures reported separately. Comparison against BR 209 (2022) 25%/5% thresholds, with window-by-window interpretation against the applicable criteria.

NSL Working Plane Analysis

No Sky Line analysis of affected rooms, showing the proportion of the working plane (at 0.85m) receiving direct skylight before and after the proposed development. Comparison against BR 209 (2022) 0.8× threshold, with room-by-room interpretation against the applicable criteria.

Shadow Diagrams at LPA-Required Dates

Shadow footprints at the equinox (21 March / 21 September) and winter solstice (21 December) at all required time-points. Existing and proposed shadows distinguished by colour. Matched to the scale and north point of your submitted drawings.

BRE 209 Methodology Statement

Written methodology confirming software and sun-path model, OS grid reference, applicable BRE 209 section, reference dates, and true north basis. Written for planning officers — ready to paste into your Design and Access Statement or planning statement.

One Free Planning Amendment

If your LPA requests amended daylight calculations as a planning condition, one amendment round is included at no additional charge. We review the officer's comments, update the analysis and re-issue the amended set within 48 hours.

Common Reasons for Challenge

✕
Windows Omitted from the Assessment Scope
Assessments that omit windows which should have been included — typically ground floor windows, windows in secondary rooms, or windows on elevations not facing the development — can require a full re-assessment once identified.
✕
No interpretation against the applicable threshold
Presenting VSC, APSH or NSL results in tabular form without interpretation against the applicable BR 209 (2022) criteria is insufficient for most LPAs. Officers need to be able to read the assessment without cross-referencing the BR 209 text themselves.
✕
Geometry not verified against OS data
BRE 209 assessments built from approximate building heights or unverified site geometry are challengeable at appeal. True north, building heights and neighbouring property footprints must be verifiable against OS map data or survey.

The CasStudio Standard

✓
Full window schedule confirmed before modelling
We confirm the full schedule of windows to be assessed — including ground floor, secondary rooms and all affected elevations — with your planning consultant before modelling begins. The scope is agreed in writing before any calculations are run.
✓
Window-by-window interpretation, referenced to BR 209 (2022)
Every VSC, APSH and NSL result includes interpretation against the applicable threshold, referencing the specific BR 209 (2022) section. Written for planning officers, not architects — ready to paste into your DAS or planning statement without modification.
✓
OS-Verified Geometry & Methodology Statement
True north confirmed against OS map data. Building heights and neighbouring property footprints verified against OS data or survey before modelling. A full methodology statement confirms the verification basis used.

Send Your Brief

Share your planning drawings, OS map extract, LPA name and any specific requirements from the validation checklist. We confirm the required BRE 209 criteria and window schedule before starting.

Scope Confirmed in Writing

We confirm the full window schedule, applicable BRE 209 sections (VSC, APSH, NSL), reference dates, and any LPA-specific requirements in writing before modelling begins. No surprises mid-project.

First Outputs

3D model built and verified against OS data. VSC, APSH and NSL calculated for windows in scope. BR 209 methodology statement prepared. First outputs are typically issued within 48 hours for confirmed shadow-plot scopes; timelines for full VSC, APSH or NSL modelling are agreed per project.

Final Report & Source Files

After your review and one amendment round, final print-ready PDF, results tables and DWG source files delivered. BRE 209 methodology statement ready for DAS inclusion. One amendment included at no charge.

Standard BRE 209 Assessment

^£450

Up to 10 windows assessed. VSC + APSH + NSL. Single planning application.

VSC for all affected windows
APSH for main living room windows
NSL working plane analysis
Shadow diagrams at equinox & solstice
BRE 209 methodology statement
Window-by-window interpretation against the applicable criteria
1 free planning amendment · NET 14

Most Common

Extended / Major Application

^£850+

11+ windows, multi-storey development, or complex site geometry.

Everything in Standard package
Extended window schedule (11+)
Multi-storey / complex geometry
Detailed methodology statement
Sensitivity analysis where required
Priority 48-hour turnaround available
Quoted per project scope

All prices in GBP. No VAT — invoiced from KAS STUDIO DOOEL, Skopje, North Macedonia. Fixed quotes within 24 hours of brief received.

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A daylight and sunlight assessment for UK planning quantifies the amount of daylight and sunlight reaching rooms and open spaces in both the proposed development and neighbouring properties. It is carried out in accordance with BRE 209 using three main metrics: Vertical Sky Component (VSC), Annual Probable Sunlight Hours (APSH) and No Sky Line (NSL) analysis.

A full BRE 209 assessment is typically required for larger planning applications — particularly multi-storey development, residential schemes near existing dwellings, or any application where the LPA's validation checklist specifies one. Many London LPAs require full BRE 209 assessments for applications above a certain height. We check your LPA's requirements before starting.

VSC stands for Vertical Sky Component. It measures the proportion of the sky vault visible from the centre of a window. BRE 209 suggests a VSC of 27% or more indicates adequate skylight. If a proposed development reduces a neighbouring window's VSC by more than 20% of its former value, the impact may be noticeable. Both the absolute value and the percentage reduction are reported.

VSC measures how much of the sky is visible from a window (daylight). APSH measures hours of direct sunlight over the year (sunlight). NSL measures what proportion of a room's working plane receives direct skylight. All three assess different aspects of daylight and sunlight — which metrics your LPA requires depends on the development type and local policy.

We need your planning drawings (DWG, PDF or ArchiCAD preferred), an OS map extract for true north and site boundary verification, and your LPA name or planning reference. For multi-storey development, floor plans and sections for all levels. IFC and Revit files are also accepted.

No. CasStudio is based in Skopje, North Macedonia (KAS STUDIO DOOEL). Invoices are issued in GBP from a non-UK entity and are not subject to UK VAT. Payment terms are NET 14 from invoice date.

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