Knowledge Base

Daylight & Sunlight Planning Guide: BR209 & Right to Light

Reviewed: January 2026  |  By: The team at Daylight Sunlight Expert - Specialists in BR209 (BRE guidance) & BS EN 17037 assessments for UK planning

📖 15 min read  •  3,000+ words  •  13 sections  •  27 FAQs

Planning decisions in the UK often hinge on a single, critical question: "Will this development allow adequate light to reach neighbours?"

Whether you are planning an extension, developing a new property, or designing a high-density scheme in London, the guidance regarding light emphasizes the need for a technical assessment with context-specific flexibility.

We're able to help you navigate the nuances and find the best path forward for your project amidst the complexity, with an understanding of the various allowances or exception cases based on local conditions. This guide serves as a comprehensive resource for understanding the BRE BR209 (2022) guidance, the BS EN 17037 standards for internal light, and the important distinction between Planning Permission and the legal Right to Light.

We have successfully used these methodologies to help clients across the UK-from simple rear extensions in various London boroughs to complex high-rise developments in Luton as well as projects in dense urban settings like Manchester, Birmingham, Leeds, and Bristol.

Related: Meet The Experts | Free Risk Calculator | Get a Quote

1. When is a Daylight & Sunlight Report Required?

The 45-Degree Rule (Householder Extensions)

For residential extensions, the most common screening test is the 45-degree rule. This is a "rule of thumb" used by planning officers across the UK to identify when detailed assessment is appropriate.

• The Test: Imagine drawing a line at 45 degrees from the centre of your neighbour's nearest ground-floor habitable room window (living room, bedroom, kitchen, or dining room- not bathrooms or hallways). This line is drawn in two ways: looking down from above (plan view) and looking from the side (elevation view).
• The Trigger: If your proposed extension crosses either of these 45-degree lines, it indicates that a more detailed technical assessment may be beneficial to demonstrate the actual impact on light levels.

The 25-Degree Rule (Opposite Buildings)

For new buildings facing existing windows (such as a new block of flats across a street), we use the 25-degree rule.

• We draw a line at 25 degrees from the horizontal, starting at the centre of the lowest existing window.
• If the new building is underneath this line, light is likely adequate.
• If the new building cuts above this 25-degree line, a full computer simulation (VSC calculation) is required.

2. Vertical Sky Component (VSC) Explained

What is VSC?

VSC measures the percentage of the sky that is visible from the centre of a window. It does not calculate sunlight (direct sunbeams); it calculates "skylight" (the diffuse light available on a cloudy day). A completely unobstructed window looking at an empty field would achieve a VSC of nearly 40%.

The Assessment Criteria (BR 209)

The BRE Guidelines (Site Layout Planning for Daylight and Sunlight, 2022) provide benchmark targets. A window is considered to have "good daylight" if it achieves a VSC of 27%.

However, in dense urban environments like Central London, achieving 27% is often not feasible. In these cases, the BRE guidance includes flexibility. If the VSC is below 27%, the development can still be acceptable if the new value retains at least 0.8 times (80% of) its former value (i.e., the reduction is less than 20% from the current state).

Result	VSC Score	Planning Outcome
Pass (Ideal)	≥ 27% VSC	Good daylight. Unlikely to be refused.
Pass (Relative)	< 27% but > 0.8x Existing	Loss not noticeable. Acceptable.
Fail (Minor)	0.7x - 0.79x Existing	Minor adverse impact. May need justification.
Fail (Major)	< 0.6x Existing	Significant loss. High refusal risk.

Example Scenario

• Before: Neighbour has 20% VSC.
• After: Neighbour drops to 17% VSC.
• Calculation: 17 ÷ 20 = 0.85.
• Result: PASS. Even though it is below 27%, the reduction is only 15%, which is less than the 20% threshold.

3. Sunlight (APSH) Analysis

Which Windows Are Tested?

Importantly, not all windows require a sunlight test. The BRE guidelines specify that we only test windows that face within 90 degrees of due South (this includes south-facing, south-east, south-west, and windows oriented up to 90 degrees either side of due south).

• If a neighbour's window faces North, North-West, or North-East, it does not have a "reasonable expectation of sunlight" and is excluded from this test.
• This is an important detail in assessment-windows outside this orientation range are not expected to receive direct sunlight and therefore aren't assessed for sunlight impact.

The APSH Targets

We measure sunlight in two periods: Annual (total year) and Winter (short days between September 21st and March 21st).

Period	Target Hours	Percentage
Annual (Jan-Dec)	1,486 Hours	25%
Winter (Sep 21 - Mar 21)	297 Hours	5%

Just like VSC, if a window falls below these levels, the new value should retain at least 0.8 times (80% of) its former sunlight hours to be considered acceptable.

4. Garden Overshadowing (The 2-Hour Rule)

The Methodology

We build a 3D model of the site and simulate the path of the sun on March 21st (The Spring Equinox). We calculate how much of the garden receives at least 2 hours of direct sunlight.

The Pass Mark

• At least 50% of the amenity area (garden/patio) should receive 2+ hours of sun on March 21st.
• If the existing garden is already overshadowed (e.g., small courtyards), the area receiving 2 hours of sun should retain at least 0.8 times (80% of) its former area.

Metric	Passing Score	Significance
Sun on Ground	≥ 50% of garden with 2+ hours sun	Adequate outdoor amenity
Proportional Loss	> 0.8x existing sunlit area	Acceptable if already constrained

Design Mitigation

If your extension fails this test, using a sloping roof or chamfered corner can often allow sunlight to pass over the top of the extension, saving the neighbour's garden amenity and securing planning permission.

5. BR 209 vs BS EN 17037: Which Standard Do You Need?

BR 209 (Site Layout Planning)

• Focus: Protecting Neighbours
• Used For: Planning Applications, Objections, Impact Assessments
• Key Metrics: VSC (Vertical Sky Component), NSL (No Sky Line), APSH (Sunlight)

BS EN 17037 (Daylight in Buildings)

• Focus: Quality of life for Future Residents
• Used For: New Build Design, BREEAM Credits, Internal Layouts
• Key Metrics: Illuminance (Lux), Daylight Factor, View Out

Standard	Purpose	Who Benefits
BR 209	Neighbour impact assessment	Existing neighbours
BS EN 17037	Internal daylight quality	Future occupants

6. Design Tips to De-Risk Planning

Practical Design Strategies

1. Chamfer the Corner: If you are failing VSC on one specific neighbour window, "chamfering" (angling or cutting off the corner at 45 degrees) the top corner of your extension closest to them can often regain the 1-2% VSC needed to pass.
2. The "Stepped" Massing: Instead of a sheer vertical wall, step the upper floors back by 1-2 metres. This allows more skylight to reach the ground-floor windows of the neighbour. (Massing refers to the overall shape and volume of the building.)
3. Light Coloured Render: In tight courtyards, changing the facade material from dark brick (10% reflectance) to white render (80% reflectance) can double the internal light levels.
4. Mansard Roofs: Angling the roof away from the neighbour at 70 degrees often clears the "No Sky Line," allowing the development to pass where a vertical gable would fail.
5. Parapet Heights: Reducing a parapet height by just 300mm can sometimes be the difference between a Pass and a Fail for a neighbour's garden sunlight.

Focus First On:

• Worst-case receptors (lowest VSC / highest loss)
• March-21 garden shadows and percentage change
• Proportional impacts vs existing baseline

7. Homeowner checklist: Do I need an assessment?

• ☐ Height: Will your extension be taller than the middle of your neighbour's ground-floor windows?
• ☐ How far back: Will your extension stick out from the back of your house more than 3 metres (about 10 feet) for a terraced or semi-detached house, or more than 4 metres (about 13 feet) for a detached house?
• ☐ The 45-Degree Line: Picture drawing a 45-degree angle line from the center of your neighbour's window-both looking down from above and from the side. If your extension crosses either of these lines, an assessment is recommended.
• ☐ Orientation: Is your neighbour's house to the north of yours? (If your extension is on the south side of their property, it's more likely to affect their sunlight.)
• ☐ Land Levels: Is your neighbour's garden or house lower than yours? (Sloping sites can increase shadow impact.)
• ☐ Dense Urban Area: Are you in a densely built-up area where buildings are close together?

8. The Difference Between "Right to Light" and Planning Permission

1. Planning Permission (Public Law)

• The Focus: "Amenity." Is the new building acceptable for the neighbourhood? Does it provide neighbours with adequate light for comfortable living?
• The Standard: Local Councils use the BR 209 Guidelines (Vertical Sky Component, Sunlight Hours) as a benchmark.
• The Outcome: If the assessment shows unacceptable impact, the Council may refuse the application or request design modifications. If acceptable, you receive permission to build.

2. Right to Light (Private Law)

• The Focus: "Property Right." A Right to Light is a form of property right (an easement), similar to a Right of Way. It is typically acquired under the Prescription Act 1832 after 20 years of uninterrupted light access.
• The Standard: The Courts use the "50/50 Rule" (0.2% Sky Factor), which differs from BR 209 planning targets.
• The Outcome: Even with Planning Permission, neighbours may pursue civil action if their Right to Light is affected. Courts may award compensation or require design modifications to protect established rights.

9. Balancing Daylight with Overheating (Part O)

If you design large glass facades to pass your daylight assessment, you might fail your Part O Overheating assessment.

Our Solution: Fabric First Approach

We recommend a "Fabric First" approach to solve this conflict:

• G-Value: Use high-performance solar control glass (e.g., G-value 0.40) to reduce heat while maintaining light transmission (LT-value 0.70).
• Overhangs: Deep window reveals or balconies act as "Brise Soleil," blocking the high summer sun (heat) while allowing the lower winter sun (light) to enter.
• Ventilation: Ensure that large windows can open effectively to purge heat.

10. Integrated Building Performance: Beyond Daylight

Why Integrated Assessment Matters

Designing for daylight alone can create conflicts with other building regulations:

• Large windows for daylight can cause overheating (Part O)
• Solar gains affect energy performance (Part L, SAP)
• Ventilation strategies impact both light and air quality
• Material choices affect embodied carbon (LCA)

Our Holistic Approach

We work alongside specialists in:

• Part O Overheating Assessment - TM59 compliance
• Part L & SAP Calculations - Energy performance
• Energy Statements - Planning policy compliance
• Life Cycle Assessment (LCA) - Embodied carbon
• Ventilation Strategy - Air quality and comfort

The Passive Design Advantage

Early coordination means we can:

• ✓ Identify conflicts before they become problems
• ✓ Design passive solutions (orientation, shading, materials)
• ✓ Avoid costly retrofits at compliance stage
• ✓ Optimize for comfort, not just compliance
• ✓ Reduce overall project costs and timeline

11. Internal Daylight (BS EN 17037)

From ADF to Illuminance

Historically, architects used "Average Daylight Factor (ADF)" (e.g., 1.5% for living rooms). This is now outdated. The new standard requires a more complex calculation using climate-based daylight modelling (CBDM) to determine Target Illuminance (Lux).

Current Targets for New Dwellings

To achieve compliance, 50% of the room area must meet the following lux levels for at least half of the daylight hours in a year:

Room Type	Target Illuminance (Lux)
Kitchen / Dining / Living	200 - 300 Lux
Bedroom	100 Lux
Home Office	300 - 500 Lux (Recommended)

12. Local Authority Variations (London vs. UK)

Strict Boroughs (Inner London)

Boroughs like Camden, Westminster, and Kensington & Chelsea are historically strict on daylight matters. However, they often accept VSC values below 27% if the "Mirror Image" argument can be proven (i.e., matching the massing of an existing extension on the other side of the building).

These boroughs also tend to have their own Supplementary Planning Documents (SPDs) that add local requirements on top of BRE guidance. Always check the borough-specific design guides before submitting.

Regeneration Areas (Manchester, Birmingham, Leeds)

In high-growth cities outside London, planning officers often prioritize regeneration and housing delivery over strict BRE compliance. We frequently successfully argue for Alternative Target Values based on the "dense urban context" clause in Appendix F of the BRE guide.

Key cities where we apply flexible arguments:

• Manchester: City centre regeneration zones accept higher density with proportional daylight reductions.
• Birmingham: Big City Plan areas have relaxed daylight expectations for strategic sites.
• Leeds: South Bank regeneration allows contextual daylight assessments.
• Bristol: Temple Quarter and harbourside schemes benefit from design-led flexibility.

13. Architect's Pre-Application Guide for Daylight

The Key Angular Rules

These screening tests determine whether a full BR 209 assessment is needed:

Rule	Application	What It Means
25° Rule	New buildings facing existing windows	If your building lies wholly below a 25° line from the centre of a neighbouring window, it's unlikely to cause noticeable daylight reduction.
45° Rule	Extensions perpendicular to neighbours	If your extension subtends more than 45° from a neighbour's window (in plan or elevation), detailed VSC analysis is required.
43° Rule	VSC approximation	When obstruction exceeds 43° above horizontal from a window centre, VSC typically falls below 27%, triggering assessment.

Common Design Mistakes That Cause Failures

1. Excessive Building Depth: Floor plates over 12-14m cannot achieve adequate daylight to their cores. Single-aspect units deeper than 9m struggle to meet ADF requirements.
2. Deep Balconies: Balconies projecting more than 1.5m with solid balustrades can reduce VSC to rooms below by 30-50%. Use glass balustrades instead.
3. Single-Aspect North-Facing Units: These cannot receive direct sunlight and struggle with both ADF and APSH. Avoid units facing between NNE and NNW.
4. Inadequate Floor-to-Ceiling Heights: Ceiling heights below 2.5m significantly reduce daylight penetration. Every 100mm of additional height improves daylight by 3-5%.
5. Recessed Windows: Window reveals deeper than 200mm reduce visible sky angle, significantly impacting VSC calculations.

Pre-Application Checklist for Architects

• ☐ Identify all residential properties within 50 metres of the site
• ☐ Obtain or create 3D model of surrounding context (minimum 100m radius)
• ☐ Run 25° and 45° screening tests on initial massing options
• ☐ Test VSC to all potentially affected neighbouring windows
• ☐ Verify all proposed units have sunlight-receiving living rooms
• ☐ Review Local Plan policies on daylight/sunlight
• ☐ Confirm with LPA if formal assessment is required at pre-app

14. Planning Submission Pack: What Daylight Documents Are Required?

Required Drawings for Daylight Assessment

• Existing Site Survey: Site location plan showing your property and surrounding buildings (typically within 50-100m)
• Proposed Floor Plans: Room uses clearly labelled, accurate window schedules with dimensions
• Elevations: All elevations clearly showing window head and sill heights, and ground levels
• Sections: Minimum two sections showing relationship to neighbouring properties
• Context Drawings: Street elevations showing proposed development in context

3D Model Requirements

Planning consultants typically require models in .3DS, .DWG, .SKP, .RVT, or .OBJ formats:

• Accuracy: Reasonably accurate representation of existing buildings
• Coordinate System: Model should be properly located and oriented to match the site
• Proposed Development: External envelope, all balconies, projections, accurate window positions
• Surrounding Context: All buildings within 50m radius with window positions on affected neighbours

What Information Architects Must Provide

Item	Details Required
Window Schedule	Dimensions, head/cill heights, frame width, room served, glazing spec
Room Schedule	Reference numbers, room use, net floor area, ceiling height
Site Information	Topographical survey with AOD levels, tree survey, boundary treatments
Design Intent	Façade materials (reflectance), balcony specs, external shading devices

Common Validation Issues to Avoid