Daylight and Sunlight Assessment: Natural Light Calculations

Assessing Daylight and Sunlight

A Daylight and Sunlight Assessment evaluates how a proposed development will affect the natural light received by existing buildings and by the new spaces you are creating and sets out whether those impacts fall within accepted best practice standards. It is widely relied upon by planning authorities, designers, surveyors and consultants to help balance development needs with the amenity and wellbeing of people in and around the site.

Local planning policy and technical guidance do not mandate a single national rule for daylight and sunlight, but most councils require or expect assessments to be submitted in accordance with recognised industry standards, particularly latest guidance from the Royal Institution of Chartered Surveyors (RICS) and the Building Research Establishment (BRE).

What is a Daylight and Sunlight Assessment?

A Daylight and Sunlight Assessment is a detailed technical analysis and report that evaluates the impact of a proposed development on:

• Daylight received by internal spaces: primarily rooms used for living, dining, sleeping and cooking.
• Sunlight received by both internal spaces and external amenity areas such as gardens or communal courtyards.
• Overshadowing assessments are made of neighbouring properties, open spaces, and key amenities.

This assessment focuses on amenity and planning considerations rather than legal property rights (such as “Right to Light” civil law matters).

The methodology used in these assessments is rooted in nationally recognised guidance, most commonly the BRE Report: Site layout planning for daylight and sunlight: a guide to good practice (BR209, 3rd Edition – 2022) and associated British Standards like BS EN 17037:2018 that inform tests for internal daylight and sunlight assessments.

Why do you need a Daylight and Sunlight Assessment?

These assessments are typically requested by:

• Local authorities as part of a planning application, especially for residential, mixed-use or large developments.
• Developers and design teams who want to demonstrate compliance with planning policy and avoid delays or objections relating to loss of light.
• Architects and consultants aiming to optimise design in response to potential light impacts.

Assessments are essential in situations where a development could:

• Reduce existing levels of daylight or sunlight to surrounding properties or communal spaces.
• Cause overshadowing of gardens, playgrounds or amenity areas.
• Influence the internal light quality of new dwellings or offices.
• Trigger objections from local residents or statutory consultees.

Without a proper assessment, a planning application may be delayed, refused, or require redesign, even if the rest of the application is technically sound.

How a Daylight and Sunlight Assessment Must be Carried Out

Define the Brief and Gather Base Data

A quality assessment starts with a clear brief and collection of baseline information, including:

• Architectural plans and 3D models of the proposed new developments and surrounding context.
• Existing site surveys showing the position, size and orientation of neighbouring buildings, windows and amenity spaces.
• Identification of receptors: the windows, rooms and outdoor areas that could be affected by the development.

Professional sunlight surveyors, planners or environmental consultants will often co-ordinate with architects at this stage to ensure the climate based daylight modelling is accurate and representative.

Model the Site and Proposed Development

Modern assessments use 3D modelling and specialist software (e.g., CAD or radiance-based simulation tools) to recreate:

• Existing levels of light before development.
• Proposed conditions: light levels with the development in place.

This enables precise, reliable calculations of how daylight and sunlight will change for receptors around the site.

Apply Standardised Tests

Assessors typically use a combination of tests from the BRE guidance to quantify daylight and sunlight impacts:

Daylight Tests

• Vertical Sky Component (VSC): measures the proportion of visible sky from a window; indicates how much daylight is available outside a window.
• No Sky Line (NSL): assesses how much of a room’s interior floor area can receive daylight from that window.
• Average Daylight Factor (ADF) / Illuminance Metrics: used for proposed new units, especially under British Standard BS EN 17037:2018.

Sunlight Tests

• Annual Probable Sunlight Hours (APSH): measures how much direct sunlight a window receives over the year and in winter months.
• Sunlight to Amenity Spaces: assesses if outdoor areas retain adequate levels of sunny hours on representative dates such as the spring equinox (21 March).

These checks are benchmarked against advisory thresholds provided in the BRE Guidelines. While not mandatory, these thresholds help planners evaluate the significance of change in light levels.

Analyse Results

The assessor compares baseline and proposed values for each receptor and determines whether any reduction:

• Exceeds a percentage that is likely to be noticeable to occupants.
• Falls below recommended values for healthy, usable spaces.

Results are interpreted with professional judgement, taking into account site context (e.g., dense urban vs suburban settings) and material planning considerations.

Report Findings in a Structured Format

A daylight and sunlight report typically includes:

• Introduction and scope: why the assessment was commissioned and what it covers.
• Methodology: standards and software used (e.g., BRE Guidance BR209 2022).
• Baseline conditions: existing VSC, NSL, APSH and other metrics.
• Proposed conditions: predicted changes and numerical results.
• Illustrations: shadow diagrams, 3D models, and receptor maps.
• Assessment commentary: interpretation of results against BRE guidelines.
• Conclusions and recommendations: whether the impact is acceptable or if mitigation is advised.

Key tests explained

Vertical Sky Component

VSC assesses how much direct view of the sky is available to a window. A higher value generally indicates better daylight availability. If the VSC at a neighbouring window drops below 27% or less than 80% of the baseline value, this is likely to be noticeable and may be considered unacceptable without justification or mitigation.

No Sky Line / Daylight Distribution

The NSL test measures how much of a room’s internal floor area can “see” the sky through a window. Reductions below 80% of the original daylight distribution suggest that the room will rely more on artificial light.

Annual Probable Sunlight Hours

APSH considers existing levels of sunlight received both annually and in key periods such as winter. A room typically should receive a minimum annual percentage of sunlight, and significant reductions (again often benchmarked at 80%) may indicate an impact on amenity and comfort.

Sunlight to Amenity Spaces

Outdoor spaces are tested to determine whether they continue to receive a minimum number of sunlight hours on key dates (e.g., 21 March). Reductions below thresholds may lead to resident objections or planning concerns.

When are Assessments most Commonly Required?

Local authorities often ask for daylight sunlight assessments when:

• The development is close to new and existing buildings, especially residential properties.
• Multiple storeys or higher-density developments are proposed.
• Changes to window positions, height or orientation are included.
• There is potential for overshadowing of gardens, courtyards or public spaces.
• Significant objections on grounds of light loss have been raised.

Some local authorities explicitly set out thresholds (e.g., number of residential units) where assessments are expected, and a daylight factor target, but as requirements vary between authorities, early engagement is recommended.

Best Practice and RICS standards

The RICS has published professional standards on daylighting and sunlighting that support competent practitioners in delivering robust assessments. These standards emphasise understanding the original briefing, adopting appropriate methodologies and aligning reports to planning policy, BRE guidelines and other accepted guidelines.

While the RICS document on daylighting and sunlighting was originally issued as a professional guidance note and more recently reissued as a standard, its core purpose remains to ensure that practitioners apply consistent, professional methods in analysing and reporting on daylight sunlight impacts.

Get Support for your Planning Application

A Daylight and Sunlight Assessment is more than a technical requirement: it is a tool for demonstrating responsible and sustainable design that respects existing communities while creating healthy, well-lit spaces for future occupants.

By applying recognised methodologies and professional standards such as those advocated by the RICS and BRE, developers and their teams can prepare robust evidence that supports planning applications, reduces the risk of objections, and ultimately contributes to high-quality built environments.

If you’d like professional support preparing or interpreting a daylight and sunlight assessment for your project, we partner with experienced professionals around the country who can assist at every stage, from scoping to final reporting. Simply contact us and we will provide a free quote for your work.