Dark Skies in South Downs National Park: Achieving compliance without over-dimming

By Nikolaos Protogeros, Technical Director, SRE

The South Downs National Park (SDNP) is more than just rolling hills and breathtaking views; at night, it transforms into one of the UK's most precious astronomical theatres. In May 2016, it was designated an International Dark Sky Reserve (IDSR), a testament to the stunning quality of its starry nights and a commitment to protect them as one of only around 25 such sites globally. For developers, architects, planners and lighting designers, this special status presents a unique challenge: how do you provide safe, functional and aesthetically pleasing outdoor lighting while adhering to the stringent policies designed to combat light pollution?

Thankfully, compliance doesn’t mean plunging projects into darkness or accepting over-dimming which could compromise safety and design intent. The reality is that achieving compliance is not about eliminating light but about implementing intelligent, considered lighting design.

In this blog, I’ll provide clear, actionable guidance on how to evidence compliance with the SDNP's dark sky policies. I’ll explore how taking a proactive approach, backed by a robust Lighting Impact Assessment (LIA), can lead to successful planning applications without sacrificing lighting quality.

Understanding the SDNP's ‘Dark Sky’ Policy: What you need to know

The core purpose of the SDNP's lighting policy, detailed in documents like the Dark Skies Technical Advice Note, is to protect the special qualities of the park. Artificial light, when poorly designed, has significant negative impacts on nocturnal wildlife, human health and the simple, profound beauty of a star-filled sky.

The planning policies embedded in the SDNP's Local Plan are designed to control and mitigate these impacts. To comply, you must address the three main forms of light pollution:

• Skyglow: The brightening of the night sky over inhabited areas, caused by light scattering in the atmosphere. Fully shielded, downward-directed lighting is the primary solution.

• Glare: The uncomfortable brightness of a light source, which can impair vision and create safety hazards. This is managed by controlling brightness and ensuring light is well-directed.

• Light Trespass (or Intrusion): When light spills beyond the boundary of a property, creating a nuisance for neighbours or disturbing habitats.

The principles of ‘smart’ lighting design: More than just turning down the lights

Effective compliance starts at the design stage. It’s not just about dimming lights as an afterthought; it’s embracing a philosophy of smart, efficient and considerate illumination. The guiding principle should be: the right light in the right place, at the right time.

Here are the key design considerations to embed in your project from day one:

• Shielding and Direction: All external luminaires must be fully shielded and downward-facing to ensure zero upward light spill (a 0% Upward Light Ratio). Light should be directed precisely where it is needed, preventing it from escaping above the horizontal plane.

• Colour Temperature: The colour of the light is critical. Blue-rich white light is more disruptive to nocturnal ecosystems and contributes more to skyglow. Planning guidance consistently points towards the use of warm colour temperatures, typically 3000 Kelvin (K) or less. For particularly sensitive areas, especially near wildlife corridors, a warmer 2700K is often recommended.

• Intensity: Use the lowest effective light level for the task since over-lighting is a primary cause of light pollution. British Standards provide guidance on appropriate illuminance levels (measured in lux) for different applications. A ‘one size fits all’ approach rarely works; lighting levels must be justified and appropriate for the specific context.

• Controls: The most effective way to reduce light pollution is to ensure lights are only on when needed. The use of controls like timers, motion sensors (PIRs), and dimming systems is a fundamental part of a dark sky friendly lighting scheme. For example, lighting in a car park could be dimmed to a low level after hours and only brought to full brightness when motion is detected, or when activated.

• Internal to External Spill: Light spill from windows is a significant, often overlooked, contributor to light pollution, especially in designs featuring large glazed elements. While challenging, this must be addressed. Mitigation measures include specifying glazing with lower visible light transmittance (VLT) values, integrating internal blinds or curtains into the design and accounting for their use in the assessment, and installing occupancy sensors so internal lights operate only when the space is occupied.

Evidencing compliance: Your toolkit for a successful planning application

A great design is only half the battle - you must be able to prove its compliance to the planning authority. From our experience at SRE, a detailed and robust evidence base is the key to a smooth planning process and avoiding costly Requests for Information (RFIs).

Lighting Impact Assessment (LIA)

A LIA is the cornerstone of your evidence. This technical report, prepared by a competent professional, provides a comprehensive evaluation of your proposed lighting scheme. A robust LIA should always include:

• A Baseline Survey: This establishes the existing lighting conditions on and around the site, identifying the environmental zone and any sensitive receptors.

• Identification of Sensitive Receptors: This includes neighbouring residential properties, ecologically sensitive habitats (like bat corridors) and important viewpoints within the landscape.

• Detailed Luminaire Specifications: Provide full technical data for every proposed light fitting, including wattage, lumen output, colour temperature (CCT) and Upward Light Ratio (ULR).

• Lighting Design and Modelling: Using lighting design software, produce detailed plans showing the location of each light fitting. These should be accompanied by isolux contours - maps that visually demonstrate the predicted light levels and clearly show that light spill is contained within the site boundaries.

• Mitigation Measures: Clearly state the measures taken to minimise light pollution, referencing the design principles of shielding, direction, colour temperature and controls.

Supporting documentation

Your LIA should be referenced within your Design and Access Statement, which outlines your lighting strategy and explains how it aligns with SDNP policy. You should always include the manufacturer’s product specification sheets for all proposed luminaires as appendices to your application. This provides the planning officer with indisputable proof of the fittings' performance characteristics.

Top 5 causes of non-compliance: A quick check

1. Upward Light Spill
Pre-Check: Are all luminaires fully shielded with zero light output above the horizontal?
Fix: Specify only fully shielded, IDA-compliant style fixtures. Check manufacturer data sheets for a 0% ULR.
2. Incorrect Colour Temperature
Pre-Check: Is the Correlated Colour Temperature (CCT) of all lamps 3000K or lower?
Fix: Specify lamps with a warm white colour (max 3000K, ideally 2700K in sensitive locations).
3. Over-lighting
Pre-Check: Are the proposed light levels (lux) appropriate for the task and location?
Fix: Justify all lighting levels against relevant British Standards and use the minimum light necessary for safety and function.
4. Poor Controls
Pre-Check: Are lights left on all night unnecessarily?
Fix: Incorporate timers, dimmers and motion sensors to ensure light is only used when and where it is needed.
5. Unmitigated Glazing
Pre-Check: Has light spill from windows and rooflights been assessed and addressed?
Fix: Assess internal light spill in the LIA. Consider lower transmission glass or integral blinds as mitigation.

Conclusion: A collaborative approach to a star-studded future

Protecting the dark skies of the South Downs National Park could be seen as yet another planning policy with which to comply. However, it can also be viewed as an opportunity to lead development towards more sustainable lighting solutions which are energy-efficient, respectful to their context and technologically advanced.

The key takeaways:

• Compliance is about smart design, not just dimming. A proactive, principles-led approach is essential.

• Thorough evidence is vital for planning success. A detailed LIA is your most critical tool.

• Early engagement with lighting professionals is key. Expert advice can save time, reduce costs and de-risk the planning process.

Navigating the complexities of Dark Sky Assessments requires specialist knowledge. If you are working on a project within the SDNP or another dark sky area, seeking expert advice will ensure your lighting scheme is not only compliant but also a high-quality addition to the built environment.

To find out more about Lighting Impact Assessments and Dark Skies Assessments or to discuss how we can support your upcoming projects, contact Nikolaos Protogeros.

Night sky image by Jamie Fielding.