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How Do I Know a Daylight Analysis Report Has been Prepared Properly and Honestly?


You may be in the process of designing a building, studying a planning application for a new building near your home, or considering undertaking your own development. When faced with a long technical report, how do you know it’s trustworthy? Will it stand up to scrutiny through the planning process? What can you do if you are suspicious of its findings?

Daylight and overshadowing are primary concerns for building designers, but are also emotive surrogate issues for land owners competing for comfort and space in our urban areas. Despite there being no “right to light ” law here, “light” forms the substance of many planning “objections”, observations and even legal disputes. This can (and should) be discussed rationally, using readily verifiable factual analysis. This article describes the quantitative process which should be used to create proper professional opinions as to the adequacy of light in and around new buildings.

Here’s what to look for:

  • The geometry of the 3D modelling will be directly based on the drawings upon which planning permission has been approved, there should be few discrepancies.
  • Dimensions should be shown on the model used for analysis, for verification by independent readers of the report.
  • The report will contain all key numerical values input into the analytical software, which could be used to independently recreate the results
  • Namely values for: latitude, longitude of the site, orientation (angle to north), material reflectance values for walls, roofs, floors, and ground surfaces should be realistic; light transmission values for glass should also be realistic also.
  • The name and accreditation of the analytical software will be provided
  • Any undertakings as to the adequacy of light levels will be based on an independent system of assessment, such as BRE or LEED.
  • Findings will refer to specific sections of the BRE or LEED documents, and these will correspond to the physical reality of the scenario proposed.
  • Interpretation of values is an important part of the process; this should refer to objective third party standards wherever possible.

We provide daylight analysis services, typically in support of planning applications for housing developments. In addition, as agents to parties involved in complex planning processes, we have occasionally been required to comment on the efficacy of other firms’ reports, which has caused us to consider how a person might seek to mislead a reader or misrepresent findings. This article is based on that experience.

Daylight analysis has historically been relatively rare in the Irish planning process. In the UK, legislation has existed to quantify the “right to light” for about a century, using a variety of paper based geometric tools, but no such law exists here. However, the assessment standards which exist in the British system are available for use here, and are often relied upon by planners to quantitatively support decisions in the increasingly contentious world of multiple housing and office developments, in dense urban settings.  

Misrepresentation is Possible

It is possible to force analytical software to output favorable daylight metrics for a proposed building design, by strategically mis-estimating input values for the calculations. It can be difficult to spot this if you haven’t actively been involved in the preparation of one of these reports.

How the Process Works

Daylight analysis is based on creating a “digital twin” of the building using the drawings upon which planning permission is based, then using software to simulate the effect of sunlight within the spaces, as the sun moves through the sky, changes through the year, is obstructed by buildings and trees, or is reflected off surfaces inside and outside buildings. 

Typical Report Contents

As such, there are several steps in the work which must be completed competently and honestly, there are key points where honest errors can occur, or erroneous values can be inputted which skew the output values.
A report like this typically comprises of:

  1. A digital model of the building and adjacent homes and other structures
  2. Assignment of values used for the calculations such as reflectances of materials, physical latitude and longitude, orientation and other parameters required for accurate results. 
  3. In some cases, paper based analytical tools are used, but the process is generally based on analytical software. 
  4. The software creates tables of values for key metrics like sky component, daylight factors and luminance in internal and external spaces which are of concern to users of the report. 
  5. These are compared to official standards to determine the suitability of the scheme design and the adequacy of the light remaining after construction of the new building.
  6. There are various “rule books”, to set calculated values against; none of which have particular standing in Irish law.

What Can be Done to Double- Check?

To assess the accuracy of the report, you could complete all or some of these steps, I have listed them in ascending order of difficulty and cost:

  1. Check the calculated values provided against the various official standards
  2. Obtain the full text of the reference document the report uses as the basis of its assessment.
  3. Consider the logic of the interpretation of the values: is the scenario described in the official standards really the same as the proposed development? Is there another set of requirements in the reference document which has been overlooked in the report or which better reflects the scenario?
  4. Confirm the geometry of the model used by checking key dimensions of the building. (Variances in building geometry less than 100mm are unlikely to prove significant to the overall finding of the report.
  5. Check the accuracy of values they used for material reflectances in the calculations; they may not match the actual properties of materials which are used in the construction. 
  6. Recreate the digital model and conduct your own independent calculations. 

In practice, the software can be manipulated to create more favourable values, by making external surfaces (like the walls of the new building) more reflective, (anything close to 1 is unrealistic) and by making glass in windows more transmissive (any value above 0.8 is unrealistic). In some cases, the reports provide an overview of the method, and detailed conclusions, but do not provide key numeric values needed to independently verify the results.    

Obviously, step 6 above is a lot of work, and would take several weeks for another consultancy firm to complete. It is crucial to be realistic about the available budget for this process? If you are concerned about the contents of a daylight analysis report pertaining to a new building in your area, the best course of action may be to make an observation through the proper planning channels quoting specific concerns; the onus is ultimately on the owner of the proposed scheme to provide any clarification which may be needed, and indeed to commission reports which are readable and unambiguous.

A good starting point, if you have general concerns about the efficacy of a daylight analysis report could be to hire an independent professional to review the report on the basis of an hourly rate, and give you some pointers as to any obvious “red flags” and likely avenues in your favor. 

Proper Use of Daylight Analysis in Design

These tools were developed not to settle disputes, but to assist designers in designing buildings to optimize light quality by natural and artificial means. Ideally, they would be employed not after a building has been submitted for planning permission, but during the design phase, when calculations could be used to enhance design quality.

It must be said that all qualified and experienced architects have the ability to determine natural light quality from drawings with tremendous accuracy, completely unaided by software.

Legitimate Steps to Mitigate Poor Light Quality in Difficult Locations

When a problem area is identified, it often results from site constraints such as proximity to a north facing wall, a tall obstruction or the simple spatial reality of building in a city. At a developed stage of design many factors have already been balanced and considered to determine the shape and size of a building; it is not always practical or desirable to reinvent the scheme to improve light reaching a small number of problem spaces.

Ideally, massing on the site and the overall shape of the building would guarantee good light to all spaces, but this is seldom a sustainable use of space in urban areas.

As a general rule of thumb:

  • Increase window sizes
  • Increase quality of glass to increase transmission of light
  • Use interior finishes with lighter colors and more reflective surfaces
  • Re-purpose areas with poor light quality for uses which either require little light, OR which demand very high levels of artificial light.
  • For example, cooking areas always need lots of artificial light, but dining areas benefit from natural light, and the appearance of being naturally lit.
  • Consider the color and reflectance of external obstructions, such as the color of tree foliage, courtyard walls and pavings. These all contribute to indoor light quality in problem areas.

Avoid Spurious or Vexatious Planning Observations on the Basis of Light

“Light” is a term often used interchangeably with “overbearing”, “overlooking” and “overshadowing” in planning observations. When considering any observation to a planning authority on the basis of light, it is advisable to study the issue in detail, be sure of the relevant terminology and be satisfied that there is either: an absence of information that should have been provided, or a finding which belies common sense to the extent that a demonstration of the efficacy of the analytical process is required.

Creating a daylight analysis report is an expensive undertaking, to the extent that it could easily double the professional fees needed to secure planning permission for a small project like a house extension or even a small apartment scheme.

This is a significant imposition of costs, which should not be taken lightly.

DKAD offers daylight analysis and consultancy services.

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