16th January 2017

FAQ: Is There a Limit to the Amount of Glazing I Can Have in an Extension?

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Posted by: Carly

There is a stated guideline for an extension that the glass area must not exceed 25% of the overall floor area. However, this is a guideline and not a rule and there is a common misconception that this cannot be overcome.

There are many ways to get around this issue to maximise the glazing in your extension and these are achieved by ‘trade off’. This is all to do with heat loss and energy efficiency. You can have a fully glazed contemporary extension if you desire. This could be in the form of a conservatory, Livin Room or Wintergarden. Or you could have a solid roof structure with masses of double glazing, incorporating Bi-Folding or Sliding Doors as well as large fixed or openable windows and Glazed Roof Lights.

Essentially, a nominal brick and tile structure with a maximum 25% glass area will perform to a certain level. It will require an amount of energy to sustain it under conventional methods, using materials and standards laid out within the guidelines of building control. Any changes made to the design or use of the structure that have a negative impact on the performance of the extension, must be offset by something that has a positive improvement: ‘Trade Off’.

Any material or specification that is over and above standard requirement would mean an improved performance to off-set against the glass. Therefore, any building method used that is better than standard, would aid the increased glass area. Alternatively, changes can be made within the existing home to justify, or balance out, a lower performance in the extension.

The latter is more easily achieved in an older property. These days, houses are built to more efficient standards and anything built after 2004 will be more difficult to upgrade. However, this ‘Trade Off’ is justified by SAP Calculations.

A SAP Calculation is very easy to obtain. All you need is a floor plan of the existing house, plans of the proposed extension and a report on things such as the current boiler, central heating system, fuels used to run the house, etc. Your architect can supply this information to an Energy Consultancy, or you can pay for them to carry out a survey on your behalf. A basic SAP calculation based on information supplied by you or your architect can be as little as £150 + VAT. Any extension can be made to work with these calculations. Once you know exactly what is required, you can decide whether the increased work is worthwhile, or whether reducing the glass area slightly is a better compromise.

Some architects are up to speed with the modern guidelines, in which case, this process should be very easy for you and they will no doubt guide you through it. However, some more traditional architects are not so familiar with it and can be known to wrongly advise potential clients of a restriction limiting the design. This limits the potential for the home owner to truly explore their desire for a light and airy addition or transformation of their home.

Some examples of ‘Trade Off’ and how it can be achieved to improve or increase glass area are incredibly simple. For instance:

  • You can add any glass area being replaced on the existing house in to the amount of glass for the extension. For example, if you are extending off the kitchen and an old set of patio doors and a window are being covered by the new extension, then the square area of that glass can be added to the calculation for the extension
  • Upgraded wall and roof insulation in the proposed extension
  • Use improved glazing within the extension such as triple glazing or more energy efficient double glazing for an improved U-Value
  • You can replace your existing boiler for a more energy efficient one if it is an older model
  • Add thermostatic controls to your existing radiators if you do not already have them
  • Add cavity wall insulation to the existing house or increase loft Insulation
  • Replace existing windows with more modern, energy efficient windows
  • Use LED or Low Energy Lighting in the extension, or replace some of the existing lighting in the home with the same

These are just a very few of the basic options available without spending silly money on super modern gimmicks or massively expensive renewable solutions. Many of the things on the list above could well already be involved in your project due to carrying out some work in the existing house. All you need to do is show this and justify it.

Dependent on the size of the extension in comparison to the size of the original house and the amount of glass you’re looking for, this calculation can become more demanding. However, you can achieve up to 100% glass provided you are able and prepared to meet the Trade Off required.

If you’d like to discuss your project, please contact us.


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