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The Regs: How to get a highly glazed extension to comply with Building Regulations

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If you have ever wondered how an architect got that glass box extension past building control, then Geoff Wilkinson has the answer

There has been a huge trend in demand for extensions that join internal living spaces to the garden by way of large areas of glazing and ever larger opening bi-fold doors. However this sleek and transparent look desired by homeowners can be at odds with Building Regulations Part L, which sets targets for the building envelope’s energy efficiency.

Part L limits the total area of openings or glazed elements, such as windows, roof windows and doors, to a maximum of 25 per cent of the extension’s floor area. A small extension with a set of patio doors and a lantern light can easily use up this 25 per cent allowance. 

An extension that exceeds the allowance is generally referred to as an ‘over-glazed extension’, but they can also be known as sun rooms, orangeries or even glass-box extensions.

Architects will find that, where the 25 per cent rule is exceeded, building control will either reject the plans or request further justification for the over-glazing. So how do you get an over-glazed extension to comply?

Well, although the extension is not compliant on an individual element basis, it could still pass L1B regulations using one of the alternative methods set out in the approved document.

The first thing to do is to deduct the total area of any windows or doors that, as a result of the extension works, no longer exist or are no longer exposed. This is because the heat lost through these elements is no longer being lost. In many cases you will find that this is all you need to do, and a quick schedule of openings versus covered openings may be all that you need. 

For example: if an extension has a floor area of 30m² and is covering an existing patio door with an area of 4m² then the extension is allowed to have 6m² (25 per cent rule) plus 4m² (existing openings rule) making an allowable area of 10m². Submit that with your Building Regs application and it’s likely to sail through the checking process.

But if this still isn’t enough then the next option is to demonstrate that the proposed extension is no less compliant than if an extension of the same size and shape was built according to the 25 per cent limit. For example, you could increase the thermal resistance (U-value) of the walls, floor, roof, or glazing elements above the default values in order to increase the area of glass.

From experience I can say that in most cases the heat loss though the glazing will dominate, so start there. Given that the default U-values for walls, floors and roof should all be in the range 0.16-0.28W/m2K, there is not much you can do to significantly improve these. However, the notional glazing has a U-value of 1.6-1.8W/m2K and means that high-performance, gas-filled, low-e-coated glass could see the U-value tumble to 0.9 or better, allowing you more than 40 per cent glazing. Take out some covered-up openings and a 50 per cent glazing ratio is easy to achieve.

But if the extension is very highly glazed (more than 50 per cent), you may need to use the Standard Assessment Procedure (SAP). These calculations should be carried out by an accredited SAP assessor, and demonstrate that the calculated carbon dioxide (CO2) emission rate from the dwelling with its proposed extension would be no greater than the dwelling and a notional fully compliant extension of the same size and shape. This method allows you to keep almost 100 per cent glazing by upgrading existing elements and services within the original part of the house instead – for example, by installing a more efficient heating and hot water system, or increasing the insulation within the existing roof space.

The SAP method allows you to keep almost 100 per cent glazing by upgrading existing elements within the original part of the house

The building control officer may well confuse matters by asking for ‘excess glazing calculations’, ‘SAP calculations’, ‘heat loss calculations’ or ‘thermal calculations’, but in truth all four terms mean the same thing. If you are not qualified to carry out these calculations yourself then I would recommend Googling ‘heat loss calculations for over-glazed extensions’ and you will find a number of consultants that can produce them for you.

In many cases it’s only a couple of hours work for them and a budget of £250 can usually cover it – a small price to pay for that award-winning, highly glazed look on your project.

Geoff Wilkinson is managing director of Wilkinson Construction Consultants www.thebuildinginspector.org

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Readers' comments (2)

  • Which is fabulous if entering fashion awards is all you aspire to.

    If, on the other hand, you have one of those difficult clients who expects his home extension will be fit-for-purpose i.e. actually habitable throughout winter and summer, you may want expend more of your creative design skills carefully integrating appropriate insulation, and solar shielding into the roof.

    Part L exists to set a minimum standard of energy efficiency. Raw compliance does not demonstrate competent design.

    Glass boxes can be made to scrape through current regs if you bolt on enough compensatory measures. The stunning example pictured above would unfortunately fail most comfort metrics e.g. thermal comfort (radiant heat asymmetry), visual comfort (disability glare, daylight uniformity), acoustic comfort (reverberation time), air quality (openable windows/ducted ventilation..?!) etc.

    Great design usually costs more than a £250 Get-out-of-Jail card.

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