Overglazed Extension and Latest Part L

[carson321]

Hi All, 

 

we will be having an 'overglazed' extension built, big sliding doors and roof lanterns. Looking at various calculators available for doing the average area weighted u calculations, and comparing against a notional extension, they all make reference to an allowance for the openings that will no longer exist due to being incorporated into the new building. But what I cannot find for myself is exactly where in the Part L document that it states this! Is anyone able to point me to where this bit of information is recorded? It makes a huge difference to the calculation if these openings can be accounted for or not. 

 

Also, when working out the new glazed areas, how do you calculate it for a roof lantern, where the glass structure is pyramidal and not just simply flat at the size of the opening? Do I need to work out the area of each pane of glass in the lantern and add it together?

 

[Redbeard]

I cannot find it immediately in Part L but how about this which shows the method and refers specifically to Part L: https://www.hertfordshirebc.co.uk/guidance-note/designing-domestic-extensions-guide-to-compliance/ 

?

 

 

[ADLIan]

From memory it’s towards the end of AD L1, section on extensions. Max glazed area of extension is 25% of floor area of extension plus the m² of any openings made redundant as a result of the extension. With roof lanterns/pyramids the glazed area is based on the area of each glazed section, not just the hole in the ceiling. With big sliding doors and a roof lantern you may fall foul of the ‘simple’ methods and need the SAP calculation route.