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.

[Roger440]

4 hours ago, ADLIan said:

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.

 

Is it really that low?

 

How are people doing these extensions with bi folds the full width and sky lights/lanterns etc?

[Redbeard]

 

2 hours ago, Roger440 said:

How are people doing these extensions with bi folds the full width and sky lights/lanterns etc?

From the link I posted above:

2 – Whole Dwelling Standard Assessment Procedure (SAP)

 A designer can exploit even more design flexibility by carrying out a SAP (version 10) energy rating calculation for the whole extended dwelling. Using this method a comparison is made with the expected carbon emissions and energy performance of a ‘notional’ dwelling with an equivalent (same size and shape) extension that complies with the fixed limits set out above. This method gives a designer the potential for ‘trade-off’ by improving the performance of the existing house. The upgrades must at least meet the standards in table 3 below. We ask that all SAP calculations are carried out by an accredited energy assessor.

Table 3 – Minimum Standards for Upgrading Existing Thermal Elements

                                                                        U-Value (W/m² ^oK)

Wall – cavity insulation	0.55
Wall – external or internal insulation	0.30
Floor	0.25
Flat or pitched roof – insulation at ceiling level	0.16

[carson321]

Yeah, I might be going down the SAP route just for the ease, but I think I have now found the section in the most recent document, 10.7 d, it is just not as explicit as it used to be. Current revision states

'The total area of windows, roof windows, rooflights and doors in extensions should not exceed 
the sum of the following. 
i. 25% of the floor area of the extension.
ii. The total area of any windows and doors which no longer exist or are no longer exposed 
due to the extension.

 

Which i assume means i + ii so does account for the removed openings. 

 

Thanks guys.