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25% Glazing Rule!

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#1 nickam


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Posted 10 April 2013 - 12:41 PM

A question for those other technicians out there doing plans (Temp?). I know this one has been addressed before but I'm interested to know other's experiences. This is for when an extension is added to an existing building and the opening into the new is opened up so they become one room. As far as I understand, the rule it's worked out by calculating the combined floor areas of the new room plus the existing room. Then you get 25% of this combined floor area plus the area of any existing doors and windows removed in the conversion before you need to do any offset in the element approach to calculating U-Values?

#2 temp


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Posted 10 April 2013 - 04:18 PM

It's not clear in the regulations what happens if an extension enlarges an existing room but I don't think you are right.

If the extension were to form a seperate room then page 12 para 4.2 would apply..


That says 25% of the area of the extension plus any external doors or windows that no longer exist.

That's consistent with the old 25% rule for new houses. It's equivalent to moving the window that was covered up (so still 25%) then adding an extension with it's own 25%.

In the case where it's all one room I believe they would limit you so that when finished the total area of all external windows and doors was 25% of the new total floor area. Thus still 25% of the floor area. Your method would result in more than 25% of the total floor area.

#3 nickam


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Posted 10 April 2013 - 05:14 PM

Hi Temp,
Yes, the Local Authority has the same opinion as yourself but I seem to remember that we disussed this very same problem a couple of years ago on this forum and at that time you seem to have been of the opinion that both the new and existing floor areas could be summed together. Anyway, I was just wondering if some Local Authorities interpreted it differently but ultimately it's, of course, down to them to approve it anyway so I'll have to comply. Thanks for your reply.

Edited by joiner, 10 April 2013 - 07:45 PM.

#4 cds


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Posted 10 April 2013 - 08:37 PM

Building regulations are very clear it is 25% of the floor area of the extension (Part L1b Section 4.2 a)

Edited by cds, 10 April 2013 - 08:37 PM.