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Green Building Bible, Fourth Edition
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Building Regulations and Standards: Part L1B 25% rule

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      CommentAuthorJustin
    • CommentTimeJan 18th 2008
     
    Hi,
    Section 15 reads:

    "..plus
    b. The area of any windows or doors which as a result of the extension works, no longer exist or are no longer exposed"

    So can "windows which no longer exist" be in a different part of the dwelling to the area of the extension? I'm unsure quite what's meant by "as a result of the extension works". Clearly openings specifically covered over by the extension are additional basic allowance, but do openings which I might shrink or cover over elsewhere in the dwelling during the same work, also add classify as an additional allowance against the 25% rule?

    (Yes I'm aware of the "optional approaches", and have been looking carefully at those, but want to ensure I've fully understood this basic one first)
    • CommentAuthorCWatters
    • CommentTimeJan 18th 2008
     
    The 25% rule for a NEW house was on a room by room basis. I believe the section you refer to is trying to extend that to an extension of a room.

    I suspect in complex cases the BCO will probably take a step back and look at the extended room as a whole (as if it were a new house built to the 25% rule). So after the room is extended.. is the total area of windows and doors less than 25% of the total area of the new room larger room.

    If you are building new rooms rather than extending an existing room they will be treated in isolation.

    So I think overall the answer to your question is no. You can't block up a window in a kitchen and add that to the allowance for say an upstairs bedroom under the old 25% rule. As you say, you might be able to achieve a similar effect under more recent rules that look at the overall CO emissions of the house. eg section 20 the SAP calculation.

    If in doubt have a word with your BCO, they are generally quite helpful and the more you work with them the more likely they are to bend the rules when you really need help. For example they probably won't make you change a window if the area works out at a llittle over the 25%. Particularly if they can see you are putting more insulation in the walls than necessary to comply.
  3.  
    Hi Justin. I don't think you can offset glazing areas in the way you suggest, as doing so could result in huge areas of glazing on a single elevation, which in turn could result in disproportionate heat gains and/or losses.

    As with all the AD's however, I have the feeling that this is down to the interperitation of your Building Inspector.
    •  
      CommentAuthorJustin
    • CommentTimeJan 19th 2008
     
    Thanks for those replies. The "one room" case sounds like it could have value (which would help me out), so I'll keep that in mind when I talk to BCO.

    Regards,

    Justin
  5.  
    I went for the "weighted average U-value" approach on an extension I did last summer. BC accepted it. Perhaps that is one of the "optional approaches" you are already aware of?

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