Technical Booklet F2 Conservation of Fuel and Power in Buildings Other Than Dwellings·Page 51·3.20
Opening areas – maximum glazing percentages
The total area of windows and rooflights in an extension must not exceed specified percentages of exposed wall or roof area, unless the adjoining existing building has a higher glazing percentage, in which case the extension may match that higher percentage.
The total area of windows and rooflights in an extension should not exceed the values given in Table 3.1 unless a greater percentage of glazing is present in the elevations of the adjoining part of the existing building to which the extension is attached. In such cases, the area of glazing may be increased but should not exceed the percentage of glazing in the adjoining part of the existing building.
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