Building Control Fire Regulations - effect on windows

Hi all

First post, seems like a knowledgeable place so thought you may be able to help.

I'm building a 2 storey side extension to a semi detached house. It comes to 80cm from the boundary (i.e. I'm building on my driveway). There is a door and several windows on the wall (obscured glazing for bathrooms/wc/utility room/kitchen and landing).

Building control have informed me that it needs to conform to their technical booklet regarding fire spread. Essentially this limits the area of unprotected openings on the wall (i.e. windows and doors). I'm having difficulty working out how much total opening I can have as I may have to modify plans (reducing window sizes) to achieve this.

Does anyone know the max. area of openings I can have at 80cm from the boundary? The wall is 13m long and approx 5.5m high to top of windows. The area which actually has windows within it is around 37.5m2 (referred to as the enclosing rectangle in the regulations) and the total area of all openings is around 6.5m2. All of the windows are less than 1m2 each apart from one which is 1.5m2 but could be reduced if needed.

I appreciate this is a very technical question but hopefully someone is familiar with this part of the building regs and can share their expertise?

I'm hoping there is an easy way to work it out rather than getting into heat transfer calculations!

Cheers

Refer to AD B (From Page 44)...

http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADB1_2006.pdf

You would have to refer to Diagram 20 as you are less than 1000mm from the boundary.

If you used Table 4 as a base, then for 800mm, you would be talking about a percentage of unprotected areas of 6.4%. However, they state Table 4 cannot be used if the distance between the boundary and wall is less than 1000mm, hence why I said use it as a "base".