Fire proof windows and frames

[everylittle]

We need to know what the distance between our house and our next door neighbours is that determines whether we need a fire window and frame in our new bathroom upstairs. And also where the measurement needs to be taken from. Any help greatly appreciated.

[buildersmate]

The relevant distance is one metre. As far as I can read from the Regulations, you measure it between the two walls. If the walls happen not to be parallel to one another, there are prescribed ways of working out the 'equivalent distance'. There also seem to be concessions regarding the proportion of 'unprotected areas for small residential buildings' (within a wall). I take this to mean the maximum size of any windows within an otherwise (fire) protected (brick or rendered) wall that one can have, without having to resort to a fire resistant window. This appears to mean to me that you may not need to have a fire resisting window at all, depending on the sizes involved.
The Regulations are within Part B of the Building Regulations. Part B is concerned with Fire Safety and section B4 (within this) is concerned with fire spread. Since you are clearly extending your house, you would need Building Regs approval anyway, and the Building Control Officer will assess your proposed drawings to determine that the design meets the Regulations. Whilst I am happy to help further, your question puzzles me because you are going to have to use the appropriate experts to rule on the acceptability of the design anyway.
PS Whilst I use and understand Building Regs, this section crops up less regularly as a potential issue, so I've never actually dealt with a design that needed this aspect to be considered. Hope that helps. BM