Double glazed uPVC on outer skin of cavity?

[SJRSJR]

Hello, hope you can advise.

I've read that a window should be at least 30mm into the cavity from the outside skin, but given the position of the stone window ledge and external render, my double glazed unit is going to have to sit on the outside skin completely. What are the building regs / thermal implications of that? There's a cavity closer.

Thanks

 

[Wayners]

That's a question for window surveyor. They have to fit to regs so see what they say