Windows challenges - My House Extension

A one storey extension
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Windows challenges
One of the more significant challenges to building the extension is going to be the window. The reason for this is because the windows in the rest of the house consist of real lead light windows which are draft proofed with secondary glazing as shown below.

In April 2006, new regulations superseded those laid out in 2006. The new and old regulations can be seen on the comprehensive Pilkington Glass web site. In brief, this stated that U values 2.0 for wooden windows were required in all new and replacement windows.  This meant that low emissivity glass was necessary for compliance. Certainly, secondary glazing would not meet these new planning regulations!

Here is an example of an existing window. By the way, I have had to replace most of the window frames now made out of hardwood and most of the lead light windows themselves which I made my self. I will have to do the same for the extension. The question is - how can I build windows to match the existing house AND meet the new regulations. No, I will not consider the use of PVC units!
One of the windows in the main house (yes, I need to do some painting!
The window as seen from the inside of the house
Showing how the existing windows fit into the frame and showing the the secondary glazing
Clearly the new wood windows will need  to incorporate a double glazed unit of some sort. Would I be told by  building control that I would have to go to 22mm air filled unit or could I go for a thinner - 14mm Argon  filled unit from someone like Pilkington Glass? I need to find out!
An oak sill made to replace a window in the house
This is a sill I made last year to replace one of the windows in the main house. It was relatively easy to make with a router, a circular saw and planer.
A leftover from one of the central uprights     
The uprights were made from 69 x 94mm  planed softwood. The extension window will have a plane chamfers on the  inside rather than an ogle.     
A leftover from one of the outside uprights     
Copyright: Chris Gare 2007 - 2018
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