Horses for courses here I think. Where an extension to a traditional building is at rear ground level, I have no strong preference as between traditional materials, conservatory, or glass extension - and am planning a glass extension to the back of my Victorian house. Once you get higher up, or with something visible from the road, then I am strongly in favour of the traditional look with as much care as possible being taken to make the extension look as if it was always part of the building. I am aware that "reproduction" is sometimes considered a rude word, and also of the school which considers good modern design in an extension preferable to reproduction, and tends to equate glass with good modern design. In my view, that is not always the case. As a rule of thumb (but subject to the requirements of the particular site), I would prefer to see modern design in clearly visible extensions to older buildings confined to larger, usually non-domestic, sites.

I think the style of extension is dictated by purpose, location and privacy. The image above appears to use a glass box extension as a link between the main house and what appears to be a converted barn. The glass box allows the historic fabric and character of the original building to be retained. The chair in the glass link indicate the space is not used for much more than a link and possibly a place to sit and read. The use of the glass box is ideal and perfect for this use. A sitting room with little privacy from passers by, may not be so practical to be built from a glass box, unless a tinting or privacy glass option is used, to avoid the goldfish bowl syndrome.

As an alternative consider a timber frame extension, you can create almlst any style from contemporary to the house to ultra modern using sustainable and energy efficient wood. One company I know of is Vision Developments but there are others.

Google timber frame extensions and look at images, you will be amazed by the variety and can buy some as kits.