@nella85,
I'm probably not much help as I am currently building and wouldn't dream of anything other than aluminium because of "nil" maintenence but if the rest of your home has timber then it would probably look better to maintain the same look unless the new reno is exposed to the elements??
Double glazing is usually twice the cost to standard windows and if the rest of the house has single glazing then I wouldn't spend big money on the windows just on this reno, as there isn't any serious benefit.
Good luck.
Barbara

If you are considering using aluminium windows then ensure they are thermally-improved or thermally-broken frames, standard aluminium windows conduct a huge amount of heat into and out of your building, decreasing comfort and increasing running costs.

Timber windows and thermally-broken aluminium windows are similar in terms of thermal performance.

Well painted timber windows should be maintenance free for 10-15 years, yes they will require more effort over time than aluminium but it is not a weekly event. This is still a major factor for many people in making this choice. Accoya timber (have a Google) is relatively new but does appear to allow a painted finish to last even longer.

Double glazing, yes it costs more but not twice as much. If you buy good quality frames why would you not also buy good quality glass (and vice versa).

It can be relatively easy to replace the single glazing in your existing frames with double glazing although this depends on the frames themselves.

These are probably the largest areas of glass in your home, if you use single glazing you will lose more than 50% of your heat through them (depending the level of insulation in the remainder of the building, possibly more) hence choosing the right glass is incredibly important.

Building comfort is a product of many many factors, each decision contributes to the whole. Be mindful of 'death by a thousand cuts'.

I hope this helps a little.

If the door is not going to be exposed to the western sun, then I would not use double glazing, due to the considerable expense. Money saved could be spent in other areas of the renovations. As all other doors and windows are timber framed then I would install a timber framed door on the extension.