Thank you @Beige, this link is very helpful as I think it’s relatively new guidance, which I’ve been hoping for for many years.
Previously, they outlawed UPVC sliding sash windows in conservation areas but I noticed an Appeal case in North London a few years ago, which I believe must have caught the attention of other Borough Councils.
The thing, for me, is that the actual material that a window frame is made of wouldn’t be apparent from the pavement or road so it’s the general proportions that matter. Unfortunately, UPVC window fame sections tend to be slightly thicker than the proper old traditional timber but, that aside, the really important thing is to have a deeper bottom rail and run-through horns on the top sash as these 2 features really do help for a good overall appearance.
Apart from aesthetics, there’s the practicality factor, which leads on to cost. These days, tradesmen aren’t always allowed to work from a ladder so you might need to factor-in scaffolding when wooden frames need to be repainted every 6 years, which makes the slightly thicker, but maintenance-free, UPVC option worth a bit more than a second look.
As with most things, it’s normal that suppliers want to cash-in on people’s dilemmas, so if a timber sash is £1,000 most UPVC ‘heritage’ window suppliers would come in at a [supposedly] very reasonable £695 for instance. However, I’ve found a brilliant firm who charge under £400 for any window up to 2.1m high by 1.1m wide - and that’s with slide & tilt openers but you add on a bit for glazing bars. Pre-Covid, they were running at 10 days from order to delivery and all totally bang on time. Hopefully it’s not outside of posting rules for me to give the heads-up on this firm (who also supply timber windows but I can’t vouch for those) - Sash Windows – Colin's Sash Windows