Uvpc replacement windows - casement v sash

[whatwouldyoudoifisangoutofkey]

We have had some windows replaced with UVPC sliding sashes and now considering replacing more at back of property.
Existing windows are metal frame casement with a fixed bottom pane and an opening top page with side hinges.
The builder who is quoting has given a much cheaper price for casement then sliding sash.
He has tried to explain and I think he's saying that the new casement could sit in new sub frame inside the existing frame .
The existing frame is I think boxing in where the old weights etc remain from the original wooden sash windows .
And that new UVPC sashes would involve taking out more of the old wooden frames.
Is there anyone who can help me understand and whether my interpretation is right ,??

It doesn’t sound right to me, nor have I ever heard of it been done that way. I can kind of see why it would be cheaper to fit the new window into the existing frame (If I understand the situation correctly) Not only is there more to take out there could be a lot of remedial work building the substructure back up, replastering etc. What I don’t understand is why he can fit casement into the existing frame but not sash. Either way you would be fitting the new frame of the window up against the existing frame so what’s the difference. Bottom line I would clarify with him exactly what he is doing and why, I would also get another quote to see if they suggest the same method. Personally, I don’t like the sound of what he is suggesting it just introduces more joints than necessary and you have dissimilar materials going through a hot cold cycle. Again, I am surmising as I’m still not 100% of the situation maybe a photo might help.

Surely you want to go back to the brick frame and make windows that fit the original opening. Rather than a window frame inside another frame?
what period of property do you have? If the house originally had sash windows, then replace with sash windows.

i used a firm called “Colin’s sash windows” that I found on eBay. And whilst they are upvc I think they are very decent, certainly for the price. I installed ours 9 years ago and don’t regret it at all.

id ask him for pictures of where he’s done this before or ask if you can go round and see.

I have upvc sash at the front and tilt and turn at the back of my period property (and also casement in a modern side extension). Both tilt and turn and sash included "full box removal" of the old wooden sash windows. This was done to maximise the size of window glass and therefore light. The tilt and turn and sash can both be opened in ways that mean they cleaned inside and out from inside the house. This is not possible with the casement windows.

Casement UPVC windows are generally the cheapest option, then tilt and turn, then sash.

Go for a walk in your neighborhood (assuming it is made up of similar properties) and have a look at the different window styles. What seems to work best? What looks most in keeping with the property style? And definitely ask for pictures from the builder of both styles fitted by them. Ideally get more than one quote too...

I'm very grateful for all the replies. The guy in question has done quite a bit of work for me and I do trust him .
I'm going to speak to him again and see pics . Meanwhile I've , hopefully, attached pics of windows .