Do you mean an 'oriel' window? I don't know about pricing, but you can probably contact a couple of suppliers to give you estimates? A quick google and amongst loads of others these came up too. The IQ glass one gives some useful info on sizes I think: https://www.finepoint.glass/product/oriel-windows-london/ https://medium.com/iq-glass-uk/what-to-consider-when-specifying-an-oriel-window-cb39beedce04

The options with the joinery are limitless, its bespoke after all. You can have it as storage etc but in this case it's also casing for the room's radiator. I dont know the pricing for Oriel, but you'd have to replicate that on site with structure and insulation, and the end cost could be similar. I may have someone capable to replicate this on site. Let me know if you need a bit of help with design and managing

Thank you so much for your reply. we actually need to build out the right side of that wall to fit a dimplex electric fire, which would be a bit higher but we could extend that along and go along the wall a bit like this maybe? it would be different heights though…i love the idea of oak cladding thank you

The windowsill is low (lovely big window) so I think an ottoman that you can also sit on if you want is a good idea as it wouldn’t be high enough to cover the bottom of the pane of glass. possibly oval shape? Otherwise there are corner storage units that are a similar shape to the alcove tho I don’t know if that is a bit unecessary, a standard bench/ottoman seems like a nice idea.