New window next to where the old window was.

Hi, we had a conservatory and used the kitchen window aperture (marked in yellow on the pic) for one of the doors which was narrower than the window so breeze blocked the remaining hole, now we want a kitchen window (marked in red) right next to where the window was.
My question is about the lintel (marked in black) how would this work? would i need a lintel above a 500-600mm wide window and if so how should it be fitted please?

Is it a single storey ?

No, it has a 4' bedroom window directly above, outside is rendered and I would like not to have to replace the existing lintel with a wider one which is probably the correct method, but this will turn it into a mammoth job, as opposed to cutting an opening and fitting a window like shown.

and after hopefully