Peter_in_Hungary

What usually happens with old houses with out of square window apertures is that the old windows/doors are removed and the aperture lined to make it square then a door/ window fitted, Sometimes the aperture is squared up first and sometimes it is done at fitting. Either way it is very unusual to make an out of square window / door to fit the hole, apart from anything else all the joinery machinery is set up to do square joints and to alter everything to make corners a few (variable) degrees off would be prohibitively expensive if you could find someone to do it.
But before doing anything you should ensure that movement that made it out of square is historic and not on-going.

E.G. a 2 - 5 cm of of square is an easy fix, the new window is fitted square and then the gap would be foamed in and a decorative beading shaped to cover the foamed in gap.

Sachina

There's only one window in my house that's out of square. Someone had either lost their spirit level or maybe had had too much palinka when they built it.

I've got double windows with 20cms of wood in between, so I'm not sure how you'd manage to square the window without it being noticeable?

Peter_in_Hungary

There are a couple ways to replace such windows, One is to place the new window in the outer return on the wall and remaking the internal reveal after the old window has been removed. This moves the new window outboard of the existing and gives a much wider window ledge. This may or may not hide the out of square of the original. Alternatively the new window can replace the existing and the remaining reveal made good. It depends where the out of square is located (and how much it is) If the out of square in in the brick work then remedial work can be done to the wall prior to the window installation, whether this is by addition which means a smaller window or chiselling out which gives an equivalent size is up to you. The method I said above would give a beading that did not have parallel sides and how much that annoyed is up to you. If the brick work is square and the window and frame are out then removing the window solves the problem

duztee

Only an aside.
If you have opposing windows doors open make sure you have some form of door stop fitted to all internal doors, because it don't half scare the bijibbers out of you when they slam shut! Especially important if any of them have glass panel.