EN wrote:

I want to put a window into the bathroom wall - I can't imagine why it
wasn't ever done before, but still. The window will be about 3ftX3ft, or
thereabouts double glazed wooden frame. The main problem is that it needs
to be set into a stone wall which is about 2ft thick.

Would it be prohibitively expensive to have this done by a builder? And is
there anything else I should think about while planning this?

Although the obvious thing would be to get a builder in for an estimate I'd
rather have a rough benchmark first, I believe I need to check planning
permission too as I'm in a National Park

Yes you will (ie just ask the planners if it's necessary to apply;
doesn't cost to do that.

You'll need to involve Building Control too; there are various
stipulations as to lintel specification, minimum height of window,
distance from corner of building etc etc which you'll need to adhere to.

Wouldn't like to guess as to the likely cost of labour (you're probably
talking about 200-250 GBP for materials). I suspect your main problem
will be finding a decent builder interested in taking it on though.

David