The reference in the Planning and Development Regulations 2001 still appears to be relevant. This relates to windows in extensions, but I'd be surprised if this didn't relate to new windows in existing houses, so picorette's comment is probably spot on.
Column 1: Description of Development: Class 1
"The extension of a house, etc..."
Column 2: Conditions and Limitations
"6. (a) Any window proposed at ground level in any such extension shall not be less than 1 metre from the boundary it faces."
Just remember, your neighbours may not understand the fine points of planning law and may refer you to the local authority if they take offence.
I usually advise clients that the best way to avoid such hassles is to take a professional approach, taking the neighbours on both sides into their confidence and showing them a little sketch of the proposed works; then discussing the timing, apologising in advance for any disturbance or disruption due to noise and builders traffic.
That usually smooths the way even where planning permission isn't required. And if they take offence, politely point them in the direction of the planning and development regulations.
The first real problem you will face if this is referred to the local authority by your neighbours is the area planner's definition of "the boundary". Some planners may take this to be the physical boundary, i.e. the inside face of the wall. Some may take it to be the notional boundary, i.e. the legal boundary, usually the centreline of the wall. All this gets complicated when there are piers every few metres on one side only and/or its not clear where the legal boundary is located.
Estate houses, even detached ones, sometimes have very tight side passages, and you should check that the distance to the notional boundary is 1.0M clear, then you can at least make an argument that you comply in principle, and argue the toss about what the regulation means, whether the real or notional boundary distance complies.
The sad fact is that may houses have only about 800mm clear or 900mm to the centreline of the wall and this is where planning permission can be needed.
Do ensure that you emply a competent reputable builder and you should also take engineers advice on the contrustion of the ope, head/beam placement and specification as this is in a loadbearing external wall. Do not take on this work yourself as a layperson.
Overhead work is dangerous. It may require propping of internal floors and temporary supports, as well as a whole raft of safety protocols on top of the non nuisance measures about working hours, keeping noise to a minimum and keeping dust, dirt and debris within your own house site.
Even works as minor as this can benefit from an architects input in terms of frame and glass specification and the prevention of cold bridging if yours is cavity wall or hollow block contruction.
Be very careful if your builder has to break a hollow block to reach your desired head height, as the walls of these are only 25-40mm thick or so and a beam usually required 150mm of bearing at either end, but again, your engineer can advise on this.
Once you have a builder on site you may find you go mad and start doing loads of things you never intended to do - at their "suggestion". Be careful and have all such ideas checked by a professional and properly costed before commencement, because in the current climate a paying client who will expand the brief will be like gold dust for a builder who is surviving on bits and pieces.