All windows and doors replaced will need to comply with a minimum standard of energy efficiency.
Any replacement windows or doors must not have a U value which may exceed 2.0 unless you live in a listed building and then other restrictions may apply.
U value is measured by the rate of heat loss from the warm air inside the property escaping through the glass of the window to the outside. The lower the U value the less heat is lost.
All glass should comply with current building regulations. All glass 800mm or less from floor level should be toughened safety glass. This would also mean from a stairway or inside a bath or shower cubicle.
All glass fitted into a door needs to be toughened glass. Also 300mm from the edge of the door surround. This could be adjacent windows or quarter panels. This does not apply if the glass unit falls below a minimum size.
One window within a habitable room is classed as a secondary fire exit. If you are changing one of these windows, you can’t reduce these opening sizes but you can enlarge them.
These are just three of the main regulations giving you an insight into window installation.