Replacing windows and doors now needs to meet certain standards in the Building Regulations to reduce energy loss.


This means that you need to comply with the Building Regulations when installing replacement windows, doors, or rooflights in both domestic and non-domestic buildings.



Thermal Insulation

Replacement windows will need to meet the new thermal insulation requirements. If the replacement windows have wooden or plastic frames, then the glazing needs to achieve a maximum U-value of 2.0 W/m² K, while for metal frames a slightly higher U-value of 2.2 W/m² K is allowable.


Please note that these values are very difficult to achieve and many double glazing units currently on sale will not meet these new standards.

Means of Escape

All first floor windows in dwelling should ideally have opening lights large enough to allow you to escape through them if you were trapped in the room by a fire. This also applies to rooms in bungalows, which open into a hall (unless the hall itself has an external door through which you could escape).


To meet this requirement, all such windows should have an unobstructed openable area of at least 0.33 m² and be at least 450 mm x 750 mm in either direction. The bottom of the openable area should not be more than 1100mm above the floor.

Safety Glazing

Low level glazing i.e., glazed areas within 800mm of floor level and glazing in doors within 1500 mm of floor level should generally be of the type so that if broken, it will break safely.


In practice this means such glazing should be laminated or toughened glass. Ordinary glazing can still be used in small pane sizes, however, provided the glass is sufficiently strong to resist breakage.


The Approved Document to Part N of the Building Regulations gives maximum sizes according to the thickness of glass, e.g., in a single pane less than 1.1 m² - 8 mm glass would be satisfactory.

Structural safety

If the replacement windows are wider than those they replace, or involve the replacement of bay windows, then the Building Control Officer will need to be satisfied that proper structural support is provided above the window.


In older buildings, the timber frame of the window was often sufficiently strong to carry the load of a wall or roof above it without a lintel. Obviously in these cases, a lintel needs to be installed when the window is replaced, or the new frame carefully reinforced to carry the load.


The Building Regulations require that adequate ventilation is provided for the occupants in the building and this should be considered when deciding on the size of the opening lights in the replacement windows.


For most rooms, one or more opening windows equalling 5% of the floor area, with background ‘trickle’ vents of 8000 mm² will be adequate. For kitchen, utility rooms and bathrooms an extractor fan is also normally required.