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Building Control Guidance Note No.4 - Replacement Windows/Doors to Existing Dwellings

I thought that replacing windows did not need Building Regulations consent?

For many years, it has not been necessary to apply for Building Regulations consent when replacing windows in existing dwellings unless a structural alteration was involved. This usually only occurred when the existing opening was being made wider (which made the installation of a new support lintel necessary), or where the windows were part of a structural bay. However, from the 1 April 2002, all owners replacing windows and doors require consent, and the new windows themselves will need to fully meet the requirements of the Building Regulations. So even if you are replacing only one window bought from a local DIY centre you still need to follow the guidance below.

It is important that householders obtain the necessary consent as solicitors will make specific checks for this when the property is sold.

What do I have to do?

You have a choice, you can either:

How much will it cost?

If you use a contractor registered under the FENSA scheme the cost should be built into the quotation for the works you are given. In all other cases your Council will make a charge for dealing with your Building Notice application. Your local Council will be able to tell you the charge for your project.

What regulations do the windows have to comply with?

Owners of listed buildings and buildings in Conservation Areas may not need to satisfy these requirements in all cases. See What Can I Do In A Conservation Area? for further information. However, it is still necessary to apply for consent through the local authority or the FENSA scheme. Please remember that separate Planning Permission may be required from the local authority for these works.

Thermal Insulation

The replacement windows will need to meet the new thermal insulation requirements of the regulations. If the replacement windows have wooden or plastic frames; then the glazing needs to achieve a maximum U-value of 2.0W/m2K while for metal frames a slightly higher U-value of 2.20W/m2K 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.

Take great care when ordering new windows that your supplier can prove the glazing units will satisfy this requirement, also, all 1st floor bedroom windows require an escape window with an unobtrusive opening of 0.33m2, the Building Control Surveyor will need to see this proof before issuing a completion certificate. We would particularly recommend that you leave any labels on the glazing until after a satisfactory inspection has been carried out by the Building Control Surveyor.

In some cases it may be permissible to use glazing units which do not meet the above specifications, but to do so you (or your supplier) would have to submit calculations to prove that the overall insulation requirements of the regulations would still be met. This may be possible if other insulation measures are undertaken at the same time as the window replacement - for example; installing cavity wall insulation or 'topping up' loft insulation. The benefits gained by installing this extra insulation can be used to offset the higher losses through the glazing, but this should not be undertaken lightly. We would strongly advise you get any such calculations checked by the Building Control Surveyor well before the replacement windows are installed, so that expensive mistakes can be avoided.

Means of Escape

All first floor windows to habitable rooms in dwellings 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) and all inner room situation. To meet this requirement all such windows should have an unobstructed openable area of at least 0.33m2 and be not less than 450mm high and 450mm wide (the route through the window may be at an angle rather than straight through). The bottom of the openable area should be less than 1100mm above the floor.

If your existing windows do not have opening lights which meet the above requirements, we would strongly recommend for your own safety that you take the opportunity to provide them in the replacement windows. This is not, however, a requirement of the regulations, which simply state that the replacement windows must be no worse than those they replace in this respect. Where the existing windows already having opening lights which are larger than the above requirements, those in the new windows can be reduced in size provided they are not reduced to less than the escape windows size mentioned.

Safety Glazing

Low level glazing (glazed areas within 800mm of floor level) and glazing in doors within 1500mm of floor level should generally be of a type so that if broken, it will break safely. In practice this means such glazing should be either laminated or toughened. Ordinary glazing can still be used in small pane sizes however, provided the glass is sufficiently strong to resist breakage. Approved Document N of the Building Regulations gives maximum sizes according to the thickness of glass - for example, in a single pane less than 1.1m square - 8mm glass would be satisfactory.

Structural Safety

If the replacement windows are wider than those they replace, or involve the replacement of bay windows, then proper structural support is required above the window. In older buildings, the timber frame of the window was often sufficiently strong enough to carry the load of a wall or roof above it without a lintel. Obviously in these cases, either a lintel needs to be installed when the window is replaced, or the new frame carefully reinforced to carry the load. Further advice on structural stability can be obtained from your local authority Building Control Section or from any member of the FENSA scheme.


An important factor to consider is that of adequate ventilation for the building and this should be considered when deciding on the size of opening lights in the replacement windows. For most rooms, one or more opening windows totalling 5% of the floor area, with background 'trickle' vents totalling 8000mm2 will be adequate. For kitchens, utility rooms and bathrooms 4000mm2 will be adequate. These rooms should have adequate mechanical ventilation to alleviate problems associated with condensation.

Updated February 2011.