Replacement windows

I thought replacing windows did not need building regulation consent?

For many years, it has not been necessary to apply for Building Regulation consent when replacing windows in existing dwellings. This is unless a structural alteration was involved. From the 1 April 2002, however, all owners replacing windows need to make an application. The new windows will have to meet the requirements of the Building Regulations. If you are replacing only one window with one bought from a local DIY centre, you still need to follow the guidance below.

You have a choice, you can either:

  • Use a contractor registered under the FENSA, BSI or Network VEKA competent persons scheme. This does not apply to buildings in Conservation Areas or Listed Buildings, or,

  • Use us and submit a Building Regulations application. The simplest way would be to submit a Building Notice. The Building Notice form must be fully completed. It must be returned to us together with the appropriate charge. The charge is currently £120, including VAT for domestic installations and a site location plan at least two working days before removing the old window.

Once we receive the Building Notice, a Building Control Officer will normally visit your property twice. This will be once before and once after the replacement windows are installed. If all is found to be satisfactory, a Completion Certificate will be issued to confirm this. You may well be asked by your solicitor to produce this Certificate if you sell your property. If your windows are being replaced by a specialist window firm, they may well make the application on your behalf. Be careful to check this with them in advance to make sure.

What regulations do the windows have to comply with?

Thermal insulation

The replacement windows will need to meet the thermal insulation requirements of the regulations. Replacement windows needs to achieve a maximum U-value of 1.40W/m²K. Take care when ordering new windows that your supplier can prove the glazing units used will satisfy this requirement. The Building Control Officer will need to see proof before issuing a completion certificate. We would particularly recommend that you leave any labels on the glazing in place until after a satisfactory inspection has been carried out by the Building Control Officer.

In some cases it may be permissible to use glazing units which do not meet the above specifications. 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. This should not to be undertaken lightly. We would strongly advise you get any such calculations checked by the Building Control Officer well before the replacement windows are installed, so that expensive mistakes can be avoided.

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. The Approved Document to part 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.

Means of escape

All first floor windows 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 a bungalow that open into to a hall. This is 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 0.33m². It should not 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.

Requirement for window to be used as a means of escape

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 a requirement of the Regulations, provided the replacement windows are no worse than the ones replaced in this respect. Where the existing windows already have 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 dimensions above.


Replacement windows are now required to have trickle ventilators fitted even where the original windows did not have trickle ventilators installed, the Building Regulations require that adequate ventilation is provided for people in the building. 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 5000mm² will be adequate.

For kitchens, utility rooms and bathrooms an extract fan is also normally required. In some cases the existing windows may contain a permanent vent to supply combustion air to a heating appliance, although this is now rare. If this is the case, you should ensure that either the replacement window contains a similar permanent vent, or that some other means of providing the required ventilation is installed at the same time.

Structural safety

If the replacement windows are wider than those they replace, or involve the replacement of bay windows, 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. 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.

Last updated: Wednesday, 5 October, 2022.