Building Control


Technical guidance – replacement windows and external doors

What you need to know and do when you want to replace your windows and doors

Replacement windows and doors in existing dwellings

You can find on this page information relating to replacement of windows and external doors in existing dwellings.

I thought replacing windows and exterior doors did not need building regulation consent?

For many years it has not been necessary to apply for building regulation consent when replacing windows and exterior doors 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.

From the 1 April 2002 all owners replacing windows and exterior doors will need to make an application and the new windows etc themselves will have to fully meet the requirements of the building regulations. So even if you are replacing only one window or external door with one bought from a local DIY centre you still need to follow the guidance below.

What do I have to do?

You have a choice, you can either –

1. Use an installer registered under either the FENSA scheme, or the CERTASS scheme, in which case you do not need to make a building regulations application to the council. Your installer should then ensure your windows and doors fully comply with the Regulations and will supply you with a certificate confirming this when the installation is complete. If you wish to use this option please ensure that your installer is properly registered under the scheme before you place an order. You can check this and find more details of the schemes by visiting the websites at or

2. Make a Building Regulation application. In most cases the simplest way of doing this is to submit a building notice. You must complete the building notice form and return it to us together with the appropriate fee at least two working days before removing the old window(s) and exterior door(s).

Once we receive the building notice a building control officer will normally visit your property twice on your request – once before and once after the replacement windows / doors 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, so please follow the guidance given in this document to ensure that one can be issued to you.

How much will it cost?

If you use a contractor registered under the FENSA or CERTASS schemes the cost should be built into the quotation for the works you are given. In all other cases the council will make a charge for dealing with your building notice application. The current charges are as follows –

All domestic installations involving the replacement of windows, doors or rooflights £130.00 inc. VAT per application.

What regulations do the windows and doors have to comply with?

Thermal insulation

The replacement windows will need to meet the new thermal insulation requirements of the regulations. Replacement windows with wooden, metal or plastic frames the glazing needs to achieve a maximum U-value of 1.6W/m2K while for doors with less than 50% glazing and solid doors a slightly higher U-value of 1.8W/m2K is allowable. If doors have over 50% glazing the U-value of 1.6W/m2K applies. Please note that these values are very difficult to achieve and many double glazing units on sale will not meet these new standards. Take great care when ordering new windows that your supplier can prove the glazing units used will satisfy the requirement, as the building control officer will need to see this 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, 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 officer well before the replacement windows are installed, so that expensive mistakes can be avoided.

Replacement window

Means of escape

All first floor windows in dwellings should 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 that open into a hall (unless the hall itself has an external door by which you could escape). 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 that 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 should be no worse than those they replace 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.

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 that such glazing should be either toughened or laminated. Ordinary glazing can still be used in small pane sizes however, provided the glass is sufficiently strong enough to prevent breakage. The Approved Document to part N of the building regulations gives maximum sizes according to the thickness of the glass – for example in a single pane less than 1.1m2, 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 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 either 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 people in 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 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 however 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.

Building Control

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