Replacing or adding windows

There are three main options when it comes to upgrading windows: refurbishing the existing units; fitting new glazing into the retained existing frames; or complete replacement. The best option will depend on your budget and the age and condition of the property. You may also want to add a new window opening to boost the amount of light to a room or benefit from a view.


Planning permission isn’t normally required for straightforward window replacement, except in conservation areas and to listed buildings. However adding a new opening for a window can be more contentious, particularly to the side or anywhere a new window would overlook neighbouring properties. Also, adding new windows may be prohibited in the planning conditions dating from the time your house was built. Where all else fails, fitting discreet roof windows or light tubes etc. may be feasible.

The Building Regulations define new windows and doors as ‘controlled fittings’ (Part L1B). However, in most cases installation work will be carried out by FENSA registered installers (Fenestration Self-Assessment) who can ‘self certify’ the installation. So an application to Building Control only needs to be made when windows are replaced by an installer not registered as a ‘competent person’ or where the opening is new or enlarged.

To comply with Building Regulations maximum permitted heat loss standards apply and new windows must meet a minimum performance standard based on either a ‘C’ Window Energy Rating (WER) or a minimum whole window U-value of 1.6 W/m2K.

Replacement windows also need to comply with requirements for ventilation, e.g. with trickle vents in window frames. For new window openings to habitable rooms there are minimum size requirements equivalent to at least 1/20th of the room’s floor area. Building Regulations consent will also be needed for any structural alterations, such as widening an existing opening and fitting a new lintel.

Apart from looking good and keeping your home warm, bright and well ventilated, there are other factors that need to be considered when installing replacement windows:

  • Escape from fire – on upper floors
  • Security, especially to ground floors and windows facing flat roofs
  • Danger from broken glass

To cut the risk of accidents, safety-glass is required in critical locations. This includes any glazing within 800mm of floor level and if there are any windows adjoining doors (within 300mm) then any glazing lower than 1500mm from the floor must also employ safety glass. Safety glass comes in the form of laminated or toughened glass which shatters into relatively safe small pieces (BS 6206 class A). Safety can be further improved by fitting special high-level child-proof handles.

Picking windows

The choice of new windows on the market is extensive. You can buy anything from quaint replica box sashes to conventional side or top hung casements or even exotic ‘tilt-and-turn’ units. These are available in a variety of materials such as softwood, hardwood, UPVC, painted aluminium or galvanised steel, as well as in a range of colours and glazing styles. It’s worth noting that although UPVC windows are sold as ‘maintenance free’, and don’t need periodic decoration, they typically have a useful life of only around 30 years. But double glazing offers other important benefits, such as improved security, sound proofing and reduced condensation. 

As a general rule it is worth trying to emulate the original window architecture of the house. Fitting cheap plastic windows into a period cottage is likely to slash its market value as well as, quite possibly, contravene planning laws. The quality of original windows in older properties is generally far superior to modern equivalents. They can also be an important part of the building’s character, so it’s usually a better option to restore them. In contrast, softwood windows dating from the 1960s to 1980s can be especially prone to rot, and likely to require complete replacement.


Glazing technology has made great advances in recent years, and even super slim units can achieve excellent performance. There are three key features that help thermal performance:

  • Gas filling reduces heat transfer across the glazing cavity.
  • Low-E (emissivity) coatings reduce heat loss across the glazing cavity.
  • Super-insulated frames incorporate ‘warm edge’ spacers to reduce heat loss via thermal bridging.

Where historic windows in older properties need to be retained, a good alternative is to install internal secondary glazing. These can comprise double glazed units that open inwards to the room. Secondary glazing also has superior sound deadening qualities to double glazing.

Alternatively, special super-slim double glazed units can sometimes be fitted to existing window frames in period houses, retaining much of their period charm.

Cold bridges

The vertical sides of the walls around window and door frames (the reveals) sometimes suffer from damp and mould. This is due to ‘cold bridging’ where the brick or blockwork is ‘returned’ around the corner forming a bridge between outdoors and indoors. To avoid this when re-fitting windows, special insulated plastic ‘cavity closers’ filled with polystyrene foam can be inserted to ‘close the cavity’. This also has the advantage of forming a vertical DPC and some types can provide a fixing point for window frames.

Structural issues

It’s important that replacement window contractors carry out key checks in advance. For example, many properties built from the 1940s to 1970s have no lintels over window and door openings because the original frames were designed to support the walls above, with no need for lintels to the outer leaf. But replacement windows aren’t designed to support such loadings. So if there is no lintel, suitable temporary support must be provided and a new lintel inserted.

A similar but more serious problem can occur with bay windows particularly to 1930s houses. The original windows often had integral columns supporting heavy loadings from roofs etc. So it’s important that replacement windows are designed to provide sufficient structural support.


As well as checking in advance whether there’s a suitable lintel supporting the masonry walls above the window, there are a number of key points to bear in mind before cutting out the old windows:

  • Before removing the old windows ensure you have the new replacement windows ready on site and sufficient temporary propping in place (where necessary).
  • When removing old glazing protect yourself by wearing goggles and gloves. A useful tip is to cover both sides of the old glass with cling film to reduce the risk of small shards flying off.
  • The frame can then be cut into manageable lengths and prised away from the brick reveals and the opening made ready for installation of the new replacement unit.

Frame fixing

Various methods have been used to anchor frames into the surrounding masonry such as special frame fixing screws, galvanised steel brackets or dual cavity closer/sub-frames. But whatever method is used, the screws must not be over tightened and cause distortion to the frames.

With UPVC frames it is especially important to leave a suitable gap to allow for expansion. A strip of DPC should be provided around the opening, including sills, prior to installing the frame. This is essential for timber frames, which also need to be primed, knotted and undercoated before fixing. The gaps between the frames and the surrounding wall are then sealed with a suitable silicone mastic. If the outer wall surface below the window is tiled or timber clad, a lead ‘apron’ should be fixed under the sill and dressed down over the tiling or cladding.

One design factor that affects the look of the house but is frequently overlooked, is the question of how far back the new windows should be recessed within the opening. Traditionally, windows were set back about 100mm which helped protect them from the weather, whereas modern windows are typically rebated only about 25mm. The best approach is normally to match the original pattern of the existing windows, which means that for many older properties they need to be set fairly well back. This decision will also affect the outer sills, which must project well clear of the wall below so rainwater can disperse without causing damp.