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Conservation area window design guidance

Design guide: Replacement windows

There are now 6 conservation areas in Blackpool as well as more than 40 listed buildings and nearly 300 locally listed buildings. Stanley Park Conservation Area (and by June 2020 Raikes Hall Conservation Area) has an article 4 direction in place which is intended to better manage changes to all visible elevations including windows and doors. Upvc windows are here to stay, at least for the foreseeable future, and this informal guidance is intended to assist when enquiries or applications are received which will impact on designated and undesignated heritage assets.

Energy efficiency

Improving energy efficiency is one of the main arguments put forward for replacing historic single glazed windows with modern upvc double glazed units. However, certain classes of historic buildings are expressly exempted from the need to comply with the energy efficiency requirements of building regulations where compliance would unacceptably alter their character and appearance. These include listed buildings, buildings in conservation areas and scheduled ancient monuments. The regulations also include ‘special considerations’ which can apply to locally listed buildings.

Energy efficiency can be improved with draught stripping and other measures such as secondary glazing and/or heavy curtains



Height restrainer window lock.
Height restrainer window lock (Image from Historic England Traditional Windows)


Meeting rail window bolt
Meeting rail bolt (Image from Historic England Traditional Windows)

Not only will secondary glazing reduce heat loss and noise levels but it will enhance building security if this is a concern. A variety of ironmongery can also be added to original windows to improve security, most of it unobtrusive. Window locks, dual screws, anti-lift devices, mortice bolts and sash chains can be fitted, while still allowing the window to be opened for ventilation and cleaning. Special bolts may be drilled into the meeting rail of sliding sash windows so the two sashes lock together.

Listed buildings

Replacing original windows in listed buildings should not be permitted unless there are exceptional circumstances, and should always instead be retained and repaired appropriately by a timber window specialist. 

If it can be demonstrated (i.e. via a condition report from a joiner) that historic timber windows are beyond economical repair (e.g. more than 3/5 rotten) they should be replaced with timber windows which should match the form, detailing and operation of the windows they are replacing. Under these circumstances, and only if it can be carried out so that the original profile can be retained, consideration can be given to replacing single glazing with timber double glazing to improve energy efficiency. Extra slim double glazed units with krypton-filled 4mm cavities are available.

If listed buildings have previously been fitted with standard upvc double glazing (for example the Imperial Hotel) if and when the units fail they should be replaced with slimline timber double glazing, with sliding sashes if that was the original design.

A few local joinery companies have a track record of making and repairing timber windows

Conservation areas 

Original windows in our conservation areas are increasingly rare, and should be retained and repaired wherever possible. The older the building and its original windows

Typical Sliding sash with glazing bars
Typical Sliding sash with glazing bars

the more important they become to its character and the evidential value of historic design they provide. Even if many other properties in the area have upvc windows, planning appeal decisions from other local authority areas indicate that, where original windows exist, they can and should be retained. Rotting timber frames can be repaired by splicing in new timber, and broken sash cords can be replaced. 

If it is accepted that replacing original windows is justifiable (see advice for listed buildings) the replacement windows should as far as practicable resemble the originals in design and materials and operation (e.g. sliding sashes). Slimline upvc may be acceptable but replacing original sliding sashes with top hung windows is not acceptable in a conservation area.

Where standard upvc units have previously been installed and are being replaced because they have failed, or as part of a wider development scheme, owners should be

Masterframe Vintage frame with 'run through' horn
Masterframe Vintage frame with 'run through' horn

encouraged to replace them with a more appropriate design. Standard double glazing units usually have a bulky frame to accommodate the two panes of glass and wide spacer bars, rather than the slim profile of original timber windows. In addition, modern ‘storm’ windows, which overlap the frame rather than sitting flush, add to the width of the frames which look out of place in an historic building.  

Where sliding sashes have been previously replaced by top hung windows with standard profiles, they should be replaced with timber or upvc sliding sashes with a slim profile. They should ideally have run-through sash horns (as opposed to ‘planted on’). In the late 19th-century vertically sliding sash windows began to make use of larger and heavier panes of glass. The weight of the glass, coupled with the disappearance of the glazing bars that had given support to the horizontal meeting rails, led to the development of ‘horns’ which strengthened the joint between the meeting rail and the stiles. This would have been the most common design in Victorian Blackpool. Most popular paint colours would probably have been white, dark green and dark brown


Flush casement upvc window
Flush casement upvc window

Ideally the upvc sliding sashes should also use profiles which replicate the traditional putty line chamfer externally, be mechanically jointed (rather than welded joints) and have no trickle vents. Building Regulations only require a trickle vent in a new replacement window if there was trickle ventilation within the window being replaced.


Flat glazing bars inserted within double glazing units should be avoided because of their poor visual quality. 

Original stained glass in domestic properties can be encapsulated but be aware that this may result in the loss of some of the edges.

When replacing any window the opening should be sized to provide at least the same potential for escape as the window it replaces. If the original window that is being replaced was larger than necessary for the purpose of escape then the new window could be reduced.

There are a growing number of double glazing manufacturers producing upvc sliding sash and flush casement windows which would be acceptable in conservation areas and locally listed buildings if the above criteria for replacement are met. 

Window ironmongery


Straight arm sash fastener
Straight arm sash window fastener


Fitch sash window fastener.
Fitch sash window fastener

Sash lifts and fasteners would probably be cast iron or brass and Sash weights were generally cylindrical and made of lead or cast iron. Sash cords were made of
cotton during the Georgian period, but by the later 19th century these were sometimes by linked chain robust enough to operate the very large plate glass sashes
then in use. In 1930 the spiral sash balance was patented. New timber sliding sashes can be fitted with the spiral balance mechanism, but the cords and pulleys in
original windows should be replaced when necessary on a like-for-like basis by companies such as Ventrolla which specialise in overhauling traditional timber sliding sash windows.

Metal windows

Although simple wrought iron casement windows were used from around the 16th century, the development of hot rolled steel in 1856 meant that inexpensive window frames could be produced in mild steel rather than wrought iron. After the First World War firms such as

W F Crittall were responsible for the development of the ‘universal suite’ of hot-rolled steel sections that formed the basis of what we now regard as the classic metal windows. A number of our public buildings constructed in the 1930s have, or had, metal windows.

Metal windows come with their own set of problems, and repair methods may depend on the type of metal i.e. ferrous (cast iron, steel) or non-ferrous (bronze, copper). Window replacement may be proposed because frames are distorted, stuck or corroded. See for details on how to remedy relatively simple problems and deal with surface corrosion

The same criteria should be applied as for timber windows when assessing whether or not original metal windows might be replaced, or when replacement upvc windows have failed or are proposed to be replaced as part of wider development. Bronze anodised aluminium windows have been used successfully in local refurbishment schemes of 1930s buildings including the Municipal Buildings, although marine grade powder coating may be acceptable depending on the host building and its location. Original metal windows in listed buildings should only ever be replaced like for like, and only if they meet the criteria as for timber windows.


It goes without saying that historic glass in listed churches should be repaired and retained. If needed, laminated glass window guards are preferable to polycarbonate, and should be fitted with a gap to prevent condensation – see below.


Protection of Stained Glass window using Lead Bracket Method
Protection of stained glass window using lead bracket method

Locally listed churches, or churches in conservation areas, should also be encouraged to repair and retain original stained glass windows, even when adaption and re-use is proposed for redundant churches.