Replacing (Sash) Windows in Listed Buildings

Owning a listed property has many rewards, however there is a downside to having an old house and that is maintenance. Additionally if your old house is listed then maintenance can be a paper work headache. Applying for listed building consent & planning approval can be a drawn out task, with statements and drawings being submitted for every minor detail.

We work on listed buildings daily and our knowledge is respected, we have recently been asked to write an article for the Listed Property Owners Club. This will be published on our website.

listed property

Consent for Slimlite double glazing is not a forgone conclusion as many think.

Their have been many instances that we have read about where the owners of Grade II listed properties have automatically assumed that “Slimlite” double glazed units are now accepted as an alternative to single glazing. On some of these occasions the owners have installed this type of double glazing without getting prior approval from planning / Listed Buildings. This has resulted in costly reinstatement of single glazing.

The reasons for these refusals are numerous, but generally slimlite units will not fit into existing sashes without leaving the double glazing spacer exposed. An original sash is constructed to accept 3 – 4mm glass (Slimlite 11mm-12mm). Thus the unit will take up the space needed for the putty finish which is generally 7-8mm. So the installation of  slimlite units into an originally single glazed sash will detract from that original tidy and sleek look.

We have not known any type of double glazing being permitted into Grade I listed property fronts.

So how do slimlite double glazed units get approval in Grad II Listed properties?

When applying for planning and listed building consent the conservation officers will take into account several points.

1. The existing window condition can be a deciding factor before any consideration is given for new windows or glazing upgrades. If the existing windows are original and are not  “beyond economical repair” then consent can be turned down on this point alone. If your are keen to upgrade your windows to double glazed then proving the poor condition of the original windows will be a neccesity.
2. If your windows are not original (But are copies) and are in a good condition then point no.1 may not be a deciding factor. The argument can be quite easily made that you are not replacing original windows unnecessarily. We have seldom seen replacement sashes that match the original mouldings and can be quite easily distinguished from originals.
3. The proposed window will need to be sympathetic to original designs in both construction, materials used and aesthetics.  Attention to detail in the proposed application can help smooth the way for approval, ovlo or lambs tongue mouldings, getting the meeting & bottom rail depth correct are just some of the points that a little bit of research can clarify.

• The window stock size on an original Georgian Sash window is usually around 35mm (Sashes)
• The stock size required on a sash with slimlite units made to an original design will need to be 42mm + to enable an authentic putty finish.
• The glazing bar width of an original sash is generally between 15mm & 18mm. A slimlite unit has a spacer depth of at least 7mm. So 7mm each size of the glazing bar plus the timber divider between the units (Feather) is 4mm. This adds up to 7mm x 2 + 4mm = 18mm. However a tolerance to fit the units of 1-2mm per unit is required thus giving a total glazing bar width of 20mm +. Its this added tolerance that the slimlite section details don’t show and  conservation officers are sometimes aware of. The argument for this added width could be that if you have original windows, the paint on the glass normally protrudes the edge of the putty on to the glass, this can make the glazing bar width on original sash windows look considerably wider. So this point can used to argue the aesthetics.

• If you have original windows and think that slim line units would not be allowed or you have been previously turned down in a previous application then their is an alternative. Pilkingtons have developed a completely new type of double glazed unit that is only 6mm thick! It is called Pilkington Spacia It is expensive but it will inevitably lead to the dimise of slim line double glazing in period windows. Further detils available. Please contact us
• At Pembroke & Nash have an extensive knowledge of fitting windows into listed buildings.  We are only to happy to help with an initial chat & survey.

Examples of drawings that we can provide –

Contact us for some informative advice.

We can replace sash windows in areas such as Brighton & Hove, Sussex, London, Surrey & Kent.

by Pembroke Nash.