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Frequently asked questions

We answer the most commonly asked questions about sash window restoration and installation.

New double glazed sash windows

If we install new double glazed sashes that replicate the originals into existing sash boxes, this is classed as “refurbishment”, and is therefore not subject to planning permission or building regulation approval.

If you want or need complete new sash windows then these may require planning permission if your property is in a conservation area or is classed as being of ‘Townscape Merit’.  We would always advice you to contact your local council Planning Department to seek their advice.

In most conservation areas if we install new double glazed sashes that replicate the originals into existing sash boxes, this is classed as “refurbishment”, and is therefore not subject to planning permission of building regulation approval, so they can be installed in a conservation area. We would always recommend you contact your local planning department to seek their advice before changing your windows.

SashWise operates under the regulation of the Federation Self-Assessment Scheme (FENSA) this means that all of our new windows comply with current thermal performance and Building Regulation standards.  If we install new sash boxes with new double glazed sashes, they will be subject to FENSA regulations and therefore you will receive a FENSA certificate directly from FENSA once the work has been carried out and we have informed FENSA the work is complete. The FENSA certificate confirms that your new windows comply with current regulations.

If however, we install new double glazed sashes into existing sash boxes (so the original boxes are refurbished instead of being replaced), then FENSA deem this as being a “repair” and so falls outside of their remit and you will not receive a FENSA certificate.

Unfortunately it is not possible to change your existing sashes from being single glazed to double glazed. However, in most instances we can make replica timber sashes that are double glazed to fit within your existing original sash box frame. This is a more cost effective and less disruptive process than replacing the whole of your sash window.

Yes, one of the many benefits of our specialist draught-proofing system is that it is very effective at reducing outside noise levels. In addition we can re-glaze your existing sashes with acoustic glass, which is denser and therefore reduces outside noise levels.

Whilst in some instances this maybe unavoidable due to the condition of your sash windows. Your original sash windows are an important period feature of your property and our company policy is to wherever possible renovate the old as opposed to replace with new, thereby retaining the original character and fabric of your home. By refurbishing your original sash windows and installing our specialist draught-proof system we can often provide you with many of the benefits of new sash windows, ease of operation; reduced noise and air pollution; improved thermal efficiency; without the expense and disruption of unnecessary replacements.

We use a range of treated softwood and hardwood timber all of which are certified by the Forest Stewardship Council and from sustainable sources.

In June 2022 there was a change to Building Regulations requiring complete new windows to be fitted with trickle vents in order to improve ventilation within properties.

We have gone to great lengths to source trickle vents that have the minimal impact on the aesthetic look of our traditional windows.

There are exceptions whereby trickle vents are not mandatory:

  • If less than 30% of the glazing within the property is being replaced
  • If there is some other form of mechanical ventilation within the property
  • If it is just the sashes that are being replaced and not the window in its entirety.

Full restoration

All our new windows are fully painted and decorated. We are also able to paint and decorate existing sash windows. In both instances we use a special micro porous paint finish that allows the timber to breathe which reduces the risk of rot and provides a greater degree of protection from the elements. We can also use your chosen internal paint finish to match any existing woodwork.

In most instances the work we undertake is the repair and renovation of existing sash windows. This is classed as ‘refurbishment’ and is therefore not normally subject to planning permission or building regulation approval. This may vary for listed buildings and properties located in conservation areas but we will happily advise you on this.

If we are fully restoring your sash windows, we will take out the sashes and we take them off site to our workshop in Twickenham. Your windows will be boarded up securely with security boards, whilst the sashes are being work on in our workshop. We will be working on site restoring the sash boxes and once both area ready, they will be re-fitted. The windows are usually out for approximately 1 x week. The benefit of taking the sashes to our workshop means, much less mess and disruption for the client and we can get a high quality finish to the sashes restoring them in a workshop environment.

Repairs and service and draft proofing

Typically one of our craftsmen can carry out any repairs and install our specialist draught-proofing system to one or two sash windows per day depending upon the extent of the repairs that need to be carried out.


Yes, this is a common problem as over time sash windows can become clogged with paint. As part of our servicing and draught-proofing process we will ensure that both the top and bottom sash are fully operable and run smoothly with ease.


The cost of servicing and draught-proofing a sash window varies depending upon the size and style of the window, and the extent of the repairs required. At SashWise we are committed to providing a top quality service, using only the finest materials, and all at a highly competitive price. We will be more than happy to provide you with a free and friendly no-obligation quotation.


No, the draught-proofing system that we install has been specifically designed to be discreet and not to detract from the traditional look of your sash windows. All of the replacement draught-proof beadings are made from timber.


Yes, in some instances depending upon the extent of rot damage to your sash window we maybe able to repair the existing sash using a specialist resin. Should the sash be beyond repair then we can produce a replacement timber sash matching any detail moulding or profile to replicate the original window.


We do have a minimum policy of two windows per job for any of our services.   You could have one window replaced and one window repaired.


We can service and draught proof a minimum of two windows, the process includes the replacement of the sash cords.  We are not able to just replace the sash cords as an independent repair.



Generally we do not require scaffolding to refurbish your sash windows as the work is carried out from inside your property. We do require clear access to the sash windows and kindly ask that any blinds and curtains be removed prior to the work commencing.


We can fit high security ‘Sash Stop Locks’ to your sash windows that provide a high-level of protection whilst maintaining the traditional look of your sash windows. These locks are available in a variety of finishes so we can match them to any existing door or window furniture. They also have the added advantage of allowing the sashes to be left open slightly whilst still remaining secure against intruders.

For more information click here to contact us.