1. Which rules apply to you
Three questions decide what you need before any window or door is changed. Your borough’s website answers the first two; the planning portal map shows conservation area boundaries.
- Is the building listed? Search the National Heritage List for England. Listing covers the whole building, inside and out, not just the front.
- Is it in a conservation area, and is there an Article 4 direction? London has more than a thousand conservation areas. Many have Article 4 directions that remove the right to change windows and doors on elevations facing the street.
- Is it a house or a flat? Houses have permitted development rights for like-for-like changes. Flats and maisonettes do not, so changes that alter the outside appearance usually need planning permission.
2. Listed buildings
Any work that affects the character of a listed building needs listed building consent. That includes replacing windows and doors, changing glazing bars or glass, and often fitting secondary glazing. Doing the work without consent is a criminal offence, and the owner at the time can be made to put it back, whoever did the work.
- Conservation officers look for the right proportions, sightlines, glazing bar profile, horns and opening method, and for timber where the original was timber.
- Slim double glazing or vacuum glazing in new sashes is often accepted where the historic glass has already gone. Where original crown or cylinder glass survives, expect to keep it.
- Secondary glazing on the inside is reversible and is often the easiest way to add comfort, though it may still need consent.
Talk to the conservation officer early
Most boroughs offer pre-application advice. A short meeting with a sample section and a photo survey usually saves weeks of revisions.
3. Conservation areas and Article 4 directions
In a conservation area without an Article 4 direction, a house can usually replace windows with ones of similar appearance under permitted development. Similar means the same material look, the same opening style and the same pattern of glazing bars. A white PVC casement replacing a timber sash is not similar.
An Article 4 direction removes those permitted development rights for named changes, most often windows, doors and roofs on the front elevation. With one in place, you need planning permission even for a like-for-like replacement. Your borough publishes which directions apply to which streets.
4. Repair or replace?
Repair keeps the most historic fabric, and conservation officers usually prefer it. It is also often the cheaper route for a good box sash.
- Repair when the boxes are sound and most sashes are original: splice in new timber where it has rotted, renew cords and beads, and add draught-proofing brushes.
- New sashes in the old boxes when the sashes are beyond repair but the frames are good. This is where slim double glazing can go in.
- Full replacement when the frames have decayed or the windows are later, poor-quality replacements. Then you can restore the original pattern, which officers tend to welcome.
5. Building Regulations in old buildings
Replacement windows and doors are building work, so the Building Regulations apply, through a registered installer or a building control application. Old buildings get some flexibility.
Part L: energy
Replacement windows in existing homes normally need a U-value of 1.4 W/m²K or better. Approved Document L recognises that historic and traditional buildings need a different approach, so that energy work does not harm their character or cause damp. Agree the target with building control and the conservation officer before you order.
Part F: ventilation
New windows must not leave a room with less background ventilation than before. Old sashes leak air; tight new ones don’t, so plan trickle vents, a vent in the box or another source of ventilation.
Part B: escape
If the existing window is an escape window, the new one should be no worse. On upper floors of houses that usually means an opening of at least 0.33 m², at least 450 mm high and wide. Sashes can be made to meet this.
Source: Approved Documents B, F and L, GOV.UK, and Historic England guidance. Contains public sector information licensed under the Open Government Licence v3.0. Summary only, not legal advice.
6. Survey and measurement
We survey the existing windows and doors: sizes, sightlines, glazing bar profile, horns, opening method and condition. Then we measure every opening. That record shows what is original and is the basis for repair or like-for-like replacement.
Questions
Do I need permission to replace windows in a conservation area?
For a house, replacing windows with ones of similar appearance is usually permitted development, unless an Article 4 direction removes that right, which is common for front elevations in London conservation areas. Flats do not have householder permitted development rights. Check with your borough before you order.
Can I put double glazing in a listed building?
Sometimes. Any change that affects the character of a listed building needs listed building consent. Slim double glazing or vacuum glazing in new timber sashes is often accepted where the original glass has already been lost, but it is decided case by case by the conservation officer.
Do replacement windows in an old building still have to meet Building Regulations?
Yes, replacement windows are building work. Approved Document L allows a different approach for historic and traditional buildings so that energy efficiency work does not harm their character, but the approach should be agreed with building control before work starts.
Should I repair or replace my sash windows?
If the boxes and most of the sashes are sound, repair and draught-proofing keeps the most historic fabric and is often what a conservation officer prefers. Replacement makes sense where the timber has decayed beyond repair or the windows are later, poor-quality replacements.