The short answer
- Listed building: replacing original steel windows needs listed building consent. Repair and secondary glazing are usually the starting point.
- House in a conservation area, no Article 4 direction: replacement windows of similar appearance are usually permitted development. Different materials or a different pattern of glazing bars may need planning permission.
- Article 4 direction, or a flat: expect to apply for planning permission. Flats have no permitted development rights for windows.
1. Do you need permission?
Start with your council: check whether the building is listed, whether it is in a conservation area, and whether an Article 4 direction covers it. Most London boroughs publish conservation area maps and appraisals that name original windows as features to keep.
- Listed buildings. Any work that affects the character of a listed building needs listed building consent, and that includes replacing windows.
- Houses. Permitted development lets a house be altered on condition that the materials used in exterior work are “of a similar appearance” to the existing house. Whether a new window is similar is judged on how it looks: frame size, glazing bars, opening pattern, colour.
- Article 4 directions remove some permitted development rights, often for windows on elevations facing the street. With one in place, you need planning permission.
- Flats and maisonettes don’t have householder permitted development rights, so changing windows that alter the look of the building needs planning permission.
Our conservation areas and listed buildings guide covers consent for all window types in more detail.
2. Repair, secondary glazing or replacement
Historic England’s guidance on traditional windows puts repair first. For steel windows that usually means, in order:
- Repair and overhaul. Rust treated, hinges and stays freed, new putty or glazing seals, draught-proofing. A window that leaks air often needs servicing, not replacing.
- Secondary glazing. A slim inner window keeps the original frames untouched and cuts heat loss and noise. Approved Document L accepts single glazing supplemented with low-emissivity secondary glazing where the character of the building has to be kept.
- Replacement. Where the originals are beyond repair, or planners accept a change: slim double-glazed steel systems copy original sightlines, and on many unlisted buildings planners also accept thermally broken aluminium in a steel-look profile.
Single-glazed steel frames are cold, so condensation on the frames is common. That alone is rarely a reason for consent to replace on a listed building, but it is a fair point in a planning application for an unlisted one.
3. Building Regulations
Replacing a whole window, frame included, is building work. It needs a registered installer under a competent person scheme or a building control application. The rules are the same as for any replacement window, with one important exception for historic buildings.
- Part L, energy. Replacement windows in existing homes normally need a U-value of 1.4 W/m²K or better, or Window Energy Rating band B. The 2026 edition of Approved Document L, which takes effect on 24 March 2027, keeps the same limit.
- Listed buildings and conservation areas. The energy rules don’t have to be met in full where doing so would unacceptably alter the building’s character or appearance. The work should then meet them as far as is reasonably practicable.
- Where Table 4.2 can’t be met because of the character of the building, Approved Document L gives two alternatives: a centre-pane U-value of 1.2 W/m²K or better, or the original single glazing supplemented with low-emissivity secondary glazing.
- Part B, escape. A replacement window shouldn’t make escape worse than the window it replaces.
4. What a replacement has to match
Planners look at the window from the street, so the details that decide an application are visual. Draw them before you apply:
| Detail | What to match |
|---|---|
| Sightlines | Frame and sash widths seen from outside; original steel sections are very slim. |
| Glazing bars | The pattern and width of the bars, and whether they divide the glass or are applied to it. |
| Opening pattern | Which panes open and how: side-hung, top-hung or fixed. |
| Colour and finish | Often black, dark grey or white; the council may want the original colour. |
| Position | How far the frame sits back in the wall opening. |
Our steel-look windows
We supply steel-look windows in thermally broken aluminium. They suit buildings where the planners will accept aluminium. Send photos of the windows and the council’s guidance with your enquiry.
Questions
Can I replace Crittall windows in a conservation area?
Often yes on a house, if the new windows look similar to the old ones, under permitted development. You need planning permission if an Article 4 direction covers the windows, if the building is a flat, or if the new windows look different. A listed building needs listed building consent.
Do replacement steel windows have to meet Part L?
Normally yes: a U-value of 1.4 W/m²K or Window Energy Rating band B. In listed buildings and conservation areas the rules don’t have to be met in full where that would unacceptably alter the building’s character, and secondary glazing is an accepted alternative.
Is secondary glazing allowed in a listed building?
It is often the preferred option, because the original windows stay untouched. It can still need listed building consent, so ask the council’s conservation officer first.
Will the council accept aluminium instead of steel?
It depends on the building and the borough. Many accept slim aluminium on unlisted buildings when the sightlines and glazing bars match. Listed buildings usually need steel, or repair of the originals.
Sources
The official documents this guide is based on. Check the current edition before you order.
- Permitted development rights for householders: technical guidance (GOV.UK)
- The Town and Country Planning (General Permitted Development) (England) Order 2015 (legislation.gov.uk)
- When is permission required? (including Article 4 directions) (GOV.UK)
- Traditional windows: their care, repair and upgrading (Historic England)
- Listed building consent (Historic England)
- Building Regulations and historic buildings (Historic England)
- Approved Document L: conservation of fuel and power (GOV.UK)