Double Glazed Sash Windows in a Conservation Area: What’s Allowed and What It Costs

If you live in a conservation area, you can usually have double glazed sash windows. The key is that they must look like the windows they replace. Councils across London regularly approve slim double glazed timber sashes, but they often refuse chunky units, fake glazing bars and uPVC on the front of the house. This guide explains what is normally allowed, when you need permission, which glazing options work best and what you should expect to pay.

The short answer

• Houses in a conservation area: replacing windows with ones that look similar is usually permitted development, so you often do not need planning permission. Double glazed timber sashes that match the originals generally fall into this.
• Streets with an Article 4 direction: many London conservation areas remove that right for windows facing the street. You will need planning permission, even for like-for-like double glazing.
• Flats and maisonettes: they have no permitted development rights, so any change to how the windows look usually needs planning permission.
• Listed buildings: you need listed building consent for almost any window replacement. Double glazing is often refused, so secondary glazing or vacuum glazing in the existing sashes is the usual route.

Rules vary between councils, so always check with your local planning team before you order. Our planning permission guide explains the process step by step.

What councils look for

Conservation officers care about how the window looks from the street, not the energy rating. When they assess double glazed sash windows, they usually check:

• Material: timber that matches the original. uPVC is often refused on front elevations in conservation areas.
• Opening method: the sashes must slide vertically. Casements or tilt-only windows made to look like sashes are rarely accepted.
• Frame and glazing bar sizes: slim frames and bars that match the originals. Thick double glazing units force thicker bars, which is the most common reason for refusal.
• Glazing bars: real (through) bars or bonded bars with a spacer bar inside the unit. Stick-on bars without a spacer bar look flat and are often refused.
• Details: horns, meeting rails, mouldings and the colour of the spacer bars (dark, not silver).
• Glass: some councils prefer a slight ripple or handmade-glass look on important buildings.

The best double glazing options for period sashes

Slim (heritage) double glazing

Slim double glazed units are usually around 12 to 14mm thick, compared with 24 to 28mm for standard units. They fit into new timber sashes with glazing bars almost as slim as the originals. This is the option most often approved in London conservation areas.

Vacuum glazing

Vacuum glass is only a few millimetres thick but insulates about as well as standard double glazing. Because it is so thin, it can often be fitted into your existing sashes, which keeps the original timber and avoids replacing the whole window. That makes it a strong choice where councils want the original windows kept.

Standard double glazing in new sashes

Standard units give the best insulation for the money, but the thicker glass needs thicker sashes and glazing bars. They are usually fine on the rear of a house that is not covered by an Article 4 direction. They are often refused on front elevations.

Secondary glazing

If double glazing is not allowed, secondary glazing is a slim, removable inner window fitted on the inside. It keeps the original windows untouched, cuts heat loss and noise, and rarely needs consent. See our secondary glazing guide.

Timber or uPVC in a conservation area?

Timber is the safe choice. Some uPVC sash windows now copy timber details well, and a few councils accept them on rear elevations or on modern buildings within a conservation area. But on the front of a period property, uPVC is the option most likely to be refused, and it can lead to enforcement action if fitted without permission where permission was needed. If you are unsure, ask your council or hire an installer with conservation-area experience.

How much do double glazed sash windows cost?

Typical 2026 UK prices for a supplied and fitted window of average size:

Option	Typical cost per window
New timber double glazed sash (softwood)	£1,200 to £2,500
New timber double glazed sash (hardwood or engineered timber)	£1,800 to £3,500
uPVC double glazed sash	£600 to £1,200
Secondary glazing	£180 to £400

London prices are usually towards the top of these ranges, and large or arched windows cost more. Slim heritage and vacuum glazing cost more than standard units, but they are often the only options a conservation officer will approve. For a full breakdown, see our sash window replacement cost guide.

Do I need building regulations approval?

Replacement windows must meet building regulations for energy efficiency and safety. A registered installer (for example one registered with FENSA or CERTASS) can self-certify the work. Building regulations allow some flexibility for historic buildings and conservation areas where meeting the full standard would harm their character. Our building regulations guide covers this in more detail.

How to get it approved first time

1. Check your status. Find out if your home is in a conservation area, covered by an Article 4 direction or listed. Your council website will have a map.
2. Read the council’s guidance. Many London councils publish conservation area appraisals or design guides that say exactly what they accept for windows.
3. Photograph your existing windows and measure the glazing bars, frames and meeting rails.
4. Ask for detailed drawings. A good installer will provide section drawings showing frame and bar sizes alongside the originals.
5. Talk to the conservation officer early. Many councils offer pre-application advice, which can save a refusal.
6. Use an experienced installer. Firms that work in conservation areas every week know what each council accepts.

Find an installer who knows conservation areas

The fastest way to get it right is to use a firm that already fits double glazed sashes in your area. Compare sash window companies near you, filter by timber and double glazing, and ask each one for examples of approved work in conservation areas.

Frequently asked questions

Can I have double glazing in a conservation area?

Usually, yes, as long as the new windows look like the originals. Slim double glazed timber sashes are commonly approved. Thick units, fake glazing bars and uPVC on front elevations are the most common reasons for refusal.

Do I need planning permission for double glazed sash windows in a conservation area?

For a house, like-for-like replacement is usually permitted development unless an Article 4 direction applies. Flats, maisonettes and listed buildings almost always need permission or consent. Check with your council before ordering.

What is the thinnest double glazing for sash windows?

Vacuum glazing is the thinnest, at only a few millimetres, and can often fit existing sashes. Slim heritage double glazing is usually around 12 to 14mm.

Is secondary glazing better than double glazing in a conservation area?

Secondary glazing is easier to get approved and keeps the original windows, so it is often the best choice for listed buildings. Double glazed sashes give a neater finish and are usually better if you are replacing rotten windows anyway.