Our glass roof extension guide: tips and ideas

Is there any greater wow factor than a glass roof extension?

Not only do they look stunning, but there are lots of other benefits too.

Glass roof extensions will flood your living room with natural light. They can now be self-cleaning. They’re very well insulated. They let you look up at the stars while reading a book… we could go on and on.

As glass roof extensions are becoming increasingly popular, our architects have decided to prepare this useful guide. Topics include: planning permission, building regulations, cost, the best ways to keep them warm… and much more!

We hope you find it helpful.

1. Planning permission for glass roof extensions

Choosing whether to go down the Planning Permission or Permitted Development route is a little tricky. Both the structure (the walls) and the roof of glass extensions will be treated differently by the planners.

1.1. The extension structure

When it comes to glass roof extensions, there are specific types which can be built without planning permission. This is called Permitted Development.

However, there are quite a few criteria your new glass roof extension would need to meet:

• Flats: Permitted development only applies to houses. If you live in a flat, you will need to apply for planning permission.
• Location: Only half the area of land around the “original house” can be covered by extensions or other buildings.
• Height: Extensions cannot be higher than the highest part of the existing roof, or higher at the eaves (usually the gutters) than the existing eaves.
• Proximity to a boundary: Where the extension comes within two metres of the boundary, the height at the eaves cannot exceed three metres.
• Siting: Extensions cannot be built forward of the ‘principal elevation’ (usually the front of your house) or, where it fronts a highway, the ‘side elevation’ (if you live on a corner site).
• Conservation areas: In conservation areas, the work cannot include cladding of the exterior.
• Depth: Single-storey rear extensions cannot extend beyond the rear wall of the original house by more than four metres if a detached house. Or by more than three metres for a semi-detached or terraced house.
• Height: A single-storey rear extension cannot exceed four metres in height.

If your glass roof extension doesn’t meet the above requirements, you would need to apply for planning permission. However, that’s not necessarily a bad thing. Going down the planning permission route will often give your architect more scope to be creative. This is because planning policies are less rigid than Permitted Development legislation.

1.2. The glass roof

When it comes to glass roof extensions, Permitted Development legislation is somewhat ambiguous. The section relating to permitted external materials reads:

“The materials used in any exterior work (other than materials used in the construction of a conservatory) must be of a similar appearance to those used in the construction of the exterior of the existing dwellinghouse”.

Traditionally, houses are brick built with slate or clay tiles on the roof. This means any extension you add should use those same materials if you’re going down the Permitted Development route.

However, the caveat in the above policy mentions, ‘other than materials used in the construction of a conservatory’. This suggests that extensions built of glass are indeed acceptable as the majority of conservatories are.

1.3. Certificate of Lawful Development

Before going down the Permitted Development route for glass roof extensions, we would always advise obtaining a Certificate of Lawful development from your local planning authority first. They will issue you with a certificate confirming that your proposed plans would be ‘lawful’ and would therefore not require planning permission.

2. Building regulations and glass roof extensions

Good news! If you have made it this far your glass roof extension will have been approved!

However, there is still a little more work to do before you can start building.

First, your architect’s detailed plans will need to be signed off by Building Control. Due to energy efficiency requirements, large expanses of glass in your extension can conflict with building regulations.

2.1 Building Regulations Part L

This limits the total area of openings or glazed elements in glass roof extensions. These include:

• Windows
• Roof lights
• Doors

The area of these elements should be a maximum of 25 per cent of the extension’s floor area. A small extension with a set of patio doors and a lantern light can easily use up this 25 per cent allowance. 

3. Tips for getting glass roofs through building control

3.1. Increase Thermal Resistance

One option is to increase the thermal resistance of other exposed elements. So, you might look to upgrade your proposed floor, walls or roof. If this isn’t sufficient, then it may be necessary to increase the specification of the glazing to a high-performance glass. These have better thermal efficiency qualities.

3.2. Standard Assessment Procedure (SAP)

A second option is to carry out a Standard Assessment Procedure or SAP calculation. The goal is to demonstrate that the CO2 emission rate from the house and glass roof extension would be no greater than for the house and a conventional extension of the same size. There are lots of ways this can be done. For example, better loft insulation or installing a more efficient hot-water system or boiler.

3.3. Separate your new extension

Did you know there are instances where conservatories are exempt from Building Regulations? This is subject to a few caveats, including: if they’re separated from the house by external-quality doors/windows and have independent heating controls. This won’t work if you’re looking for open plan living. But this could be an option if you’re looking to create a separate room within your new extension.

4. What are the pros and cons of a glass roof?

4.1. Pros of glass roof extensions

• They look stunning
• They flood the room with natural light
• They make even the smallest space feel more open
• There is less need for artificial lighting

4.2. Cons of glass roof extensions

• They can be too bright
• They can get very hot during the summer
• They are far more expensive than traditional roof construction techniques

5. How do I ensure privacy?

Having a glass roof does increase the chances of neighbours being able to see through your glass roof and into your living space. However, there are a number of different ways this potential issue can be easily addressed:

• Electric blinds
• External louvres
• Planting

6. What about keeping it warm?

In winter, rooms with glass roofs can get quite chilly. Fortunately, there are a number of different ways to heat glass roof extensions.

6.1. Under floor heating

With so many underfloor heating options to choose from, it’s certainly a must for any new extension. We would always suggest choosing a water system over an electric, as a water system is much cheaper to run.

6.2. Perimeter & trench heating

Trench heating can replace the need for radiators in a new extension. A trench will be included within the perimeter of your new floor and a simple ‘convector unit’ connected to the heating system.  The only visible part is the grille. The system uses natural convection to heat the entire room from floor to ceiling.

6.3. Low U-value

U–Value is the measure of the rate of heat loss through a material. With glass, the aim is to achieve the lowest U–Value possible. The lower the U–Value in glass roof extensions, the less heat will escape through your glass. This is why we would always advise picking a glass roof with a low U-Value as it will retain more heat, meaning you spend less on bills (which is always a good idea).

7. How can I keep my glass roof extension cool?

There are lots of different ways!

7.1. Gas filled glazing

There are now new and innovative coatings which can be applied to glass. New trends also include gas filled cavities inside the windows, which gives added thermal insulation. Argon is the most common gas used but others also include krypton (we hope Superman doesn’t have these). Double check with the manufacturer producing your glass roof to confirm its thermal insulation properties.

7.2. Fritted glass

A relatively new innovation is glass printed with a ceramic frit and fired into a permanent, opaque coating. This not only helps reduce glare but will also keep your extension cooler during those hot summer months. The final advantage is that they come in a host of different patterns which is a great opportunity to create something truly unique when it comes to glass roof extensions.

8. What about cleaning my glass roof extension?

This is by far the biggest area of contention when it comes to glass roof extensions. Noone likes the thought of a beautiful glass roof being covered in leaves and mud. Neither do we!

Have you ever tried to clean a glass roof?

It’s not easy. Given that it’s not safe to stand on a glass roof, they are very difficult to clean. And, lashing a couple of mops together would be a challenge in itself and probably wouldn’t get your glass clean.

8.1 Self-cleaning glass

Wouldn’t it be a dream if you could avoid cleaning your glass altogether? Well, you’re in luck. Self-cleaning glass now exists. Here is how it works.

1. Manufacturers use a ‘dirt eating’ coating which causes a photocatalytic rection. This breaks down organic deposits such as mud and grime.

2. They also use a hydrophilic coating. This means that water spreads evenly over the surface of glass to form a thin film instead of forming into droplets. This helps to wash dirt away, preventing the formation of drying spots and streaks.

9. What happens if my glass roof extension breaks?

A crack in your new glass roof would be a disaster. To prevent this from happening, there are a number of questions we would suggest asking your glass manufacturer before you finally pick one.

• Can each piece of glass be easily carried into place?
• If a fitted glass panel suddenly cracks or breaks, can it easily be replaced?
• Does the manufacturer offer any sort of warranty in case of a breakage?
• Will the manufacturer pay for the labour to replace the glass?
• Are there any specific exclusions under the warranty?

10. Does heated glass really exist?

Yes… heated glass exists for glass roof extensions!

Heated glass has a transparent metal oxide coating applied to the glass. Electricity is passed through the coating via buzz bars at the head and the base of the unit which are concealed within the glass construction. 

The heated glass coating is a semi conductive material. When electricity is passed through the coating, it generates heat. This has a few great advantages:

1. Your room is heated using infra-red radiation

2. It creates a warm surface to the glass to stop condensation forming

3. It creates a warm surface on the outside of a glass structure to prevent snow or ice build-up

How much does a glass roof cost?

The average cost of an entirely glass roof ranges between £1,800 and £2,200 per m2.

Ways to save money

Plan carefully:  The chances are your glass roof will be completely bespoke. This type of glass is expensive and is a difficult material to work with. If you haven’t planned extensively and need to make adjustments, it will be expensive to make changes once installed.

Frame vs Frameless: A framed style is likely to cost less than a frameless style. A frameless style will need to be bespoke and is going to be more complicated to work with than a framed style.

Get multiple quotes: In our experience, quotes for glass roof extensions can vary enormously. Bearing that in mind, make sure you obtain several different quotes from different manufacturers. Also make sure you ask for the same specifications from each so you can compare like with like.

Conclusion

Our architects love to be creative and design unique extensions that are one of a kind. This is why we think glass roof extensions are a great idea for those who want to stand out from the crowd.

We have a huge amount of experience working with designing glass roof extensions. If you would like any advice or you’re looking to discuss ideas, why not book a free 30-min call with one of our architects?

We’re always on hand to help.