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How Much Glass Can a House Extension Have in the UK

Updated: Apr 7

glass garden room extension UK
I designed this glass, garden room extension in Edinburgh. It was built in 2013

I have been a self employed Architect in the UK since 2009. I specialise in altering and extending private homes. One thing all my clients want to have, is as much glass as possible in their house extensions. While the planning application system is relevant to the design of any house extension in the UK, the maximum amount of glazing is largely determined by the building regulations, not the planners. If you are uncertain about the differences between the planning system and the buildings regulations, I wrote a post that explain this.

In this post I am going to explain how the amount of glazing (that is windows, doors and rooflights) in a house extension is determined by the building regulations. And how to design a house extension to maximise that glazing. Obviously this information only applies to the UK. Be aware that building regulations are different in England and Scotland and changes are made to the regulations every couple of years. Always take advice from an experienced Architect before going ahead with any work.

To start, we need to understand a few simple ideas.

First, is the idea of heat loss from a building. We all know there is a climate crisis and that new buildings must be designed to be as energy efficient as possible. Residential properties account for over 20% of all carbon dioxide emissions in the UK, so making private homes more energy efficient is a priority. One of the main ways the government achieve this is by writing building regulations which limit heat loss from new buildings and this includes house extensions. To measure heat loss the construction industry use what are called U Values. This is like golf, the lower the number, the better the score. The U value measure the amount heat lost from one square meter of a wall, floor, roof or window per hour in Watts.

At present, the building regulations in England differ from those in Scotland but not by much. The main thing to understand is that a wall, floor or roof is required to be far more energy efficient, and conserve much more heat that a window, door or roof-light. This is because of a basic and inescapable property of glass. It just leaks heat. Even with the best glazing technology on the market the most energy efficient triple glazing will have a u value of around 0.8 while building an external wall, with materials bought at any builders merchants, can easily exceed the requirements of the building regulations and achieve a u value of 0.1

That means that even the best glazing that money can buy is eight times less efficient than a regular wall. The best windows, doors and rooflights leak eight times more heat than the wall or roof they are built into. Knowing this, it makes sense to limit the amount of glass in a new building or house extension. So, how much glass can be built into a house extension in the UK. My clients regularly show me images of slick, modern buildings with floor to ceiling glazing and ask if they can have something similar.

While the building regulations differ between Scotland and England, they do take a similar approach to this issue.

There are two choices for the design of a new house extension.

  1. If you want unlimited glazing, you can have this but the extension must be thermaly isolated from the rest of the house. That means there must be an external grade door between the exiting house and the new extension. It cant be open plan with the rest of the house. In Scotland the regulations allow for what they call a “stand-alone building”, which can be up to 50 square meters and the amount of glass is unlimited. Being stand alone does not mean detached from the house, it means connected via a door. The English regulations don't have provision for a stand alone building but they do allow for conservatories and the limitations usually placed on the maximum area on new windows, doors and rooflights to not apply, provided the conservatory is thermally isolated from the house, by a door.

  2. If don't want your extension separated by a door, and instead want it to be open plan to the exiting house, you can do this but the amount of glazing is limited. The formula for calculating this is relatively simple. The area of new openings, that is windows, doors and rooflights must be no more than 25% the floor area of the new extension plus the area of any existing openings in the house covered by that new extension.

Lets explain what these two options would look like in practice, using a diagram. This is an existing house, with a patio door on the rear wall. The new extension will measure 4 meters by 6 meters and cover that existing patio door.

The existing house

Using the first option, keeping the extension terminally isolated using the existing patio door, the extension can have an unlimited amount of glazing. It can also be heated, provided that heating is on a separate thermostat from the rest of the house.

An extension with unlimited glass, notice the patio doors between the extension and the existing house.

Using the second option, the extension is open to the existing house but the glazing is limited to and area equivalent to 25% of the new floor area. 4 meters by 6 meters equals 24 square meters and 25% of that is 6 square meters. Plus the area of the existing patio door which is now covered by the extension, it measured 5 square meters. We add this number to the 6 square meters calculated earlier and we get 11 square meters.

An extension designed with 25% glass per new floor area. This can be open to the existing house

This is the total area of new openings allowed in the house extension. That can be used to create doors, windows or rooflights any way you wish but collectively the areas of those new openings must not exceed 11 square meters.

Section through the extension, no door between this and the house

So in this case, the house extension will have a roof light equalling 2 square meters, a feature window equaling 3 square meters and a new patio door equalling 6 square meters. 11 square meters in total.

It is possible to exceed the allowable area of new openings using what is know as a compensatory calculation. This will look at increasing the amount of insulation in the new walls, floor and roof of the extension, as well as specifying glazing which exceeds the minimum u-value requirements of the building regulations. I use compensatory calculations on most of my house extensions and, while it does allow the design to have more glazing, the extra amount is minimal. Usually just a few square meters more than the 25% calculations allows. The reason is that the minimum u values required by the building regulations are already very low. There is almost no heat left to save, so adding more insulation doesn't have a massive effect.

Both the Scottish and English building regulations now allow a compensatory calculation to include upgrades to the fabric of the existing house. By making the existing house more thermally efficient, an extension to that house could have more glazing, in theory. I have used this on some projects but it will drive up the overall cost of the project, because it usually requires the windows in the existing house to be replaced with modern ones, as well as insulating existing external walls and roofs, which is not always easy or cheap to do.

If you are thinking of extending your house, and you want to maximise the amount of glass in that new extension, talk to an experienced local Architect. They may know an energy consultant who can produce the compensatory calculations which allow glazing to be maximises but, remember, this will cost more money than the simple 25% calculation I explained earlier.

If you are planning to build a house extension anywhere in the UK, and you would like advice from an experienced architect, get in touch and book a consultation with me to discuss your ideas, timescale and budget. I provide a sense check on your ideas and can advise you on how to find the right local architect for your project.

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