HOW MUCH GLAZING IS ALLOWED IN AN EXTENSION?

(AI generated image depicting a conservatory)

Note: the following applies to England and Wales, check your local regulations if you read from elsewhere.

At our London studio, we find that many clients wish to fill their new extensions with as much natural light as possible. However, they are often surprised to learn that Building Regulations, rather than Planning Permission, determine exactly how much glass is permitted.

This is because glass, even high-quality modern glass, is less efficient at keeping heat inside than a solid wall or roof. To help reduce energy waste and carbon emissions, the UK government sets specific limits on the amount of glazing allowed in new building works.

1. Option One: The "Separated" Sunroom

One way to have a structure made almost entirely of glass is to keep it "thermally separated" from the main house. This means keeping the original external doors or windows between the old house and the new glass room.

When designed correctly, these rooms can use the sun to help heat the space (known as passive solar heating). However, because the doors must remain closed to save energy, this option does not allow you to create a completely open-plan living area.

2. Option Two: The "Standard" Limit

If your goal is to create an open-plan space where the house and extension are one, you must follow the standard rules found in Approved Document L.

The total area of your new windows, rooflights, and glass doors cannot be larger than:

1. The area of any glass you removed or covered up during the build.

2. Plus 25% of the new extension's floor area.

A Practical Example:

Imagine you have an old patio door (1.9 square metres) and a small window (1.3 square metres) on the back of your house. You decide to build a new 25-square-metre extension over that area.

Your allowance for new glass would be:

• 25% of the new floor (6.25 square metres)

• Plus the old door (1.9 square metres)

• Plus the old window (1.3 square metres)

• Total Allowed Glass = 9.45 square metres.

You can use this total however you like, perhaps for one large set of sliding doors and a skylight.

3. Option Three: The "Specialist" Route (SAP Calculations)

If your design requires much more glass than the standard rules allow, there is a third option. This requires a specialist called a SAP Assessor to perform a technical calculation.

The assessor must prove that your custom design will not lose more heat than a "standard" extension of the same size. To make this work, you will usually need to:

• Invest in Better Glass: Use higher-performance windows with special coatings that trap heat more effectively.

• Improve the Rest of the House: You might be required to add extra insulation to your loft or upgrade an old boiler elsewhere in the building to "offset" the heat lost through the extra glass.

Why This Matters

Ensuring your extension meets these regulations is vital. It guarantees that your home will be comfortable to live in, staying warm in the winter and avoiding over-heating in the summer, while also helping to lower your energy bills.

We hope this article has been of use and as always you can speak with us about your upcoming project by scheduling a call HERE