Understanding the 25% glazing rule for rooflights

October 31, 2023

Like all building components, rooflights are subject to certain regulations.

The huge demand for glazed extensions, conservatories and sunrooms has led to more specific building laws in the UK: Introducing the 25% glazing rule! But what does it entail? If you’re thinking of installing a rooflight in your home, how will this affect you?

What is the 25% glazing rule?

Simply put, the 25% glazing rule states that the glazing of any windows, doors, and roof lights to an extension should be limited to 25% of the extension floor area, including any windows or doors being built over.

The 25% rule isn’t just a recommendation; it’s a government-set regulation designed to combat global warming by minimising energy loss through glazing. You can find it in Part L of the Building Regulations or the government’s Planning Portal. 

How is the 25% glazing rule calculated?

Before determining if your home conversion complies with the 25% rule, you’ll need to know the total floor area. Take, for example, a conservatory with a total floor area of 20m².

Leading into the garden is a patio door that will remain unchanged: it measures 4 metres squared. In these cases, your extension is permitted 5m² (to fulfil the 25% rule) and an extra 4m² for the existing patio door (the existing openings rule).

However, you can sometimes bypass this rule if you prove that your conservatory or extension doesn’t have an energy loss above 25%. To do this, you’ll need a whole dwelling calculator as provided by a regulated energy assessor.

Or, you can provide an area U-value calculation. You can ensure your property has an energy loss of 25% by investing in a new, more efficient boiler or improving insulation around the home.

Where does the 25% rule apply?

The 25% rule is not only applicable to private homes but other types of commercial and residential properties:

1. Standard building extensions

Typical extensions to existing structures, where additional living spaces are created, must adhere to the 25% glazing limit to maintain energy efficiency and compliance with building regulations.

2. Sunrooms

Specifically designed to capture maximum sunlight, sunrooms predominantly feature glazed elements to allow ample natural light. The rule ensures that such designs also consider energy conservation.

3. Orangeries

A more traditional form of sunroom, orangeries typically have a brick base and are constructed with solid, glazed walls. The 25% rule applies to ensure a balance between light and thermal performance.

4. Glass-box extensions

Without proper regulation, glass-box extensions can experience temperature extremes, becoming too hot in summer and too cold in winter.

The 25% rule helps maintain thermal comfort by limiting the proportion of glazing, thus reducing the risk of over-reliance on heating and cooling systems.

What are the benefits of the 25% rule?

So, what are the specific benefits of the 25% rule? Why would the government impose restrictions on glazing, and how does it relate to the broader issue of global warming?

1. Enhanced natural lighting

The 25% Glazing Rule guides us in making our homes brighter and more welcoming while being mindful of our energy use. Setting clear requirements for businesses and installers tells us how to limit heat gains and losses while reducing our contribution to climate change.

2. Energy efficiency and sustainability

The primary objective of the building regulations, including the 25% Glazing Rule, is to ensure health, safety, and energy efficiency.

The rule directly responds to the need to conserve fuel and power in buildings. By limiting the glazing to 25% of the extension’s floor area, the rule ensures that buildings meet energy efficiency targets.

3. Improved indoor environmental quality

While we all love the look of glass, it’s important to remember that too much of it can lead to drastic temperature changes.

Luckily, our rooflights have an insulated barrier created by a thermally-broken frame. They’re designed to minimise condensation and the transfer of hot and cold air, leading to enhanced energy efficiency.

But if you’ve got older glazing in your room that hasn’t been replaced for a good few years, its insulating properties may not be effective. Brick walls are excellent insulators, meaning that a home with too much low-quality glazing can get chilly in winter and overly hot in summer.

The 25% Glazing Rule helps us find the sweet spot. By adhering to this government regulation, you can enjoy the beauty of a rooflight while keeping your home cosy and comfortable.