How Much Glass Can You Actually Have? (It’s a trick question)

One of the most common questions homeowners ask is, "What is the maximum amount of glazing I can have in my extension?"

The confusing answer is: It depends on which Building Regulation you are asking about.

There are actually two different "maximums" you need to worry about. One is about keeping heat in (energy efficiency), and the other is about keeping fire out (safety). You have to satisfy both.

1. The "Heat" Limit (Part L: Conservation of Fuel and Power)

This is the rule most people know. To stop us from building "greenhouses" that leak heat and drive up carbon emissions, the regulations set a strict limit on glass.

The 25% Rule: generally, the total area of your new windows, doors, and rooflights cannot exceed 25% of the new floor area plus the area of any existing windows/doors that the extension covers up.

The Workaround: If you want more glass than this (and let's be honest, most people do), you don't have to shrink your windows. You just have to prove your design is still energy efficient. You do this via a SAP Calculation. This usually involves upgrading the insulation in your roof or walls to "compensate" for the extra heat lost through the big glass doors. We will send out the design to a SAP assessor and they will happily furnish the Building Control officer with the required calculations. (this will be at an additional cost from the assessor)

2. The "Fire" Limit (Part B: Fire Safety)

This is the rule that catches people out. It’s not about how much heat the glass loses, but how much heat it lets through during a fire.

The "Unprotected Area" Rule: Standard glass cannot stop radiant heat. If your house catches fire, the heat radiating through your windows could set your neighbour’s house on fire.

The Boundary Trap:

Less than 1m from boundary: If your side wall is within 1 meter of your neighbour's fence, you are heavily restricted. You typically cannot have any significant windows on that wall (often limited to a tiny 1m² specifically designed area, or fire-rated fixed glass).
More than 1m: As you move the wall further away from the boundary, the percentage of glass you are allowed to have increases.

The "Double Whammy"

The tricky part is that these rules don't talk to each other.

You could have a super-insulated, triple-glazed wall that passes Part L (Heat) easily... but if it’s 90cm from the fence, Part B (Fire) will force you to remove the windows.

Conversely, you might have a wall 10 meters from the neighbour (passing Part B), but if it’s 100% glass, you will likely fail Part L without extra insulation elsewhere.

Summary

Part L limits glass to save energy (25% rule). Fix: Better insulation/SAP calcs.
Part B limits glass to stop fire spread (Boundary rule). Fix: Fire-rated glass or moving the wall back