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.

Our clients at atelier EURA Ltd love the idea of flooding their new extensions with natural light, however, they are often surprised when they find out that it is building regulations, rather than planning, that dictates how much glazing they can actually have.

Why is this? Because when compared to solid walls, floors or roofs; windows, rooflights and glazed doors do not perform as efficiently.

So, How much glazing is allowed in an extension? and what are your options?

"Conservatory":

Firstly, you can create a glass heavy structure (a conservatory, essentially) at the back of your home by keeping a thermal separation between the new structure and your home.

Before you knock these, be mindful that passive solar heating is used in many countries to manage heat demand, and conservatories and sunrooms in the right orientation and designed with intent can have a positive impact. They do however not allow you to create an open plan.

Extension by the book

So, if an open plan is your target, under the building regulations you can open up the space and connect the existing home to the new extension, but the total area of windows, roof windows, rooflights and glazed doors in your extensions should not exceed the sum of the openings you already have (and will be covering with the new extension) plus 25% of the new extension floor area.

To better illustrate , here is an example:

If the rear façade of your building has a glazed patio door that is 910x2100 (1.9 sqm) and a window that is 1247.5x1050 (1.3 sqm) and both will be covered by your new extension that has an area of 25 sqm.

Your new glazing allowance will be 25% of 25sqm +1.9sqm +1.3sqm = 9.45sqm.

You can use that allowance as you see fit. For example, you could have a 3800 x 2100 bifold and a 1.45sqm rooflight. 

SAP Calculations + upgrades

There is a third option that requires specialist expertise from a SAP assessor . The SAP assessor will run calculations aimed at demonstrating that the carbon dioxide (CO2) emission rate of the proposal, implemented, will not exceed the emissions of a by-the-book/ compliant extension (same size and shape). 

For this third option, you will also need additional budget because it will require upgrades to the existing house (i.e. window replacement, insulation and/or heating) and upgrades to the spec of the windows, rooflights and glazed doors.

(i.e. gas-filled and/or low-e-coated glass) 

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