How To Calculate Your Glazing Allowance

So, in a simple example, we have an extension 5 metres wide and 3 metres deep. This will extend out from the house covering an existing door of 2 m2 and a window of 1.5m2 which will no longer be external.

So our glazing allowance is 25% of our floor area: (5 x 3) x 0.25 =  3.75m2

Plus.

The total area of doors and windows that will no longer be exposed: 2m2 + 1.5m2 = 3.5m2

So our total glazing allowance is 3.75m2 + 3.5m2 = 7.25m2.

In our example above, an extension that doesn’t exceed the building regulations will look something like this.

Not perhaps exactly what you had in mind.

You were thinking more like a nice set of bi-fold doors, oh yes a couple of roof lights and some additional side glazing like Tom’s up the road would be nice.

With modern extensions, just a bi-fold door alone is likely to put you over your glazing allowance.

What To Do If Your Existing or Proposed Extension Is Overglazed

You’ll usually be pulled up by either your local building control officer or advised by your architect that your extension is overglazed.

There are a few different scenarios when the overglazing situation could be flagged such as:

• Before construction at the architect’s design stage of the building plans.
• During construction, when you get a site visit from building control.
• Once the extension is completed and you want it signed off by building control.
• When selling a property and the 3rd party solicitors asks for a copy of the sign off certificate from building control.
• When you have opened up the main house to an existing conservatory.
• When you extend the main heating system into an existing conservatory.

The first thing is no matter what stage you’re at, there is a solution. In around 90% of cases we can still get the extension to pass without the client having to do anything extra. However, the earlier you get the required reports done for building control, the easier will be the solution.

 

Step 1

First check that your extension is actually overglazed.

Building control usually won’t carry out any measurements themselves but will simply ask you to produce either what’s called an Area Weighted U-value calculation, or Sap calculations if they think it is overglazed. They may also quote from a document called Part L of the building regulations.

Don’t worry about these two terms, I will explain each in more detail later.

To recap.

Here’s how to calculate whether your extension is overglazed:

1st, Calculate your total glazing allowance.

• Calculate the floor area of your extension.
• Multiple the extension floor area by 0.25. This will give you your 25% allowance.
• Calculate the area of any doors or windows that are no longer external because of the extension. This 2nd part is your additional allowance.
• Add your 25% allowance and your additional allowance from the removed doors and windows. This is your total glazing allowance.

2nd, Calculate the glazing area of your actual extension.

• Add the area of all doors, windows, and roof lights.

If the area of your actual extension is greater than your total glazing allowance, then your extension is overglazed.

What to do if your extension is not overglazed

If your extension is within the total glazing allowance, you need to write a simple letter to building control showing your calculations in the format shown above and you should be OK.

If your extension is actually overglazed, then you’re going to need more expert help to produce either an

Area weighted u-value calculation

Or

Sap calculations, using the Whole House Method.

We’ll look at each of the methods in more detail in separate articles.

If you need advice or help with your overglazed extension, get in touch. We can help whatever the stage of your project.