Over Glazed Extensions

When building an extension, you are allowed external openings equal to or less than the following criteria:

• Openings = 25% of the internal floor area of the extension PLUS;
• Existing openings blocked up as a result of the extension.

For example, if you have an extension that is 20m2 and the extension blocks up a door and window that add up to 3.2m2, then the calculation is as follows:

• External openings = 25% of floor area x 20m2 = 5m2
• Existing openings blocked up = 3.2m2
• Total allowable external openings = 5m2 + 3.2m2 = 8.2m2
• If total openings are above 8.2m2, then you need to prove the average U value is lower OR provide a whole house dwelling calculation for carbon emissions using SAP 2012.

As accredited on-construction energy assessors, we use SAP 2012 (Standard Assessment Procedure) software predict the energy performance and carbon emissions of a new dwelling.

This is carried out on the whole house including the extension.

A total of 2 assessments are run. Using the example above (20m2 extension), then we will assume we have 10m2 of external openings, which exceeds the allowable 8.2m2. The following assessments are then undertaken:

1. Whole house energy assessment including the extension with external openings equal to 8.2m2 that meets Part L1A requirements.
2. A second whole house energy assessment including the extension with external openings equal to 10m2 which represents the designed extension.

If Assessment 2 has lower carbon emissions that assessment 1, then it passes.

SAP will look at several categories for the assessment such as:

• Insulation values through the walls, floor and roof
• Solar gains through windows and roof lights
• Air tightness (uncontrolled air leakage causing cold draughts)
• Thermal bridging (heat loss through junctions such as where the floor meets the wall)
• Boiler efficiency and fuel used to power the heating and hot water
• Ventilation fan efficiency and power used
• Low energy lighting
• If required, renewable energy and other low carbon technologies

If as part of the extension works your are replacing the boiler or increasing your loft insulation, then these improvements can be included to help you pass.

We start with assessing the annual solar gains. Sometimes the increase in solar gains can reduce annual heating demand so no further enhancements area required.

But we recommend you try and improve insulation values on your extension to retain more heat to help the assessment further.

We provide a summary report outlining the enhancements allowed for.

We would recommend considering how the additional glazing affects your comfort levels during hot summer days. While it is nice to have lots of glazing, consider the orientation and how to reduce summertime sun causing overheating.

We have carried out over 70 whole house over glazed extension assessments throughout England for a wide variety of clients with a 100% pass record.

Our lead assessor is an on-construction domestic energy assessor (accredited with Elmhurst Energy) with an MSc in Energy & Sustainable Building Design and over 20 years experience in Architecture. Our advice and service is geared around providing solutions for our clients, helping them achieve their goals and input our significant experience to make the process easy, whilst using materials and systems that are readily available.