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What are Excessive Glazing Calculations

With new extensions or conversions to a property, there's a limit to the amount of glazing permitted. You're restricted to 25% of glazing to floor area and if you exceed this limit, you must have proper and professional justification. This is due to glazing and openings losing more heat than walls, making it less energy efficient.


With many extensions these days having far more glazing on elements to let in more light, sticking to the 25% limit can be difficult. Excess Glazing Calculations can successfully prove your need for extra glazing and help keep your building project on track and on budget.

If it's deemed that your new extension or conversion exceeds regulations in terms of glazing, the Building Control Officer will ask to see official Excess Glazing Calculations.

Specifically, all extensions that have glazing that is equivalent to more than 25% of the total floor area of the extension will need to be tested.

Should the total percentage of excess glazing be recorded as higher, a full SAP calculation is required that must also take into account your existing property. This assesses the overall energy performance of your home, taking into account building materials and heating systems. Depending on the results, the rest of your current property may be taken into account and it may be suggested that upgrades elsewhere would successfully offset any additional glazing needed for the build.

What is required for Excessive Glazing Calculations?

Construction drawings to scale


Existing and Proposed Elevations


Site Location Plan


Existing and Proposed Floors Plans


Heating System to be used


Technical Sections

BGA Architects can produce the above information that would be provided to the energy consultant to allow them to carry out the calculation as well as obtaining a quotation.

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