Building Regulations and Planning Permissions

SAP Calculations and Glazing

All residential new builds and extensions (where the building is open to the extension) in the UK are required to have a SAP (Standard Assessment Procedure) Calculation if the glazing guidelines for Building Regulations are exceeded.

Building Regulations for glazing:

New Builds: Building regulation L1A (conservation of fuel and power in new dwellings) are applicable to new build

Extensions: Building regulation L1B (conservation of fuel and power in existing dwellings) are applicable for extensions to dwellings.

Building regulation L1B states that the glazing in an extension cannot exceed 25% of the extension floor area, plus the area of any openings which will be covered by the new extension. If these are exceeded significantly, then SAP calculations will be required - building control can advise exactly what will be required on each individual project.


A company like Evergreen Living calculates the SAP calculation to make sure that any extension or new build complies with all Building Regulations. The method for highly glazed extensions is to model the existing house with a ‘notional extension’ which is the same shape and size but conforms to the guidance in L1B. This sets the CO2 emission target. Then the same exercise is carried out with the existing dwelling plus the designed extension which gives the actual CO2 emission. To achieve compliance the designed calculations will need to factor in compensatory measures until it equals or betters (is less than) the notional calculations.

On ‘over glazed’ extensions, compliance can be achieved by offsetting excess carbon emissions with compensatory measures such as upgrading the glazing or insulation levels within the extension, or by upgrading elements within the existing dwelling, such as heating controls etc.

IQ Glass’s Super Insulating Units can go a long way to ensuring that your notional calculations and CO2 emissions achieve compliance.