Excess Glazing Calculations

Excess Glazing Calculations may be required for house extensions and conversion schemes, however by providing excess glazing calculations you can generally have large amounts of glazing as per your chosen design. You may need to increase insulation levels to the extension or make improvements t the existing dwelling in relation to upgrading existing insulation levels or improving heating system etc. 

For an extension not to require excess glazing calculations, it is allowed to have 25% of glazing to floor area and also the area of any existing openings which are covered or removed as part of the extension work before.

For example, if an extension has a floor are aof 20m² and has a nexisting patio door with an area of 4m² (patio door area being covered by extension) then the extension is allowed to have 5m² (25% rule) plus 4m² (existing openings rule) then the total allowable are awould be 9m².

Glazing area can be in the form of windows, doors and roof lights. If the house extension is outside of the guidance in Approved DOcument L1B, then Excess Glazing Calculations will be required.

Guidance u-values are as follows:

• Wall U-Value – 0.18
• Floor U-Value – 0.18
• Floor Swimming U-Value – 0.25
• Pitched Roof U-Value – 0.15
• Flat Roof U-Value – 0.15
• Insulated Ceiling U-Value – 0.15
• Window U-Value – 1.40
• Glazed Door U-Value – 1.40
• Half Glazed Door U-Value – 1.40
• Solid Door U-Value – 1.4
• Roof Lights U-Value – 2.20

When Building Control are carrying out plan checking or asking for information under a Building Notice, any extensions which have excess glazing beyond the above guidance will need some form of justification.

The Building Control Officer will ask for Excess Glazing Calculations, SAP Calculations or Thermal Calculations to justify excess glazing (All three mean the same thing). Guidance on this can be found in Approved Document L1B.

The two main methods to satisfy the guidance in part L1B for Excess Glazing Calculations are as follows:

• Area weighted U-Value method
• Full SAP Calculation (DER Calculations) to include the existing property

Area Weighted U-Value

This approach is suited for extensions that only have a small amount of excess glazing where the U-Values of the thermal elements can be improved over and above the general U-Values.

Two Calculations are required to achieve Building Regulations Compliance.

The first calculation is used in conjunction with the above guidance to achieve a total weighted average U-Value for the extension.

A secondary Calculation is used with the amount of excess glazing included and improved U-Values to the elements ie walls, floor and roof and glazing (excess glazing may be offset by over insulating just one element or it may be the case that all need to be upgraded.

Full SAP Calculations

For this calculation the above informatio nis still required, however in addition the existing property needs to be included.

Once again a notional calculation is required however in this apporach to achieve Building Regulation Compliance the existing property may be upgraded to offset the excess glazing.

This approach allows substantially more glazing to the extension (if any of the thermal elements to the existing property are upgraded they need to be upgraded to the U-Values given in ADL1B as bellow:

• Wall U-Value * – 0.55
• Wall U-Value – 0.30
• Floor U-Value – 0.25
• Pitched Roof U-Value – 0.18
• Flat Roof U-Value – 0.16
• Insulated Ceiling U-Value – 0.16
• Window U-Value – 1.60
• Glazed Door U-Value – 1.80
• Half Glazed Door U-Value – 1.80
• Solid Door U-Value – 1.80
• Roof Lights U-Value – 1.60

* A lesser provision may be appropriate where meeting such a standard would result in a reduction of more than 5% of the internal floor area of the room bounded by the wall.

As part of the offset in the existing house examples of elements that can be upgraded are if you are having cavity wall insulation installed as part of the works, upgrade the existing roof insulation, new glazing or a new boiler there are other measures if these are not possible.

Information Required To Carry Out Excess Glazing Calculations

• Construction drawings can be provided in a hard copy or email format (as long as these can be printed off to A4/A3 to scale), the general scales we work to are 1:50, 1:75, 1:100. Other scales are acceptable however these are the common ones that are used. We fully understand that in some circumstances not all construction drawings are complete, we can overcome this in most circumstances so simply contact us for more information (we will send you out a standard form to fill in for missing information). However, for ne wdwellings in particular full working drawings are required due to the fact we at RTS have to provide these for auditing purposes.
• Floor plans (Existing and Proposed)
• Elevation (Existing and Proposed)
• Sections (Existing and Proposed)
• Site Location Map (of orientation of Proposed Dwelling)*
• Specification (we will change your specification for you in order to achieve compliance for Building Regulation purposes, this will include SAP Calculations, DER Calculations, Thermal Calculations, Excess Glazing Calculations)*
• Heating Systems to be used – primary and secondary
• Any Low zero technology to be used i.e solar panels, Photovoltaic’s, Ground Source Heat Pumps, Air Source Heat Pumps.

*Please note that if you do not have this information for the existing house please contact us and we will send a form of information that is required to make this easier for you. Also please contact us if you have any questions regarding House Extensions of Excess Glazing Calculations.

There is no need to panic, there are lots of ways to offset excess glazing when providing Excess Glazing Calculations.