Part L1B SAP Calculations

Are you building a heavily glazed extension?

We can produce a SAP calculation to show compliance with Part L1B of the Building Regulations.

Three steps to compliance:

  1. Use SAP to calculate the CO2 emissions for the existing house along with the proposed extension.
  2. Calculate the CO2 emissions for the existing house with a notional extension. A notional extension is an extension that meets the minimum standards of Part L1B.
  3. Compliance is achieved provided the "proposed" CO2 emissions are less than the "notional" CO2 emissions.

SAP allows the greatest level of design flexibility

Compliance with Part L1B is automatically achieved if the envelope of the extension is sufficiently well insulated, the area of openings is within the 25% allowance2 and building services within the extension are of a sufficient standard.

However for greater design flexibility SAP can be used to demonstrate compliance. By conducting a whole-house CO2 calculation it is possible to show that features of the design that do not meet the minimum standards of Part L1B (such as the level of glazing) have been compensated for elsewhere. This may be by surpassing the minimum standards elsewhere within the extension, or by making improvements to the heating or the insulation within the existing dwelling.

What I need to provide to the SAP assessor

  • Plans, elevations and sections for both the existing dwelling and the proposed extension;
  • Specification for the construction of all floors, roofs and external walls for both the existing dwelling and the proposed extension;
  • Specification for all windows and doors within the dwellings external envelope, including frame material, width of gap (if multiple glazing) and emissivity of glass;
  • Details of all heating, cooling and ventilation within the dwelling; and
  • Hot water system specification, including cylinder size, details of any insulation to the cylinder and pipes, and associated controls.

New regulations in force from 1 October 2010

For all applications submitted for Building Regulation approval post 1 October 2010 the new 2010 version of Part L will apply.

There are two key points to note within Part L1B 2010 relevant to heavily glazed extensions:

  1. The notional extension now has lower U-values, thus making it tougher to surpass Building Regulation standards within the extension and/or increasing the likelihood of having to upgrade the existing dwelling in some way
  2. There are prescribed standards that must be met when upgrading thermal elements within the existing dwelling3:
  Improved U-Value
Wall - cavity insulation 0.55
Wall - internal or external insulation 0.30
Floor 0.25
Pitched roof - insulation at ceiling level 0.16
Pitched roof - insulation between rafters 0.18
Flat roof 0.18

1 SAP is the Government's approved methodology for calculating the energy performance of dwellings

2 The 25% allowance is equal to 25% of the floor area of the extension plus the area of any openings that are no longer exposed as a result of the extension works.

3 If achieving the relevant U-value is not technically or functionally feasible or will not achieve a simple payback of 15 years or less, then the element should be upgraded to the best standard that is technically and functionally feasible and which can be achieved within a simple payback of no greater than 15 years.