(Type your query in the field above and press enter to search)

Extending a Home in Guernsey – Our Guide to Part L

4 Jul 2019 at 10:31 AM

Each of the Bailiwick’s of the Channel Islands apply their own rules for the conservation of fuel and power in buildings.

These cover new build homes, non-domestic space and conversions. These are a distinct set of criteria, and the procedure for assessment is very different to those in Scotland, England and Wales.

This blog will look at the detail of the carbon reduction requirements of The Guernsey Technical Standard – Building (Guernsey) Regulations 2012, Part L1 and how an extension to a home is assessed.

In accordance with L1 there are three options available to demonstrate compliance where an existing dwelling is extended. These are the Carbon Index Method, the Elemental Method and the Target U-value Method.

The Carbon Index – Relic from SAP 2001

In accordance with 1.11 of the technical standard, the carbon index method can be used only if applied to the whole enlarged dwelling. The carbon index method is defined within the technical standard as follows:

“The Carbon Index adopted in this method is defined in the SAP, and the requirement would be met if the Carbon Index for the dwelling (or each dwelling in a block of flats or converted building) is not less than 8.0. 1.28 The edition of SAP used for the calculation of the Carbon Index should be the edition having the UK’s Secretary of State’s approval at the relevant time in the particular case.”

The Carbon Index was a feature of SAP 2001, however the replacement SAP 2005 removed this feature upon its release. The current version of SAP approved for use in the UK is SAP 2012, with SAP 10 expected shortly. As such it no longer possible to follow this method and it can be ignored.

Elemental Method – Unsuitable for Most Homes

According to 1.3 of the technical standard, the elemental method is suitable for alterations and extension work, and for new build work when it is desired to minimise calculations. This method consists of ensuring that U-values do not exceed limits set in Table 1 for the extension. This table is reproduced below:

Guernsey Part L1 table 1

When using the elemental method to demonstrate compliance, there are also requirements concerning the areas of openings. These are as follows:

  1. That areas of openings in the extension do not exceed 25% or floor area of the extension plus any covered openings; or
  2. That the area of openings in the enlarged dwelling does not exceed the area of openings in the existing dwelling; or
  3. The area of openings in the enlarged dwelling does not exceed 25% of the total floor area of the enlarged dwelling.

As each of these points is often not met by a proposed extension, and it is often not desirable to reduce the size of the openings. We are instead directed to the target U-value method by section 1.10 of the technical standard (“…areas of windows, doors and rooflights larger than that given [above] may be adopted in particular cases by using the Target U-value Method to demonstrate compliance”).

Target U-value Method

In accordance with 1.11 of the technical standard, the target U-value method can be used only if applied to the whole enlarged dwelling. This means that both the existing property and the new extension must be assessed as a whole.

Fabric Performance

The target u-value method is outlined from 1.16 to 1.20 of the technical standard. Compliance is met if the calculated average U-value of the dwelling does not exceed the ‘Target U-value’, or UT value, adjusted for the proposed method and solar gain of heating. The target U-value is determined from the following equation:

UT = [0.35 – 0.19(AR/AT) – 0.10(AGF/AT)  + 0.413(AF/AT)]

  • UT is the target U-value (prior to any adjustment for heating system performance or solar gain)
  • AR is the exposed roof area;
  • AGF is the ground floor area;
  • AF is the total floor area (all storeys);
  • AT is the total area of exposed elements of the dwelling (including the ground floor).

Several adjustments must be made to the UT value in the following order before compliance can be determined.

Target Adjustments for Heating Systems

In accordance with the approved document, these target values must be adjusted depending upon proposed heating performance. Where the reference boiler SEDBUK value as indicated below is used, no adjustment to the UT for boiler efficiency is necessary:

  • Mains natural gas: 78%
  • LPG: 80%
  • Oil: 85%
  • Oil-fired combination boilers: 87%

Where the proposed boiler SEDBUK value used is better or worse than values above, the target U-value must be converted by multiplying the UT by the factor fe where:

fe =   Proposed boiler SEDBUK (%)

          Reference boiler SEDBUK (%)

For boilers for which the SEDBUK is not available, the appropriate seasonal efficiency value from Table 4b of the SAP 2001 may be used instead. Where a heating system other than those outlined above is proposed, or where the heating system is not yet decided, the UT value is made more demanding by dividing by a factor of 1.15.

Target Adjustments for Solar Gain

For dwellings whose windows have metal frames (including thermally broken frames) the UT value can be further adjusted by multiplying by a factor of 1.03. This increase takes account of the additional solar gain due to the greater glazed proportion.

By default, the UT value equation assumes an even distribution of glazing the Northern and Southern elevations. Where the area of glazed openings on the South elevations exceeds that on the North, the benefit of solar heat gains can be considered and used to ease the UT value by adding ΔS. ΔS is calculated and then added to UT as follows:

ΔS = 0.04 x [(AS – AN) / ATG]

  • AS = Area of glazed openings facing south;
  • AN = Area of glazed openings facing north;
  • ATG = Total area of all glazed openings in the building;
  • South-facing is defined as facing South ± 300;
  • North-facing is defined as facing North ± 300;
  • The area of glazed openings includes the area of the frames.

The final UT value can then be compared with the actual performance of the property and proposed extensions. For this, an average U-value figure is calculated from every floor, roof, wall and opening.

Poorest Acceptable U-values

As well as meeting the UT value, the design must be such that no U-values fall below the poorest acceptable U-values. These are as follows:

  • Roof: 0.35
  • Exposed Walls: 0.70
  • Exposed Floors: 0.70

Daylight Considerations

Paragraph 1.16 of the technical standard outlines considerations which must be paid to daylighting when considering the sizing of openings. These are as follows.

“In adjusting the areas of windows, doors and rooflights, however, consideration should be given to providing satisfactory daylighting. BS 8206: Part 2 ‘Lighting for buildings: Code of practice for daylighting’, BSI, London, 1992. gives advice but in general total opening areas of less than 17% of the total floor area might be inadequate”.


The rules outlined within the approved document effectively mean that every extension must involve an assessment of the whole home, as few extensions will have glazing smaller than the sizes given in the Elemental Method, and the Carbon Index option is now obsolete.

This means that anyone extending a home must compensate for the performance of the existing property, which is not always the case in the Building Standards and Building Regulations enforced elsewhere on the British Isles.

Need further help?

Call us for a chat on 0330 055 34 05 or email


Contact us

We are always there to help. We'd love to hear from you!

Find Us